QRV09 DIY Headphone Amp

INTRO: A gentlemen in Sweden, Per-Anders Sjöström, has an interesting audio website and number of high quality diyAudio designs. I think he strikes a useful balance between solid engineering and audiophile excess. One of his most recent designs is the QRV09 Ultra High Performance Headphone Amplifier. The design generated a lot of interest at diyAudio and resulted in a QRV09 group buy of PC boards and various parts. Per-Anders has been very generous with his time considering he’s not likely to make any profit from the QRV09. I really admire “DIY Audio Philanthropists” sharing their knowledge and designs. But the one thing missing, from my perspective, was a proper instrumented test of the QRV09. So I decided to contribute some of my own time and try to fill that void.

THE DESIGN: The QRV09 uses very high-end Analog Devices AD8610 op amps for the input gain stage and the Texas Instruments TPA6120 as the output buffer. The TPA6120 is also used in the FiiO E9 I recently tested. The QRV09 has a relatively low gain of 2X (6 dB) but that’s easily changed. It includes an on-board AC power supply and jumpers to change the output impedance from 10 ohms to 120 ohms. You can read more on the Sjöström Audio Website.

NOT INCLUDED: The QRV09 is somewhat unusual in that it lacks an on-board volume control, power switch, and input/output jacks. So you’re on your own to implement those. The good news is that allows more flexibility configuring the QRV09 into a wider variety of enclosures and applications. It also allows using connectors and a pot of your choice rather than being limited by the PCB footprint. But its more difficult to use the board “standalone” by itself. I added temporary RCA input jacks, an IEC power receptacle, and a 1/4” headphone jack on a short “pigtail” for testing.

SURFACE MOUNT DESIGN: This project isn’t for beginners. You need experience working with surface mount technology (SMT) parts—specifically 0805 components which are only 0.08 by 0.05 inches or about 2 mm x 1.2 mm. The entire part is smaller than the tip of most pencils and there are about 60 of them on the QRV09 board. Three 0805 resistors in their paper tape are shown in the photo to the right with a normal pencil. Most find they need a special soldering iron tip, very fine gauge solder, really good tweezers, a magnifying lamp, flux, a very steady hand, and lots of patience. The other problem with SMT is once a part, even a resistor, is fully soldered it’s very difficult to remove or reposition without specialized equipment or unusual techniques. It’s a bit like gluing a model ship together inside a bottle. As one builder said on diyAudio, he was ready for brain surgery after assembling his QRV09.

TPA6120 SOLDERING: The TPA6120 output IC is a relatively small surface mount part that can, in theory, provide over 1 watt per channel of output but this requires it to dissipate nearly 3 watts of power. That means it needs a decent amount of PC board copper as a heatsink. TI, of course, thought of this and there’s a large metal thermal pad on the underside of the package. You can find various suggestions for how to solder this inaccessible pad but my personal opinion is it’s relatively difficult for a DIY’er to do correctly. And regardless of your technique, it’s even more difficult to verify you got it right. Per-Anders was kind enough to offer PCBs with the TPA6120 pre-soldered. My suggestion would be a large plated through hole big enough to get a soldering iron tip and some fine gauge solder through under the part. Then it would be possible to wick solder in from the bottom of the board and ultimately fill in the hole. The QRV09 board, however, has only tiny feed-through holes under the heatsink pad. TI assumed the part would be soldered with a commercial IR reflow oven and solder paste but these are not an option for most DIY builders. The TI datasheet notes if the pad is not correctly soldered to a suitable heatsink (copper PCB area) the IC may fail. See the Tech Section for more details.

USABILITY: If you don’t want to leave the QRV09 powered on 24/7, it does create a bit of a “thump” if your headphones are connected. How loud the transient is will depend on your headphones.  Also, I was surprised the Pulse Electronics transformers from DigiKey (553-1630-5) emit a soft buzz (from the transformer itself, not the headphones) that’s audible from a few feet away with the PCB exposed in a very quiet room. They’re the only readily available 120 volt transformers I know of in the USA that fit the PC board. Per-Anders used 240 volt European transformers that don’t have this problem. The QRV09 uses about $5 per year of electricity (USA rates) if left on all the time.

GAIN: The gain of the standard design is lower than normal at 2X (6 dB). This means to get the 2 volts RMS needed to drive say a pair of Sennheiser HD650 headphones, you need 1 volt RMS of input. That’s usually not a problem with a home pre-amp, home DAC, etc. But it might be a problem for a lot of portable devices and even some PC audio outputs. A gain of 3X or 4X is better if you want to use a wider variety of sources such as an iPod. I ran tests with both 2X and 4X gain. Many home sources are limited to about 2 volts of output and 2X gain limits the amp to 4 volts when it’s otherwise capable of over 7 volts. Considering how difficult it is to replace surface mount resistors, jumpers for selectable gain might have been useful. So choose your gain carefully before you build. The downside of too much gain is more noise, a less useful volume control range, increased volume tracking error and possibly more distortion. The FiiO E9 has a gain switch on the back to choose between 10 dB (3X) and 18 dB (8X) gain. See my article on amps and DACs for more.

SOUND QUALITY: The stock design sounded fine with my higher impedance headphones. I mainly listened with my Sennheiser HD650s and didn’t notice any problems beyond the power on/off glitches mentioned above. It has more than enough power and, in casual comparisons, sounded a lot like my $1600 Benchmark DAC1 Pre’s headphone output. With my Ultimate Ears Super Fi 5 Pros, however, it sounded notably different than the Benchmark due to the QRV09’s 10 ohm output impedance. With a modification to lower output impedance it again sounded very similar to the Benchmark (see Tech Section for details).

HISS & NOISE: Even with my very sensitive Ultimate Ears the QRV09 was quiet but, as explained in the Tech Section, that’s at least partly due to the lack of a volume control and the abnormally low gain.

MEASUREMENT SUMMARY: The QRV09 measured very similarly, but generally a bit better, than the FiiO E9 which isn’t surprising as the FiiO E9 uses the same output IC. The E9 has a higher current power supply and delivered more power into 15 ohms but even the QRV09 delivers more power than anyone is likely to need at any impedance. The QRV09 is quieter and has lower distortion into higher impedance loads. They both suffer from the same 10 ohm output impedance which is far from ideal but OK for most higher impedance headphones (see my Impedance Article for why and the Tech Section for a possible fix). And they both have higher than expected distortion at low frequencies into low impedances. As usual, all the geeky details are in the Tech Section but here’s a summary and comparison

Measurement	QRV09	FiiO E9	AMB Mini3
Frequency Response	+/- 0.1 dB Excellent	+/- 0.1 dB Excellent	+/- 0.1 dB Excellent
THD 1 Khz 150 Ohms	0.002% Excellent	0.005% Excellent	0.002% Excellent
THD 1 Khz 15 Ohms	0.022% Good	0.037% Good	0.017% Good
THD 20 hz 15 Ohms	0.07% Good	0.05% Good	0.01% Very Good
THD 20 Khz 15 Ohms	0.02% Very Good	0.003% Excellent	0.45% Poor
IMD CCIF	0.02% Good	0.05% Good	0.043% Fair
IMD SMPTE	0.0015% Excellent	0.002% Excellent	0.009% Very Good
Noise (ref 400 mV)	-99 dB Excellent (1)	-88 dB Fair	-94 dB Excellent
Max Output 15 Ohms	450 mW Excellent	1067 mW Excellent	104 mW Excellent
Max Output 150 Ohms	345 mW Excellent	317 mW Excellent	38 mW Fair
Output Impedance	10 Ohms Fair (2)	10 Ohms Fair	0.9 Ohms Very Good
Crosstalk 15 Ohms	67 dB Excellent	63 dB Very Good	40 dB Poor
Channel Balance Error	N/A (3)	1.8 dB Fair	1.14 dB Fair

1. The noise measurement isn’t directly comparable, see the Tech Section
2. A modification is discussed in the Tech Section reducing this to 0.08 ohms which is excellent
3. There is no volume control to cause tracking error

FIRST CLASS:

• Excellent performance into higher impedance loads (greater than 64 ohms)
• Lots of output voltage for high impedance headphones
• On board power supply is convenient
• Jumper for selectable output impedance (10 or 120 ohms)
• Protected from RF and DC at the input

ECONOMY:

• 10 ohm output impedance is a problem with many headphones
• Disappointing distortion at low frequencies into low impedance loads
• Gain is too low for many headphones using a portable or PC source
• Does not include volume control, jacks or power switch
• Requires considerable surface mount soldering skills to build
• Inaccessible output IC thermal pad difficult to solder correctly
• Uses some expensive parts (most notably the AD8610 op amps)
• No headphone protection and has some “thumps” on power on/off

BOTTOM LINE: Unlike say the AMB Mini3, the QRV09 is a very solid design on paper. Per-Anders did an excellent job and the QRV09 follows solid engineering practice rather than attempting to solve problems that are more audiophile myth than reality. It also includes useful features the Mini3 lacks like RF and DC input protection. But, in my opinion, the QRV09 is another example of a design that looks great on paper but does not perform as well as expected in reality. My measurements uncovered some concerns that may disqualify the QRV09 as “Ultra High Performance”. But it still is very good into higher impedance loads. The performance is very similar to the FiiO E9. It’s a DIY project most can be reasonably proud of. But if you’re looking for the most bang for your buck, the FiiO E9 is probably cheaper when you consider all the costs. The E9 also adds several features including adjustable gain, line out, and a protection relay. In terms of ultimate performance, the Benchmark DAC1’s headphone amp is considerably better than either one. And just the headphone amp section of the DAC1 could be built for around the same price as the QRV09. But, again, things like PCB layout, grounding, etc. are critical if you hope to match the Benchmark’s performance. Ultimately, there’s no substitute for making the right measurements.

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TECH SECTION:

DESIGN: The gain stage op-amps are individual left and right Analog Devices AD8610s that cost nearly $10 each at DigiKey. The TI TPA6120  is a somewhat unusual stereo current feedback power op amp with gain (it’s not a pure buffer like the National LME49600 or TI BUF634). Here it’s used at unity gain with only local feedback. There is no global feedback loop. The main downside to the TPA6120 is TI recommends at least 10 ohms of series output resistance to isolate reactive loads for stability reasons. As I’ve explained many times, including in my Impedance Article, I think that’s unacceptably high for many headphones—especially balanced armature IEMs such as those from Ultimate Ears, Etymotic, Shure, Westone, etc.  But it will also affect the sound of many full size dynamic cans.

INPUT FILTER: The QRV09 intelligently includes an RF filter on the input. In this age of ubiquitous mobile phones it makes good sense. RF is easily rectified to DC at the input of an op amp and that can create substantial DC on the output causing unpleasant noises, and worse, damage expensive headphones. The QRV09 uses 1K in series and 100 pF to ground to help kill RF. It also wisely includes a coupling capacitor to block DC from the source. Per-Anders used 0.1uf Polyphenylene Sulphide (PPS) 2416 SMT film capacitors. The amp I tested used 0.12 uF Panasonic ECH series PPS caps. There’s also a “bleed” resistor on the input side of the capacitor eliminating any residual DC. The 0.1 uF cap and 470K resistor form a high pass filter that blocks DC with negligible effect in the audio range. Good film coupling caps have been demonstrated to be sonically transparent in blind tests, measurements, and audio differencing. So I very much agree with Per-Anders’ design decisions addressing real world problems of stray RF and DC offset at the input without any sonic sacrifice.

NICE LITTLE TOUCHES: The power supply has extensive filtering capacitors and even uses a MOV and fuse for protection. There are board-to-wire connectors for all inputs and outputs. Per-Anders clearly sweats the details.

GAIN MODIFICATION: I ran most tests with the “stock” 2X gain. I also spot checked several applicable measurements after changing the gain to about 4X by paralleling a 470 ohm resistor on top of R4 and R16 (1K to ground on the negative op amp input).The parallel result is 320 ohms which gives a gain of 1 + 1000/320 or 4.1X (12.2 dB). Most of the graphs indicate 2X or 4X. As expected the noise is about 6 dB worse at the higher gain setting. The ultimate crosstalk into 100K was also reduced by several dB. But the distortion measurements were mostly very similar indicating the input op amp is not the dominant source of distortion.

OUTPUT IMPEDANCE MODIFICATION: I tried several ways to reduce the output impedance. I ultimately found some specific 0805 chip ferrites that seem to provide sufficient reactive load isolation and stability while providing a near zero ohm output impedance. The ferrites are from Laird and the part number is HI0805O121R-10 (DigiKey 240-2393-1). These are rated at 120 ohms at 100 Mhz and 3.5 amps DC with a 20 milliohm DC resistance. I tried several ferrites and several inductors including 0805 chip inductors, conventional through hole inductors, and an air core inductor. The ferrites provided the best square wave performance, and perhaps more important, offer easily reproducible results. Any “off board” solution would be more prone to implementation details, and where stability is concerned, that can make a big difference. The ferrites are simply paralleled and soldered on top of the 10 ohm output resistors R10 and R22 as shown in the photo to the right (click for a larger version). The 10 ohm resistor remains to provide damping for the ferrite’s resonance. There are, however, some trade offs to reducing the output impedance this way. I discuss them in the measurements below.

THERMAL ISSUES AND A SOLDERING CHALLENGE: As mentioned earlier, the TPA6120 heatsink pad is very difficult to properly solder. I played typical pop music, hitting peaks of only 1 volt, into a 15 ohm load for 15 minutes and the TPA6120 plastic package was at 120 F or about 50C. The die inside was probably over 70 C. That’s not dangerously hot, but it’s a possible sign the thermal pad is not working as well as TI intended due to poor soldering. Driving 300 ohm headphones to typical levels the pad is probably a non-issue. But in more demanding applications, or with high ambient temps, it might be. The other danger is someone could easily damage the part, perhaps in non-obvious ways, getting it too hot trying to solder the pad. One possible solution is not even trying to solder the pad and use lower supply rails (which can be done with just resistor value changes on the regulators). That would substantially decrease the dissipation but also the maximum output swing into power hungry high impedance cans. In theory, the part is thermally protected but quoting the datasheet regarding the hidden pad:

“This acts as a heatsink and must be connected to a thermally dissipating plane for proper power dissipation. Failure to do so may result in exceeding the maximum junction temperature that could permanently damage the device. “

FREQUENCY RESPONSE: There’s a slight roll off of about 0.3 dB at 10 hz from the DC blocking input cap but the response is  +/- 0.1 dB from 20 hz to 96 Khz. The –3 dB point, even with the ferrites in place, was 220 Khz. Some might argue this is excessive bandwidth but it does keep the phase error low. The channel balance, with no volume control, was excellent (within 0.004 dB). The frequency response below 48 Khz was independent of the load impedance:

THD vs POWER OUTPUT STOCK: The power output in stock (10 ohm) form was ample but significantly less into low impedance loads than the FiiO E9 using the same TPA6120. With some investigation I found the power supply is falling out of regulation (see Power Supply Constraints below). Still, 450 mW into 15 ohms should be far more than anyone needs and far in excess of what the Mini3 and most headphone outputs can manage. And the dynamic (short term peak) power is much higher (closer to the FiiO E9) due to the 470 uF caps after the regulators. THD+N into 150 and 600 ohms was very low and noise dominated the measurement below 1 volt. The distortion was quite a bit higher into low impedances but still generally below about 0.01%. The graph shows output voltage vs THD+N at 1 Khz into four load impedances. The power is shown at approximately 1% THD for each load:

THD vs POWER FERRITE MOD: With the ferrite modification you get somewhat more power into 15 ohms as you’re no longer dropping nearly half the power across the 10 ohm output resistors but the limiting factor is still the power supply. It’s also worth noting the ferrites roughly double the distortion from about 0.012 to about 0.025% across much of the range:

POWER SUPPLY CONSTRAINTS: The 12 volt rails drop out of regulation at about 1 volt RMS into 15 ohms with both channels driven by a sine wave. They’re down to 8 volts at 2.5 volts RMS into 15 ohms. The SOT223 regulators are rated for 1 amp, which is plenty, but the the small AC transformers can’t handle the load. Some might be bothered by the sag but it’s not likely to happen in real world use with music, typical loads and realistic output levels. And there is even some benefit if the rails do sag. The dissipation in output devices increases exponentially with the supply rail voltage. Amps are more efficient, and produce less heat, when the rail voltage is matched to the load. That’s why some power amps have impedance switches on them and others use multi-rail or “smart” tracking power supplies (i.e. Class G, H, etc.). Hence the QRV09’s “soft” power supply helps reduce power dissipation in the TPA6120 under extreme use when the TPA6120 is most likely to get too hot. Soft power suppliers are common in mainstream amps and receivers as they allow manufactures to use smaller heatsinks and save money. The small transformers also fit well on a reasonably sized PCB and into slim enclosures.

OUTPUT IMPEDANCE: With the 10 ohm resistors in place in the stock design, 400 mV no load dropped to 240 mV into 15 ohms giving, as expected, a 10 ohm output impedance. With the jumpers removed the output impedance was 120 ohms. And with the ferrite modification 400 mV no load barely dropped to 398 mV with 15 ohms giving an output resistance of 0.08 ohms—the lowest I’ve yet measured. Even at 20 Khz it only rose slightly to 0.09 ohms. This is excellent performance with the ferrite mod. In case you think 10 ohms output impedance is just fine, here’s what 10 ohms does to the frequency response of my Ultimate Ears SuperFi 5 Pro headphones (from the FiiO E9 which has the same TPA6120 and same 10 ohm output resistor):

THD+N 1 KHZ 150 OHMS: Here’s the impressively low distortion at 1 Khz and 1 volt into 150 ohms. For comparison, the FiiO E9 measures about 0.005% on this test. Also note the channels are well matched which indicates a symmetrical PCB layout:

THD+N 1 KHZ 15 OHMS: There’s about 10 times the distortion into 15 ohms with the ferrite. Without the ferrite, this drops to about 0.012% but that’s still about 5 times the distortion compared to 150 ohms. That said, this is still a significantly better result than the FiiO E9 which was nearly 0.04%. And it’s mainly 2nd harmonic which is the most benign. This is likely inaudible as the higher harmonics are all below –90 dB:

THD RESIDUAL 150 OHMS: Here’s the 1 Khz residual at 150 ohms and there’s perhaps a hint of crossover distortion but it’s fairly benign:

THD RESIDUAL 15 OHMS: Here’s the 1 Khz residual at 15 ohms showing it’s mainly 2nd harmonic with no signs of crossover distortion (note the different vertical blue scale compared to above):

THD 20 KHZ 150 OHMS: Into 150 ohms the high frequency distortion, measured out to 88 Khz, was very low:

THD 20 KHZ 15 OHMS: Into 15 ohms, with the ferrite, it’s notably worse but still not much to worry about with no spectral components above –80 dB anywhere in the audio range. See the Frequency vs THD graph for the distortion without the ferrite. Here the FiiO E9 has the advantage. It measured 0.003% on this test. I suspect the FiiO has a better PCB layout for routing the higher currents required at 15 ohms and/or uses different feedback:

THD 20 hz 150 OHMS: Into 150 ohms the QRV09 does very well at 20 hz with everything below –90 dB and the 2nd harmonic dominant. (Note the THD reading is slightly higher than the THD+N reading because it uses a different notch filter to remove the fundamental while leaving only the harmonics. Due to the “excessive” width of the fundamental at 20 hz, even with a 256K FFT, more of the fundamental leaks into the result. I’ll remove it from future 20 hz results):

THD 20 hz 15 OHMS: Into 15 ohms (no ferrite), just like with the FiiO E9, things get worse. The good news is the 2nd harmonic still dominates and is the only non-linearity over –80 dB but it’s a lot over at about –62 dB. Everything else is below –80 dB. The ear is less sensitive to low frequency distortion and the 2nd harmonic, so this is likely still inaudible, but this isn’t what I would call “ultra high performance”. The FiiO E9  measured a similar 0.05% on this test. If you’re thinking the dScope is somehow to blame, have a look at the Cmoy Review to see 0.0048% into 15 ohms—about 10 times lower--and the Benchmark DAC1 measures much better as well:

THD 20 hz 15 OHMS 100 mV: One source of low frequency distortion can be thermal distortion. Wondering if that’s to blame here, I tried reducing the level to only 100 mV and driving only 1 channel but it didn’t help much and the 2nd harmonic is still about –65 dB.

THD 20 hz 150 OHMS WITH FERRITE: Here’s the same test as above with the ferrite. The ferrite roughly doubles the distortion and much of the increase is in higher order harmonics. This is where an air core inductor has an advantage but, in my tests, created significantly more ringing on the output and is more subject to implementation issues. Regardless, based on the testing I’ve done, the TPA6120 forces you to trade off distortion, output impedance and ringing/impulse response. The best solution will depend on what headphones you’re using. With balanced armature IEMs a 10 ohm output impedance is unacceptable. So it comes down to more ringing vs a bit more distortion. For the sake of maximum stability and repeatability, I chose the higher distortion. At 80hz it’s only slightly above –80 dB so it’s still very likely inaudible. To me, it’s the best compromise but an even better solution is to use a better output device than the TPA6120 that’s stable without anything in the output (ignore the typo of “10 hz” in the caption—you can see it’s 20hz in the spectrum).:

THD+N vs FREQUENCY: Here’s the THD vs frequency into both 150 ohms (yellow) and 15 ohms (blue) and it looks amazingly like the FiiO E9 graph shown below. There’s a steady rise in distortion into low impedance loads below about 5 Khz. This is the stock 10 ohm design without the ferrite. I don’t know if this is a PCB layout issue or some inherent problem in the TPA6120 but it’s not very impressive. The Cmoy I tested, with it’s $0.80 op amp, does much better at 15 ohms:

FiiO E9 FOR COMPARISON: Note how similar the FiiO E9 curves are below to the QRV09 above:

THD+N vs FREQUENCY WITH FERRITE 150 OHMS: Here’s the 150 ohm plot with (blue) and without (yellow from above) the ferrite. Only below about 200 hz does the ferrite significantly add any distortion and it’s still nothing to worry about:

THD+N vs FREQUENCY WITH FERRITE 15 OHMS: Here’s the comparison with and without the Ferrite at 15 ohms and due to the much higher current it, as would be expected, causes more distortion. You can see some weirdness above about 5 Khz and overall higher distortion:

IMD CCIF: Here’s the CCIF IMD at 15 ohms. This is significantly better than the FiiO E9 which measured 0.05%. The –72 dB component at 1 Khz is a 2nd order distortion product from the difference of the 19 Khz and 20 Khz signals. Ideally, I’d like to see it below –80 dB. The good news is the higher order sidebands around the 19 and 20 Khz tones are very low at about –105 dB:

SMPTE IMD 150 OHMS: Into 150 ohms the SMPTE IMD was extremely low (just about as-good-as-it-gets—the IMD distortion is mostly lost in the noise and entirely below –110 dB):

SMPTE IMD 15 OHMS: Into 15 ohms there’s mostly just the higher harmonic (not IMD) distortion from the 60 hz component. The actual IMD is still very low at 0.0015% which is slightly better than the FiiO E9’s 0.002%:

INTERCHANNEL IMD: I ran some interchannel tests and the results were excellent as would be expected with isolated power supplies and a real ground. This review is already quite long so the graphs are not shown. The result was almost entirely dominated by the crosstalk from the headphone jack.

CHANNEL SEPARATION (CROSSTALK): I should have explained in previous reviews the crosstalk in a headphone amp is sometimes limited driving real world headphones by the 3 wire connection. Both channels share a common connection, and the lower the load impedance, the more that connection contributes to the crosstalk. Most other devices I’ve tested have had factors dominante. It comes down to the PC board layout, the resistance in the jack itself, any wiring, grounding (or, worse, virtual grounding), how the feedback is configured, etc. I minimize the resistance of any external wiring when I attach the load which simulates using headphones with 4 wire cables. Still, even the jack resistance typically limits crosstalk to around 70 - 80 dB into 15 ohms and around 80 - 90 dB into 150 ohms. The QRV09 was indeed in this range so I also measured it at 100K to show (somewhat unrealistic) crosstalk within the amplifier itself. The results are shown below into 15 ohms (blue), 150 ohms (yellow) and 100K ohms (red). All three are excellent performance and better than the FiiO E9:

NOISE: Reproducing a –80 dB signal, the noise performance was the best I’ve measured but there are two reasons why it’s not a fair comparison. First, there’s no volume control and the dScope has a very low source impedance minimizing the Johnson noise. The AD8610 is so quiet the broadband noise is nearly entirely the Johnson noise of the 1K input resistor. With a typical 10K volume control in place there would be at least twice as much Johnson noise so the noise floor would increase by at least 6 db. Second, this amp has only 2X ( 6 dB) gain. Increasing the gain to a more typical 4X (12 dB) increases the noise floor another 6 dB. Note this is referenced to the same realistic 400 mV I always use. Referenced to full output (7 V RMS), the way a manufacture might spec it, the noise is about 20 dB better (-118 dB unweighted –126 dB weighted). The nearby AC transformers from the onboard power supply appear to be the limiting factor. Somewhat odd is the amplitude of the 180 hz artifact of the 60hz AC power and 120hz ripple. But everything is still below –110 dB which should be silent. The actual hiss portion of the noise (above 500 hz) is about 16 dB quieter than the FiiO E9:

PHASE RESPONSE: The phase response below 48 Khz was unchanged by the ferrites and the result below is with the ferrites. The input DC blocking capacitor causes the expected shift but it’s still only about 8 degrees at 20 hz. At 10 Khz the shift is an excellent 0.25 degrees. But this is to be expected with a bandwidth beyond 200 Khz:

GAIN: The gain measured 2X or 6 dB in the stock configuration and and 4.1X (12.2 dB) with my modification. The higher gain slightly increased the 100K crosstalk by several dB and increased the noise by 6 dB. It had very little effect on distortion which speaks well for the AD op amp’s gain bandwidth product and distortion but also indicates the TPA6120 and/or other factors are probably the weak link in terms of distortion. Other gains are, of course, possible just by changing the feedback resistors on the op amps.

CLIPPING BEHAVIOR: Input clipping was not an issue. The output will always clip first for any gain greater than 1X. Even into reactive loads the amp was stable at clipping with and without the ferrite modification. Here’s clipping with the ferrite into 600 ohms in parallel with 0.01 uF at 1 Khz. The TPA6120 gets fairly close to the supply rails and the clipping is nicely symmetrical:

CLIPPING SPECTRUM: Worried about possible instability I checked the spectrum while clipping and there was no unexpected ultrasonic garbage out to 96 Khz with the ferrites. I didn’t try sustained clipping into 15 ohms for fear of damaging the TPA6120 if the thermal pad isn’t properly soldered:SQUARE WAVE PERFORMANCE STOCK: With the same Sennheiser CX300 headphones I always use as a load, the stock QRV09 exhibited significant overshoot on a 10 Khz square wave. I’m guessing something is not quite right here because the FiiO E9 did not have this issue. It might be related to the PCB layout or perhaps input/feedback resistor values used for the TPA6120:

SQUARE WAVE PERFORMANCE WITH FERRITES: With the same CX300 headphones as above, but the 10 ohm output resistors paralleled with chip ferrites, the overshoot is gone. The red trace is just the headphones and the blue trace is the headphones with a 0.047 uF film capacitor in parallel. You can see a small amount of 500 Khz ringing related to the added capacitance, and a bit slower rise time in both traces, but the amp is stable:

SLEW RATE WITH FERRITE: The slew rate shown below between points 1 and 2 is 11.27 volts in 688 nS. That’s about 16 volts per microsecond. This is very fast and about double the FiiO E9. But, in fact, both amps need less than 2 volts/uS. The main point here is the ferrite chip doesn’t come close to causing speed problems even at full swing of over 20 V p-p into 600 ohms:

STABILITY WITH FERRITES: I conducted a lot of tests using various ways to bypass the 10 ohm output resistors while still isolating reactive loads for stability. I tried 1 ohm chip resistors and, with added capacitance on the output, observed signs of significant instability. A 1 uH chip inductor created more ringing and more distortion in the audio band compared to ferrites. And an air core “off board” inductor also created more ringing but less distortion. The classic DIY solution for audio power amps intended to drive speakers is a wire wound coil around a 1 or 2 watt carbon composition resistor. That created more ringing than the chip ferrites. As a torture test, I used about 20 feet of headphone extension cables strung together with a “Y” cable at the end. I used up to 1.5 uF of capacitance by itself and in parallel with various real headphones and resistive loads. I also tried lots of different headphones by themselves. Into all the loads, I ran a low level 10 Khz square wave and observed the response on a 100 MHz scope, as shown above, for signs of ringing or oscillation. Overall, I was most satisfied with the chip ferrites especially when you consider their implementation is nearly foolproof (unlike handwound coils and other off board solutions). Another thing to try would be to replace (or parallel) the 10 ohm resistors with 1 ohm resistors and add through hole ferrites to the 120 ohm jumper locations.

DC OFFSET: The DC offset measured an excellent 1.8 mV in one channel and 2.3 mV in the other. The expensive op amps likely help here.

POWER CONSUMPTION: The design draws about 40 mA AC at 120 volts or 4.8 VA at idle. The bulk of this are losses in the transformers. If left on 24/7 that’s about $5 per year in electricity. The DC power consumption is also about 40 mA from the 12 volt rails or only about 1 VA.

DESIGN COMMENTS: For anyone considering the QRV09. or a similar design. here are some things to consider:

• Output Impedance - As with the FiiO E9, and likely most other TPA6120 headphone amps, the 10 ohm output impedance is a problem for many headphones. I explain why in my Impedance Article and an earlier graph shows big (6 dB) frequency response problems into my IEMs. It also degrades electrical damping and hence the bass performance of many full size headphones like the Denon AH-D2000s. In my opinion, it prevents the TPA6120 from being a “one size fits all” high quality amp. Reducing the output impedance to near zero using on-board ferrite chips increases distortion somewhat (in many cases roughly doubling it). Some further experimentation might yield a better compromise and a more ideal output network. The danger here is there may be some headphones that could trigger instability and potentially damaging oscillation in the TPA6120 when the 10 ohm resistors are bypassed. So unless you have the means to fully test the stability of your particular project, I suggest care in trying alternate techniques. I chose the chip ferrites because they’re easily reproduced by others, relatively fool proof, and seemed like the best overall compromise (see Stability With Ferrites above).
• Low Frequency Distortion - The rising distortion at lower frequencies into lower impedance loads is a puzzle. When I observed it with the FiiO E9 I assumed it might be related to the DC-DC power supply in the E9. But it’s nearly exactly the same here with the QRV09 using a very conventional power supply. The problem is either inherent to the TPA6120 or both designs somehow have a very similar design flaw. One possibility is how the power supply and ground currents are handled. The QRV09 layout uses ground fills on both sides of the PCB rather than specific ground routing. That might cause higher distortion at low impedances but I would expect it to be similar at all frequencies. Anyone have any ideas as to the cause of low frequency distortion? 
• Dual Mono Questions - I’m not sure there’s much benefit to using two single op amps and two sets of power supply regulators when the TPA6120 is a single stereo IC and the separation is ultimately limited by the 3 wire output. You could dedicate a TPA6120 for each channel (and even parallel them) but in terms of the weakest link, the headphone jack is, by far, the biggest offender. That said the QRV09 is slightly better than the FiiO E9 in a few areas and the dual mono configuration might be partly responsible.
• TPA6120 – I’m not sure the TPA6120 is the best choice. I can understand why FiiO and other commercial manufactures use it as it’s relatively inexpensive (less than $2 per channel in volume). Their products are also manufactured with commercial IR reflow ovens and screened solder paste eliminating the soldering problems. But given the difficulty in properly soldering the heatsink pad by hand, the 10 ohm output resistor requirement, and the mysterious low frequency distortion, I consider the TPA6120 a marginal choice for a high quality DIY amp. On paper it looks good. But, for the two TPA6120 designs I’ve measured, the Benchmark DAC1’s headphone amp easily outperforms both and even the Cmoy I tested outperformed it in many ways into 15 ohms.
• Op Amp Overkill - I’m not sure the AD8610 is providing any meaningful benefit over several less expensive op amps. The real world noise is limited by the Johnson noise of the input circuitry and hum components from the onboard power supply. And the distortion is apparently more limited by the TPA6120 and/or other factors. The much cheaper OPA2134 in the FiiO E9 delivers very similar performance but it’s a dual op amp and not pin compatible. But the 2134’s single channel cousin, the OPA134, and the National LME49710, would work nicely. Both are about 1/4 the price of the AD8610. I have a future article planned on “op amp rolling” to explore any differences more.
• Output Relay – The FiiO E9 has an output relay with delayed turn on and immediate disconnect on power off. Given the QRV09’s audible “thumps” on power up and down I understand why FiiO went to the added expense. An output relay can also be used as DC protection to help save expensive headphones if the amplifier fails.
• Gain Selection – 2X gain might not be enough for some applications. Think carefully about your gain requirements before selecting the values for R4 and R16. See the gain comments in the review.

TECH SECTION SUMMARY: Generally the QRV09 measures better or similarly to the FiiO E9 except in high frequency distortion and maximum power output into low impedances. The FiiO E9 is a very similar design using the same TI TPA6120 so this isn’t surprising. The FiiO has a higher current power supply while the QRV09 is quieter, has less crosstalk, and lower distortion into 150 ohms and above. The biggest issue for both of them is the output impedance. See my comments in the Design Comments immediately above. Given the output impedance issue, and the relatively high levels of low frequency distortion, some might not consider the QRV09 (or the E9) “ultra high performance” but the performance is still very good into higher impedances. And, unlike the AMB Mini3, the distortion into even low impedance loads is likely still inaudible for the reasons mentioned. But, in terms of raw performance, the Benchmark DAC1 delivers significantly better performance than either one of them. And the Bechmark’s headphone amp section could be built for around the same price. But, that said, the QRV09 and FiiO E9 both sound and perform just fine with a lot of high impedance headphones including my Sennheiser HD650s. The QRV09 might not have the ultimate specs but it’s a solid design.

Sennheiser HD 650

REFERENCE CLASS? Sennheiser bills the HD 650 as “Reference Class”. For anyone writing reviews having solid references is important. That’s partly why I bought a Benchmark DAC 1 Pre. While my Denon AH-D2000s are respected in some circles, they’re not all that well known which reduces their usefulness as a reference. A few months ago I added a much more popular headphone to my collection: The Sennheiser HD 650. After dozens of hours of listening to a wide range of music I’m rather impressed but, I have to admit, they’re a bit of an acquired taste.

CHOICES: I wanted headphones revealing and accurate enough to expose subtle differences among different gear. They also needed to be compatible with typical headphone amps which rules out Stax and other electrostatics. References get more respect when they’re well known and well regarded. That eliminated the more obscure contenders like the Audeze LCD-2. And I wanted something that was not only revealing but I’d also enjoy listening to for hours on end. That ruled out ruthlessly analytical headphones like the AKG K701 and uncomfortable ones like the popular Audio Technicas. I ended up narrowing my choice to the Beyer DT880 and the Sennheiser HD 600/650/800. The DT880s, while quite nice, suffer from some of the same problems as my DT770s. That left the trio from Sennheiser.

THREE FLAGSHIPS: It’s hard to dispute the popularity of Sennheiser’s top headphones but there are endless debates on the internet about the relative merits of the HD 600, HD 650 and HD 800. Chronologically, the HD 600 was originally the top of the class (based on the discontinued but immensely popular HD 580). Then the HD 650 was added as a new flagship. And, most recently, Sennheiser went over the top with the HD 800. To make things more complicated, near as I can tell, not all HD 600s and HD 650s sound the same. Sennheiser has apparently made various production changes along the way. Some even claim HD 600 drivers have been used in HD 650s and visa versa. See the Tech Section for some further evidence. Perhaps the 600s and 650s had more obvious differences in the past, but listening carefully to the current crop, they’re three peas from the same pod. Published measurements back that up.

MAKING A CHOICE: I ultimately chose the HD 650 over the HD 600 because, to my ears, it’s the more pleasant of the two for extended listening with typical source material. And the HD 800, while generally more impressive, it’s hard to justify the roughly three times higher price. The 600 and 650 are already approaching the point of diminishing returns.

BASICS: The HD 650s are full size circumaural (around-the-ear) headphones with an open back. They don’t provide much isolation from the outside world, and the outside world may hear whatever you’re listening to. And the HD 650’s earcups are tilted in a way that makes wearing the headphones “backwards” immediately apparently if you accidentally put them on wrong. You also get a nice hinged permanent storage box with a foam insert.

SENNHEISER PRICING POLICIES: The street price for the HD 650 has gone up with Sennheiser cracking down on dealers excessively discounting their products. They’re now refusing warranty work unless you can prove you purchased from an authorized dealer. And authorized dealers are restricted to what’s known as MAP—a Minimum Advertised Price. The MAP appears to be around $500 while the MSRP (list price) is $650. If you find a price much lower than $450 it’s probably either for reconditioned headphones or it’s not an authorized dealer. If you don’t care about the warranty, there are better deals to be found although even used prices have gone up. Be cautious, however, of possible counterfits—especially for the HD600. Chinese HD600 fakes may also explain some of the larger than life differences some claim between the 600 and 650.

COMFORT: If you read my DT770 review, you know I’m big on headphone comfort. A lot of headphones sit more on the ears rather than over them. And even headphones that sit over the ears can still be uncomfortable because they’re too heavy, have a poor headband design, get sweaty, etc. The 650s are relatively light and have breathable velour covered earpads to combat any sweat problems. Likewise the headband pads seem to work well over the long haul. The 650s, as delivered, had a bit more clamping pressure than I’d prefer. Sennheiser increased the force over the HD 600 supposedly in the interest of better sound. But to my head, after a few hours, they became somewhat uncomfortable. But there’s a fix…

PRESSURE TWEAK: Wanting a bit less head squeezing, I very carefully bent each of the metal portions of the headband (one at a time) just enough to lessen the pressure a bit. And now they’re very comfortable although still not quite a match for the even lighter HD 590s. If you try this, be extremely careful not to put much pressure on the plastic portion of the headband or earcup supports as the plastic can break. The foam in the earpads also seems “break in” and soften making them more comfortable with time. I didn’t notice a huge change in the sound, but if you press lightly while listening to simulate the tighter clamping you can hear some subtle differences. So I’d try the 650s “as is” first. And if they’re uncomfortable, only then consider trying to bend them.

CABLE & PLUG: The HD650 uses a split “Y” style replaceable cable. It’s a 4 wire cable to better isolate the left and right drivers. Other headphones, like the HD590, share a common conductor in a 3 wire cable. Some prefer “side entry” like the 590 and DT770 but that only works well if it’s on the side your source is. The cable on the 650 is notably higher quality, softer, and less microphonic than the thinner and stiffer cable on the 600. If you like to spend money on snake oil (see Subjective Debate) there are several third party cables available. Unlike nearly every other headphone I own, the HD 650 has a 1/4” large plug and comes with a pigtail adapter for use with 3.5mm mini jacks.

AESTHETICS & BUILD QUALITY: I’m not much into bling and tend to emphasize function over form. For example, it’s frustrating when products are compromised in the interest of looking better or different—like the flat keyed “island” keyboard pioneered by Apple that slows down typing. And the new HD 598 with its Pimp My Ride treatment is less comfortable than the older HD 590 it replaces. But, that said, I have to point out the HD 650s don’t look and feel like $500 headphones. They’re almost entirely plastic, the glossy somewhat mottled paint looks tacky, “Sennheiser” is cheaply silkscreened onto the headband in giant letters, and the HD 650 badges are seriously cheesy. Mine were made in Ireland and whoever painted them probably had a few too many pints at lunch. As a point of reference, the AKG K701 and Denon AH-D2000 both cost less but use higher quality materials and seem more high-end.

DRIVER BREAK IN: A few readers challenged the idea of breaking in my Beyer DT770s as being audiophile myth. So with the HD650 I decided to see if I could measure a change. I tested them right out of the box and again after 48 hours of break in. There was a small but measureable change in the bass resonance. I don’t know if it’s audible but at least the concept has some basis in objective reality. See the Tech Section for more.

INITIAL SUBJECTIVE IMPRESSIONS: I thought the HD 650 had a pleasant honesty and openness upon first listen but otherwise it didn’t make a strong first impression. To me, they’re warm, mellow, and rather laid back. It’s neither sterile and analytical like say the best Etymotics, nor exciting and enhanced like the Beyer DT770 or some of the Ultrasones. But there’s more going on than is initially obvious. And, in some ways, I think the HD 650’s main strength lies in what’s missing from what you hear. Here’s the best analogy I can think of…

POLARIZED SUNGLASSES (seriously!): Have you ever compared regular and polarized sunglasses while driving a car or out on a lake? The polarized variety dramatically cut glare and reflections. The best sunglasses remove more of what you don’t want (glare) letting you see more of what you do want. In harsh conditions, they actually enhance your vision. Believe it or not, that describes what I like most about the Sennheiser HD 650. They seem to remove glare, sibilance, harshness and other obvious flaws that are so common in many (especially pop) recordings and somehow reveal more of the music. At first, the lack of harshness can leave music sounding a bit lifeless in comparison to other headphones. But, for me, much of what’s missing usually turns out to be unwanted anyway. And once you’ve adjusted, it translates into an amazing lack of long term listening fatigue. I find myself hearing things I never noticed before.

LONG TERM LISTENING: I’ve spent hours at a time listening to the HD 650s driven from my Benchmark DAC1 Pre on all sorts of music and they’re really easy to live with. After listening to them for an hour, switching to any other headphone sounds just plain wrong. That’s partly human nature and how our brains work. But the interesting thing is the reverse isn’t nearly as true. Listening to the Beyer DT770 for an hour and then switching to the 650, instead of sounding wrong, the 650s might sound a bit dull in comparison but their strengths are immediately apparent. There’s an honesty to the 650’s sound my brain seems to immediately recognize and latch onto. And, after just a few minutes with the 650, I have no desire to go back to the Beyer.

COMPROMISES: Using yet another car analogy, on a brand new glass smooth road even a hardcore sports car can deliver a fairly smooth ride. But on more typical roads that are worn, patched, potholed, etc. a big luxury car delivers a much smoother ride. You can’t have razor sharp handling and an ultra plush ride. It might not be as exciting, but if you want to cover 500 miles of choppy worn freeway, most would consider a good luxury car the more pleasant choice. The Sennheiser HD 650 is the Mercedes S Class of headphones. The big Mercedes makes bad roads seem better than they really are while still delivering an accurate and precise driving experience and the 650 does the same with typical flawed recordings. Few roads are glass smooth and most music is is not very well recorded, mixed and mastered. Both are a great solution to a very common problem. But a Mercedes S Class isn’t a Porsche 911. And, on the right roads, the Porsche can put a bigger smile on a car enthusiast’s face. But stuck in traffic or spending hours on the freeway the Mercedes is the better choice. The same is true here. For the best recordings there are more exciting headphones than the HD 650. But in the real world, with a large variety of typically recorded music, the 650 is hard to beat if you like an honest, relatively accurate sound.

THE FLAWED TUBE ANALOGY: Some have said the 650’s sound is “tube like” but I can only partly agree and it’s a critical distinction. Some tube gear has a similar “warm” sound often caused by the frequency response of the output transformers and/or a higher output impedance. But tube gear also typically has much higher distortion (euphonic or otherwise) and that distortion, if anything, masks details in the music along with the flaws. The 650 does the opposite and isn’t masking anything with added grunge. It has extremely low distortion and reveals more detail in the music.

A HINT OF OBJECTIVITY: I used my failed attempt at a poor man’s fake measurement “head” (a box with an embedded microphone) to level match the headphones using pink noise driving two at a time from essentially identical sources with a near zero output impedance. In short, I tried to level the playing field as much as possible. I listened to the same track on the 650 and the comparison headphone, switching back and forth as necessary, noted the differences, and moved on to the next track and repeated the process for a total of five very different pieces of music. Rinse, lather and repeat for the other headphones and you have at least a relatively fair subjective comparison.

SENNHEISER HD 590 vs HD 650: The HD 590 was my previous open-backed reference headphone. The HD 650 is slightly less efficient needing a few dB more volume. The first few times I swapped back and forth the 590 seemed a bit more “alive” due to a brighter high-end and an increased “presence” through part of  the midrange. The 650 sounded a bit boring in comparison. But focusing on some female vocals I quickly decided the midrange on the 650 is much closer to what a female voice is supposed to sound like. And then I noticed the 590’s brighter high end is mostly added harshness in the high frequencies rather than actual HF content from the recording. The HD590 shares this trait with the Beyer DT770 but it’s less obvious without a proper reference like the 650. The bass is very similar between the two headphones on most recordings. But if there’s really deep bass down low the HD 650 does a better job. And the 650 is more open and spacious—which is saying quite a bit as the 590 is no slouch in that area. The 650 ultimately comes across as more honest, with a vastly better midrange, and significantly less fatiguing. But the 590 is still the most comfortable headphone I own being lighter than the 650. Sennheiser has gone backwards in the comfort department.

DENON AH-D2000 vs HD 650: The Denons are my closed back reference and are significantly more sensitive (efficient) than the HD 650. The D2000 has a more forward and intimate sound than the 650. Instead of sitting 10 rows back, it’s like you’re on stage and the musicians have huddled around you. It’s a different perspective but it’s otherwise tough to fault the Denons when listening to clean recordings. I’d say the Denon’s lower midrange and upper bass is subjectively more accurate than the 650. Like the 590, the Denon is brighter in the highs but it’s a cleaner sort of bright. But it’s still a double edged sword and, in my opinion, the Achilles heel of the Denon. On less than ideal source material the D2000 shines a relentless spotlight on high frequency flaws. The sound is more strident and can become fatiguing much quicker. The lower bass is a bit more exciting on the Denon while the upper bass is more accurate but its overall similar to the 650. So, for me, it mainly comes down to the Denon’s slightly “cooler” midrange, brighter highs, more intimate soundstage and much less forgiving nature. On clean source material they’re both very pleasing headphones in their own ways. For evaluating gear and recordings, I think the Denons are the better choice. But for enjoying typical mass produced music, the Sennheisers get my vote for their ability to smooth over the rough edges. The Denons are also significantly less comfortable because they rest more on, rather than over, the ear with their oddly shaped ear pads. They’re also heavier.

BEYERDYNAMIC DT770 PRO 80 vs HD 650: The sensitivity is close but the HD 650 is slightly more power hungry. Much like with the 590, a quick switch with the DT770 makes the 650 seem flat and boring. The punchy deep bass and “enhanced” highs of the Beyer provides an even greater contrast. And, while there’s some initial appeal to the Beyer’s lively sound, the emphasized highs mean much greater fatigue. The highs suffer from the same problem as the HD590’s only worse. It’s mostly a fake sense of detail and, to my ears, is more distortion than music. The Beyer’s upper midrange isn’t nearly as natural as the 650’s. And when I listen to the 650 for a while and go back to the DT770 its like viewing an oversaturated photo that’s been heavily tweaked in Photoshop. Some might like the enhancements, but I find myself preferring the more realistic original. I will confess, on certain music, I do enjoy the DT770’s pyrotechnic deep bass. And, when driven from a low impedance source, I think the 770’s upper bass and lower midrange are probably more accurate than the 650’s. Subjectively, the 770’s bass emphasis is much lower down the spectrum avoiding the “warm” bias of the HD 650. In terms of soundstage and openness the 770 is at least in the same league but not as transparent. Without the exaggerated thump, sizzle, and grunge, the 650 is easily more revealing. I notice many more subtle details in recordings than with the DT770. The comfort is very similar.

SENNHEISER SIBLINGS: I don’t own the HD 600 or HD 800 but I did get to spend some time with each and compare them to the HD 650. There are probably a million words already written on the 600 vs 650--complicated by the fact Sennheiser has apparently changed both along the way—but here’s my two cents worth along with an HD 800 comparison:

HD 650 vs HD 600: To my ears, the current HD 600 has noticeably less deep bass, a slightly “cooler” balance through the midrange, and a bit more sparkle (and perhaps a bit of harshness) in the highs. But the difference is not nearly as dramatic as many have argued. More significant to me, on less than pristine source material, the HD 650 is more forgiving with significantly less listening fatigue. I also prefer the slight increase in bass although, ideally, I wish the emphasis was confined to the deepest bass more like with the DT770. Despite their more forgiving nature, the 650 doesn’t give up much in the way of resolution or detail over the 600—everything is still there, it’s just presented in a slightly warmer and more flattering way. In terms of pure accuracy, however, the 600 is probably the winner. If I only listened to audiophile recordings I probably would have chosen the cheaper HD 600 instead. The comfort is the same as my headband tweaked 650.

HD 650 vs HD 800: To me, the HD 800’s highs sound more like the the HD 600 while the deep bass sounds more like the HD 650. So, in some ways, it’s the best of both. The 800 is also better made and has a much higher bling-factor for those who care about such things. The 800 seems even more revealing than the 600 while the angled drivers and larger earcup volume contribute to a more spacious sound. For an ultimate reference headphone, the 800 easily wins and I would love to own a pair. But if I have to choose just one for listening to typical real world music, I think the 650 is a better compromise. Comfort is close, but to my head the esoteric 800 is actually a bit less comfortable.

SOURCE REQUIREMENTS: The HD 650 has a high impedance (300 ohms) and needs more voltage than most portable devices and PC headphone outputs can provide. Generally you need about 2 volts RMS (5.7 volts peak-to-peak) for wide dynamic range music at realistic levels. PC and portable audio gear is typically limited to around 1 volt or less with many managing only about 0.5 volts. So you might need a headphone amp. While the 650 may deserve better, even the $20 FiiO E5 can manage about 1.3 volts which might be enough for many tastes. The FiiO E9 can do a great job with the 650 as its 10 ohm output impedance is relatively insignificant compared to 650’s 300 ohms and the E9 has a very healthy 7 volts of output. For more, see my blog articles on amps and impedance.

CAUTIONS:  If you prefer a more exciting and/or a brighter sound, the 650 might not be a great choice. Serious thump and sizzle fans are even more likely to be disappointed. And if you’re looking to reveal flaws (say for mixing work) the 650 is probably too forgiving and polite. The HD 650 needs more than an iPod to play loud enough for most tastes. The Denon D2000 is much more efficient and easier for most gear to drive. Also remember the 650 has a large 1/4 inch plug and requires a bulky adapter to use with portable gear.

MEASUREMENTS: See the Tech Section for an impedance plot and a few other details. But, lacking an artificial head, I don’t have a way to measure full size headphones properly. So I present measurements from HeadRoom instead.

FIRST CLASS:

• Very low listening fatigue
• Makes marginal recordings much more enjoyable
• Detailed and revealing sound
• Relatively accurate (leaning towards warmth)
• Very comfortable

ECONOMY:

• Might be too laid back for some tastes
• Some excitement lost with the best recordings
• Aesthetics and build quality don’t match the price
• Relatively expensive
• May need a dedicated headphone amp

BOTTOM LINE: If a lack of audible fatigue with nearly all music and physical comfort are important, the HD 650 should be on your short list of headphones under $500. If you listen to them, give yourself some time to adjust as it’s easy to confuse a lack of high frequency distortion with a lack of high frequency musical content. Once acclimated, my ears often hear more of the music with the 650 but that wasn’t immediately obvious on the first listen. Their ability to improve the sound of typical recordings is, in my opinion, their greatest strength. But just like a luxury car’s smooth ride comes with a price, the HD 650 is probably not quite as much fun as some other headphones, including the Denon AH-D2000, on pristine source material. I’m ultimately in this hobby for the music. And I don’t want to be restricted to the very limited selection of audiophile-grade recordings to thoroughly enjoy an expensive pair of headphones. So I value the HD 650’s ability to make vast quantities of margin recordings more enjoyable without masking musical details. But as an ultimate reference headphone, I think it falls a bit short.

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TECH SECTION:

SPECS: The Sennheiser provide specs are:

• Impedance : 300 ohms
• Cable: 3 Meters (10 feet) with 1/4” plug
• Sensitivity/Efficiency: 103 dB at 1 V RMS
• THD: 0.05%
• Weight: 9.2 oz (260 grams)
• Frequency Response: 10 – 39,500 hz (darn, I was hoping for 40,000 hz!)

IMPEDANCE & PHASE ON SIMULATED HEAD: Here’s the impedance and phase on my “simulated head” which is really just a head-sized multi-layer damped box with a microphone embedded in one side. In this case, it’s just closing off the earcup and you can see the bass resonance is around 90 hz. The minimum impedance is a bit above 300 ohms and peaks around 500 ohms. As proof something is going on over the years with Sennheiser making changes, HeadRoom measured a peak of about 450 ohms lower down at 70 hz. Some of the difference could be due to HeadRoom using a true HATS (head) but even wearing the HD 650 on my own head and running the plot it looked a lot like it does here so I suspect there have been changes to the drivers:

BREAK IN: To see if there are measurable differences before and after break-in I made several measurements when the 650 was new right out of the box. And then I ran them at moderately loud volume for 48 hours and repeated the measurements. I went so far as to leave them undisturbed on my “simulated head” the entire time. I also rechecked the measurements in open air and with them on my head and the difference was consistent. The bass resonance shifted slightly lower by a few hertz and that’s what you might expect as the driver’s suspension loosens up a bit. Imagine a new pair of shoes being stiff and after you’ve worn them for a while they break-in and flex easier. That’s roughly what’s likely going on here. That said, the difference in resonance frequency is so slight it’s hard to imagine it’s audible. But it’s possible other characteristics, like distortion, also improve with break-in. Collectively all the changes together could be audible. And it’s very possible other headphones, with different driver designs and suspensions, would show much bigger changes after break-in:

DRIVE REQUIREMENTS: I used an oscilloscope with a very fast update rate to capture peak signal values playing various music while listening to the headphones. As mentioned in the main article above, I think most people would be very satisfied with about 2 volts RMS of drive capability (5.7 volts peak-to-peak). This exceeds what you can get from typical 3.7 volt Li-Ion battery powered gear, or 5 volt USB/PC powered audio devices. The FiiO E5 and E7 use a DC-DC charge pump to generate a negative power supply rail to increase their output but still manage only around 1.3 volts RMS which is likely enough for typical compressed pop music, but might clip on wide dynamic range music like classical or audiophile jazz recordings. The HD 650 ideally deserves an amp with a proper split power supply like the FiiO E9 or even a good dual battery Cmoy. But it will need enough gain to get up to at least 2 volts from whatever your source can manage (typically at least 4X or about 12 dB).

HEADROOM FREQUENCY MEASUREMENTS: The Frequency Plots at HeadRoom show the HD650 has extremely low distortion and very similar frequency response to the HD600 and HD800 and even the Denon D2000. But you can see about 5 dB of enhanced bass compared to the HD600 and a significant dip above 10 Khz. A lot of the bumps and dips from 2 khz to 10 Khz are related to reflections and standing waves in the earcup. In my tests, just moving the headphones around slightly can make a big difference in this range. The brain seems to largely filter these reflections out while the measurement microphone cannot. So it’s best to draw an imaginary line through those peaks and valleys yielding an average trend. If you do that, you can see the Denon D2000 is probably the most accurate of the 4 headphones shown in the link followed by the HD800.

HEADROOM DISTORTION PLOT: Headroom’s Harmonic Distortion Spectrum is a bit odd in how the frequency axis is scaled. But it shows, for a low frequency signal, the distortion is below –90 dB which is an amazing result as that’s about 0.003%! I’ve seen similarly low distortion measurements of the 650 elsewhere. This is lower distortion than many amplifiers and helps bust the myth that headphones always have far more distortion than electronics. Some proponents of tube gear, singled ended amps, and zero feedback amps, like to believe the relatively high distortion of their amp is OK because headphones always distortion even more. But it’s far from true.

HEADROOM SQUARE WAVE: The 500 hz Square Wave Response shows typical overshoot and subsequent ringing but is a fairly decent result. If you draw an averaged line through the ringing, the square wave is fairly flat which implies accurate midrange frequency response. For comparison purposes, see the popular Grado SR 80. The Grado looks much less like a square wave.

AN EQ EXPERIMENT: I tried using highly configurable parametric EQ to tweak my Denon D2000s to be similarly forgiving and I failed. When I filter out equivalent amounts of sibilance and harshness with the Denons, more of the music is gone and the result sounds veiled and artificial. The HD 650, to my ears, somehow works more like polarized sunglasses. They largely remove what I don’t want while revealing more of what’s left. It’s not just a simple frequency response difference. My objective side wants to pin down what’s going on but I don’t have a full explanation besides the HD 650’s lack of distortion.

MEASUREMENT SUMMARY: As with speakers, headphone measurements only tell part of the story. In this case they show you may well need an amp to properly drive the HD 650, they share frequency characteristics with the HD 800/600 and have very low distortion. They also help validate claims Sennheiser has changed the design over the years. And, just barely, I’ve shown a measurable difference before and after break-in. But I can’t fully explain their ability to improve poor recordings with little loss of detail. I might do better if I had a proper artificial head and conducted more acoustic measurements comparing the HD 650 to say the Denon D2000. But, for now, I’m happy to just enjoy the HD 650.