Objective Reviews & Commentary - An Engineer's Perspective

November 16, 2011

FiiO E11 Amp

fiio e11 bench close upINTRO: Given the FiiO E11 is almost a direct clone of the AMB Mini3 I wasn’t sure it was worth my time to review it. The Mini3 has some significant problems mainly related to its 3 channel channel design. Using a similar 3 channel approach FiiO seemingly copied the Titanic complete with the big hole. But I’ve had many requests to test the E11 (and the similarly amplified but still missing-in-action E10). Plus the E11 has some significant improvements over the Mini3 including longer battery life, USB charging, two gain modes, a three position EQ switch, smaller size, lighter weight, and a much lower price of around $65 vs $150+ for an assembled Mini3. If it’s a decent amp, the E11 would neatly fill the price gap between something like the FiiO E6 and the O2 portable amp. So I gave in and decided to review it after all.

THE FiiO E11: The E11 was so light when I first held it I wondered if the battery was installed. It doesn’t have nearly the heft and solid feel of the FiiO E7. It turns out the battery was installed and the E11 is a feathery 2.3 ounces (65g)--less than half the weight of the current iPhone. If you’re going to carry the E11 in your pocket, or “rubber band” it to your iPod (it comes with a heavy rubber band), the light weight is a good thing. FiiO even protected the volume control making the E11 more “pocket friendly” helping prevent accidental volume changes. It has 3.5mm input and output jacks and a volume control that also serves as the power switch. Going beyond the Mini3, the E11 adds a mini USB port for charging, a two position gain switch, a three position EQ switch, and the battery can be swapped out without needing a screwdriver (for now at least you can get extra batteries but 2 or 3 years from now don’t count on it). It’s also smaller and thinner than the Mini3. The USB port is especially handy although it doesn’t come with a charger so you need a PC, laptop, USB charger, iPhone/iPod/iPad charger, etc. For traveling it’s one less thing you have to bring along but there’s a huge catch…

DESIGN FLAW #1 – NO DESKTOP OPERATION: Sadly the E11 cannot operate while external power is connected via the USB jack. That means it’s impossible to use it as a regular desktop amp as the battery will end up dead and you can’t listen to it while it’s charging. Ironically this might be another downside to the 3 channel virtual ground which puts the E11’s audio ground at about 4.5 volts DC while the PC’s USB ground is at 0 volts—connecting the two with the amp operating would short it out and possibly damage it. For a lot of people this is a significant limitation. For those using it at home, it also means a lot more wear and tear on the battery shortening its life. In my opinion, this makes the E11 mainly suitable only for those who want an amp to use only on the go and not at home or the office.

DESIGN FLAW #2 - THREE CHANNEL DESIGN: FiiO not only chose a similar 3 channel design but they even used the Mini3’s problematic OPA690 op amp (which isn’t even designed for audio use) for the third channel. And, incredibly, they even copied one of AMB’s false claims for the Mini3. FiiO’s website claims: “Heavy current three channels amplified circuit, effective to lower the output impedance.” In reality, using three channels raises the output impedance. Even Ti Kan at AMB has admitted as much in forum posts. This point is discussed extensively in my Virtual Grounds & 3 Channel Amps article as well as the original Mini3 review. Apparently FiiO, a Chinese company, either copied the Mini3 design without understanding its flaws (or reading my related blog articles), or they have joined forces with AMB in knowingly promoting misleading statements about 3 channel designs. The “lower output impedance” is pure myth, or purely misleading, depending on your point of view. But, in my opinion, it doesn’t reflect well on FiiO as a company.

TWO POWER RANGES: There’s a switch under the battery labeled “High Power” and “Low Power”. I did all my testing at “High Power”. FiiO lists two power specs at three impedances and they’re all 4 dB apart in the two modes. I believe the idea is to extend battery life by lowering the power supply voltage for headphones that don’t need the full output. It’s a useful feature if you plan to use it with sensitive headphones.

SUBJECTIVE LISTENING: I listened to the E11 with several headphones. It had enough output, barely, for my HD650s listening to classical music at live levels. Unlike many amps, such as the FiiO E9, the E11 sounded reasonably good with my Ultimate Ears SuperFi 5’s, but there was some hiss audible with nothing playing. Besides the hiss, I didn’t hear anything that was offensive but I didn’t spend a lot of time listening either.

BASS EQ: With some headphones, like my Etymotic ER-4 IEMs, the bass EQ was useful. On bass shy music the “2” (maximum) setting added some heft and on more normal music the “1” setting was more appropriate. But with headphones that already lean towards “boomy”, which sadly includes most of them, the EQ options just added to the boom and/or muddy bass. So it will depend on your headphones, music, and tastes, if you like the bass EQ. My personal preference is for deeper bass boost but I’ve heard worse EQ than the E11’s.

GAIN: The gains are 2.5X and 4.5X. The first is appropriate for a low gain setting, but 4.5X is on the low side for a high gain option. This is especially true for a battery-only portable amp like the E11 that’s much more likely to be used with low output portable players. Using the line out (LOD) on an iPod, or the headphone out on a Sansa Clip+, you can’t drive the E11 to full output into high impedance headphones even at the high gain setting. That’s a significant limitation for owners of headphones like the HD600/650, DT880, etc. See the Tech Section for the details. For more see: All About Gain

BATTERY LIFE: I didn’t test the battery life. FiiO rates it at 10 hours which is roughly double the real world run time of the Mini3 and slightly better than the higher output O2. It’s not clear if this is with the power switch set to the “High” or “Low” position but I’m guessing the 10 hours is at the Low Power setting.

fiio e11 benchMEASUREMENT SUMMARY: Most of the E11’s measurements are decent enough—especially for a $65 amp. The E11 gets the basics right. It has flat frequency response, a very low output impedance, is reasonably quiet, and has mostly low distortion. There are, however, some problem areas. The E11 has very odd overload/clipping behavior into 32 ohm and lower impedance loads and a bit more low frequency phase shift than is ideal. Even being generous, it fell far short of several of its claimed power measurements. It also suffers from some of the Mini3’s potentially audible Interchannel IMD distortion due to the three channel design. And it shares the Mini3’s marginal stability but, unlike the Mini3, it’s only significant with more challenging loads. There’s some RF leakage of the DC-DC converter into the audio output (but it’s much better than the E5 and E7).  Despite all these issues, the E11 still substantially outperforms the Mini3 using the same op amps and a very similar design. This is yet another example of the devil being in the details. FiiO simply did a better job with the implementation than AMB did using the same parts. For example it seems FiiO chose to use a single ground in the E11 which likely helped many of the measurements compared to the Mini3’s split ground and poor ground routing. See the Tech Section and my article on Virtual Grounds for more details. The following table compares the E11 to the O2, FiiO E9 and AMB Mini3:

Measurement FiiO E11 O2 FiiO E9 AMB Mini3
Frequency Response +/- 0.1 dB A +/- 0.1 dB A +/- 0.1 dB A +/- 0.1 dB A
THD 1 Khz 150 Ohms 0.004% A 0.0016% A 0.005% A 0.002% A
THD 1 Khz 15 Ohms 0.011% B 0.0023% A 0.037% C 0.017% B
THD 20 hz 15 Ohms 0.017% B 0.0023% A 0.05% Good 0.01% B
THD 20 Khz 15 Ohms 0.011% B 0.010% A 0.003% A 0.45% F
IMD CCIF 15 Ohms 0.002% A 0.001% A 0.05% C 0.043% D
IMD SMPTE 0.0079% A 0.002% A 0.002% A 0.009% B
Noise A-Wtd -101.3 dBu B -114 dBu A -94.7 dBu C -103.2 dBu B
Max Output 15 Ohms 63 mW B 337 mW A 1067 mW A 104 mW A
Max Output 33 Ohms 101 mW C 613 mW A 883 mW A 98 mW C
Max Output 150 Ohms 52 mW C 355 mW A 317 mW A 38 mW C
Output Impedance 0.5 Ohms A 0.54 Ohms A 10 Ohms C 0.9 Ohms A
Crosstalk 15 Ohms 48 dB B 65 dB A 63 dB A 40 dB C
Channel Balance 1.1 dB B 0.6 dB A 1.8 dB C 1.14 dB B
Battery Life ~10 hours ~8 hrs / ~30 hrs AC Only ~5 hours


  • Small and light
  • Relatively low noise
  • Flat frequency response
  • Low output impedance
  • Significantly improved performance over Mini3
  • Two gain options
  • USB battery charging


  • Cannot use the amp on external power (can only run from the battery)
  • Even the high gain setting limits output with iPod LOD input
  • Fails to meet published power specs at 1% THD both channels driven
  • Third channel creates some interchannel distortion and degrades performance
  • Some moderate low frequency phase shift
  • Shows signs of instability with difficult loads 
  • Odd clipping behavior into 15 and 33 ohms implies power supply limitations
  • Significant RF leakage from DC-DC converter into output

HEADPHONE COMPATIBILITY: For most headphones with high sensitivity the E11 should be OK. The output impedance is low enough for even balanced armature IEMs. But it’s marginal for headphones like the popular Sennheiser HD600 and HD650 which require at least about 2.5 Vrms to avoid clipping the peaks of dynamic music at typical loud listening levels. But that’s partly dependent on the type of music and how loud a person wants to listen. Forget the popular Beyer DT880-600, Audeze LCD-2, HiFiMan planars, etc. and headphones like the AKG K701/K702 will be marginal. The E11 doesn’t have enough output voltage for any of them. See my More Power article for more details on determining what headphones will work well with the E11.

BOTTOM LINE: The E11 performed better than I was expecting given it’s a virtual clone of the Mini3 and much less expensive. It’s biggest weakness for many will be only operating off battery power with the amp being unusable while charging. That makes it a very poor choice for home or office use. Many might find the adjustable gain useful although some may be left wanting for more gain—especially using an iPod LOD as the source with voltage hungry headphones. The E11 has a bit more output into high impedance loads, at least when fully charged, making it a better match for the popular Sennheiser HD600/650 and similar headphones but only if your source can produce at least 650 mV (many portables max out at 500 mV). If you don’t mind running it only from battery, and it has enough output  for your headphones, the E11 is a reasonable choice—especially if you’re on a tight budget and want a highly portable amp. Despite its several flaws it’s still a significant upgrade from the FiiO E5 and E6. If you want lower noise, lower distortion, customizable gain, and much higher output to drive nearly any headphones, the O2 portable amp is a significant upgrade but it’s larger, heavier and more expensive.



HARDWARE: The E11 uses the same AD8397 op amp design as the Mini3. The AD8397 has the advantage of high current output but it’s also rather “high strung” and, in the Mini3, it runs “wide open” (with no stability compensation) on the edge of being unstable. The E11 also uses the same OPA690 op amp for the third channel virtual ground as the Mini3 which is a poor choice. It has relatively low PSRR and isn’t even specified for operation at audio frequencies. This is discussed more in the Mini3 review. The Mini3 runs from a single 8.4V NiMh rechargeable battery with the OPA690 referenced to half that creating a “virtual” bipolar +/- 4.2V power supply for the AD8397. In comparison, the E11 uses a single 3.7V li-Ion battery and a DC-DC converter stepping it up to a higher single voltage around 9 volts which is then artificially split into +/- 4.5 volts. This puts the audio ground at 4.5 volts DC which is probably why you can’t use the amp and charge it at the same time. Connecting the audio ground to your PC’s ground would short out the E11’s OPA690 and power supply if it’s not an isolated DC-DC converter.

E11 DESIGN FLAW: If you’re going to bother with a DC-DC converter, it’s not much more expensive or difficult to have a bipolar DC-DC converter that generates a true positive and negative power supply with a real ground. Such a converter would have eliminated the need for the OPA690 entirely along with it’s rather hungry power consumption, added distortion, added output impedance, added crosstalk, and limited current capability. The extra expense in the DC-DC converter is probably less than the cost of the OPA690. In short, FiiO missed the opportunity to make the E11 a better headphone amp without increasing the component costs.

ISOLATED DC-DC CONVERTER: FiiO could have used true bipolar regulated DC-DC converter with at least the negative rail being isolated from ground. That would eliminate all the third channel problems, increase run time (getting rid of the hungry OPA690), maintain the power output as the battery voltage drops, and best of all, allow using it as a desktop amp. With such a power supply the E11 also could have been used during charging making it much more suitable as a desktop amp.

WHY THE FLAW? FiiO’s choice is difficult to understand unless they were simply bent on completely copying the Mini3 and/or believed AMB’s highly misleading claims for 3 channel designs. I suspect they were just trying to seize on the Mini3’s popularity by copying it as closely as possible. But, like the Titanic referenced earlier, the Mini3’s popularity seems to be fading fast with its real performance revealed. Many have reported difficulty selling their Mini3 for a reasonable price since my review came out. See Virtual Grounds & 3 Channel Amps and the original Mini3 review for more.

FREQUENCY RESPONSE: Much like the O2, the E11 has some minor low frequency roll off. Unlike the fully direct coupled Mini3, FiiO wisely chose to add a DC blocking capacitor to the signal path. That way the E11 won’t amplify any DC present on the input. It may however, given the E11’s size and price, be an electrolytic capacitor which can degrade the audio performance. The high frequency performance was very flat and extended. The second graph shows the Mini3 for comparison into 15 and 150 ohms and the O2 comes in somewhere between two:

FiiO E11 Frequency Response 33 Ohms Both Channels Vol=Unity using Low Gain

AMB X-Audio Mini3 200mv In Vol= 50% Freq Pink-Red=150 Ohms Yel-Blu=15 Ohms (ref ~400mV)


PHASE ERROR: The E11’s low frequency roll off created some phase error that hits about 15 degrees at 20 hz. The goal is less than 1 degree of error from 100 hz to 10 Khz where the ear is most sensitive. The E11 manages at the high end but fails at the low end with about 4 degrees of error at 100 hz. The weird kink close to 20 hz is because the settling time was set too short on the dScope—it’s a test artifact not the E11. The second graph shows the O2 which has DC blocking and stability compensation but still manages to stay under 1 degree of error over the desired range. The Mini3’s phase response is even flatter but that’s because it lacks desirable DC blocking and high frequency compensation. The slight droop above 10 Khz is evidence FiiO wisely chose to either add compensation to the AD8397 and/or included an RF blocking filter at the input:

FiiO E11 Phase Response Low Gain 150 Ohms

O2 AC Phase Response 400 mV 150 Ohms (non-inverting)


1 Khz THD+N vs OUTPUT: The E11 has similar distortion to the Mini3 into 80+ ohms but with significantly higher maximum output. The E11 produced 2.7 Vrms into 80 ohms while the Mini3 could only manage 2.4 Vrms into an even easier 150 ohms. The E11 produced 2.95 Vrms into 600 ohms or 14.5 mW where the Mini3 could only manage 2.45Vrms or 10.0 mW. This gives the E11 a noticeable 1.6 dB advantage. Into 33 ohms and lower, the E11 exhibits rather odd distortion characteristics at higher outputs. Into 15 ohms (white plot) it has much lower distortion than the Mini3 up to about 800 mV then the distortion rises sharply but stops rising around 1 volt and levels off around 0.3% before finally hitting the official 1% at 1.9Vrms—much better than the Mini3’s 1.25 Vrms into the same load. But, to be fair, I would call the real “clipping point” of the E11 around 1 volt. It exhibits similar behavior into 33 ohms as seen in the green plot. My best guess is the DC-DC converter power supply in the E11 is current limiting and/or the power supply capacitors are undersized and there is excessive power supply ripple at higher output currents. The relatively poor PSRR of the OPA690 would pass a significant amount of the ripple to the audio output creating greatly increased THD+N. The second graph shows the Mini3 for comparison into 15 and 150 ohms. While it’s not shown, the E11 produced 2.85 Vrms into 150 ohms at 1% THD vs 2.4 Vrms for the Mini3 with the same load. The third graph shows the much higher output and overall lower distortion of the O2 into all loads. In summary, the E11 produces 63, 101, 91, 52 and 15 mW into 16, 32, 80, 150 and 600 ohms respectively:

FiiO E11 1 Khz THD N vs Output 15 - 33 - 80 - 600 Ohms (top to bottom) Gain=Low BW=22 Khz

AMB X-Audio Mini3 THD vs Output Blue-Yellow=150 Ohms Red-Pink=15 Ohms (ref ~400mV)

O2 V11 Battery Both Ch 1 Khz 10mV  THD N vs Output Left to Right 15 33 80 150 600 Ohms comments

CLAIMED OUTPUT vs ACTUAL OUTPUT: FiiO Claims 300 mW into 16 ohms fully charged. Using 1 volt as the clipping point, the E11 only delivers 62 mW and even using the 1.9V value it still falls short at 225 mW. It’s also interesting 300 mW is exactly the same number AMB quotes for the Mini3 and it also falls far short. In all cases, taking the higher of the clipping points the amp at least exceeded the low number provided by FiiO but it should have been much closer to the high number. I suspect, like perhaps AMB, FiiO was somewhat cheating here and probably had the battery at an unrealistically high voltage and was only driving one channel. The gain switch made no difference in these measurements. Here are all the numbers with about 15 minutes of light average use on a fully charged battery in the High Power mode:

  • 16 Ohms FiiO claims 300/120 mW (High/Low power) but it only delivers 63 mW (225 mW at 1%)
  • 32 Ohms FiiO claims 200/88 mW but it only delivers 101 mW (180 mW at 1%)
  • 300 Ohms FiiO claims 35/13 mW but it only delivers 26 mW at 1%

100hz OUTPUT IMPEDANCE & MAX POWER: Since the Mini3 review back in May, I’ve revised my impedance measurements to use 100 hz instead of 1 Khz to more closely reflect the resonance impedance of headphones where electrical damping is most needed. At 100 hz the E11 gets close to 1% THD somewhat earlier than the 1.9 Vrms seen in the earlier graph at 1 Khz  (1.62 Vrms). Removing the load resulted in the output rising only slightly to 1.66 Vrms. This works out to an output impedance of 0.5 ohms. The Mini3 measured a higher but similar 0.9 ohms at 1 Khz. Both are “good enough” but I have to wonder if FiiO chose to use a single ground for the entire amplifier vs splitting the input and output grounds like AMB chose to do. That would allow the amplifier to partially correct for some ground impedance via feedback and would explain the lower output impedance. It’s also worth noting while the E11 matches the O2 at 0.5 ohms, in the O2 thee output isolation resistor dominate that value, the actual output impedance of the amp is much lower. In the E11, however, it’s the OPA690 virtual ground channel likely dominating the measurement:

FiiO E11 100hz Max Output & Impedance 15 ohms & 100K Low Gain


THD+N vs FREQUENCY: Here’s the THD+N vs frequency at the same 400 mV I tested the Mini3 at. Into 600 ohms it’s respectably low around 0.003%. Into 33 ohms it’s still under 0.01%. Into 15 ohms it approaches 0.02% which is acceptable but not ideal as there are distortion products above –80 dB. The rise at low frequencies might be due to power supply capacitors that are too small. Just like the Mini3, the E11 isn’t very happy into low impedance loads. And, like the Mini3, this picture gets worse at higher output levels. I didn’t run this exact test on the Mini3 but I did measure 400 mV THD+N at fixed points of 20hz, 1 Khz and 20 Khz where the distortion was 0.01%, 0.016%, and a very high 0.45%, respectively into the same 15 ohms. The E11 performs very similarly to the Mini3 at low and mid frequencies and much better at high frequencies. The second graph shows the much lower distortion of the O2 for comparison into 33 and 150 ohms:

FiiO E11 THD N vs Frequency 600 - 33 - 15 Ohms (top to bottom) Low Gain BW=22 Khz

O2 AC V11 2.5X THD N vs Frequency Both Ch 33 & 150 Ohms Vol = 100% (BW 22 Khz)


SMPTE IMD 15 OHMS: I tested the E11’s SMPTE IMD at 400 mV which is generally my current standard. It had acceptably low distortion with everything below –80 dB. The Mini3 performed poorly at 400 mV into 15 ohms so it was tested at only 200 mV, and even at half the output, had slightly higher but similar IMD. The second graph shows the Mini3 at 200 mV. The third graph shows the O2 under exactly the same conditions as the E11. Note the O2’s distortion reading is over 7 times lower and how much smaller the “mountain” around the base of the 7 Khz signal is. Note also the relative lack of distortion products above 7 Khz on the O2:

FiiO E11 SMPTE IMD ~400 mV 15 Ohms Low Gain (ref ~400 mV)

AMB X-Audio Mini3 200 mV Vol=50% SMPTE IMD 15 Ohms (ref ~400mV)

O2 AC Both Channels IMD SMPTE 400 mV 15 Ohms Vol = 50%


CCIF IMD 15 OHMS: The E11 also does respectably well on the CCIF IMD test with everything below 80 dB. This is a much better performance than the Mini3 which had 18 times more distortion on the exact same test. The E11 has everything below –90 dB in the audio band while the Mini3 had distortion products up to a likely audible –52 dB. Considering both amps use the same op amps this is a classic example of implementation is everything. Just using high-end parts doesn’t assure high-end performance. FiiO apparently did a much better job with the PC board layout, ground scheme, stability compensation, etc. The result is 18 times less high frequency distortion from the same parts. The O2 is shown in the third graph with about half the overall IMD of the E11 on the same test:FiiO E11 CCIF IMD ~400 mV 15 Ohms Low Gain (ref ~400 mV)

AMB X-Audio Mini3 200 mV Vol=50% CCIF IMD 15 Ohms (ref ~400mV)

O2 AC IMD CCIF 400 mV 15 Ohms Vol = 50%


NOISE: The E11 had respectable noise levels of –101.3 dBu A-Weighted and 13.5 uV of raw noise at full volume on the low gain setting. Using high gain added a few dB more noise. The readings were similar at 50% volume. The industry standard dBu reference is 775 mV. The Mini3 noise measurement was referenced to 400 mV. To convert it’s 20*LOG(775/400) = +5.7 dB. So the Mini3’s A-Weighted noise in the second graph is 97.5 dBr + 5.7 dB = 103.2 dBu or about 2 dB better than the E11. I suspect the Mini3 does slightly better because of the lack of a noise producing DC-DC converter. It’s also possible the EQ circuitry in the E11 contributes some noise even when it’s set to “flat”. It could also be due to the Johnson noise of the resistors. Overall, for a cheap amp with a DC-DC converter, the E11 is impressively quiet. The O2 noise is shown in the third graph in dBv referenced to 1 volt. To convert to dBu it’s –2.2 dB giving 116.2 – 2.2 = 114 dBu. So, in summary the E11, Mini3 and O2 are 101.3, 103.2, and 114 dBu respectively  A-Weighted. The E11 and Mini3 are similar while the O2 has much lower noise:

FiiO E11 Noise Vol=100% Low Gain

AMB X-Audio Mini3 Muted Input Vol= 50% Noise 150 Ohms (ref ~400mV)

O2 Battery 6.5X Gain Noise 25 Ohm Source Impedance Volume=50% dBV (ref 1V)


CROSSTALK: The E11 had crosstalk of 53.6 dB into 33 ohms and 48 dB into 15 ohms. For comparison, the Mini3 measured 46 dB and 40 dB into 33 and 15 ohms respectively. The second graph shows the Mini3 into 15 ohms and 150 ohms. Into 600 ohms the E11 was better than 80 dB up to about 1500 hz and rising from there mostly due to coupling in the volume control. I suspect the E11 does nearly 8 dB better than the Mini3 because the ground system isn’t split between the input and output like it is on the Mini3. Ti Kan claims this improves performance, but as seen here and with the E11’s lower output impedance earlier, it actually degrades performance as it removes the ability of the op amp's to correct for ground signals. And even with an improved third channel ground scheme, the E11 is still far inferior to the O2 shown in the third graph. Into the same 33 ohms the O2 managed 65 dB vs 53.6 dB for the E11:

FiiO E11 Crosstalk 33 Ohms (blue) 600 Ohms (yellow) Vol=Unity Low Gain

AMB X-Audio Mini3 200mv In Vol= 50% Crosstalk Pink=15 Ohms Blue=150 Ohms (ref ~400mV)

O2 Crosstalk 400 mV 15 (yellow) 33 (green) 150 (blue) & 600 Ohms (red) Reading 1 Khz 150 Ohms


INTERCHANNEL IMD BASELINE: The most likely audible flaw of the Mini3 is Interchannel IMD. Because the two channels interact with a common third channel (the OPA690 virtual ground amp) they share any distortion produced by the OPA690 and in the ground system in general. This is a potentially highly audible form of distortion because the distortion products are not directly related to what’s playing in that channel. The Mini3 splits the internal grounds removing the ability of the op amps to correct for certain kinds of ground signals. I measure Interchannel IMD by first showing the THD spectrum for just one channel operating at 1 volt into 15 ohms. This establishes a baseline for distortion related to just that channel and signal. The E11 produced a very respectable 0.005% THD+N figure with all the distortion products below the desired –80 dB threshold. This about 6 times better than the Mini3 managed which hit 0.03% THD+N and –62 dB as seen in the second graph. The third graph shows the O2 same signal, same load with even lower distortion than the E11:

FiiO E11 THD N 1 Khz 1 Vrms One Channel Driven (ref 1 Vrms) BW=22 Khz

AMB X-Audio Mini3 THD N 1 Volt RMS 15 Ohms One Channel Driven

O2 AC 1 Khz One Channel Driven Residual (non-THD) Noise 1V 15 Ohms


INTERCHANNEL IMD 1 Khz & 300 hz: This is where things get interesting. Nothing changes with the channel shown above, but now the other channel is also driven to 1 volt but at 300 hz. This creates peak currents that are still within the OPA690’s specifications so it should not be overloading any part of the E11 or Mini3. Ideally one would just see a component at 300 hz at the level of crosstalk present in the amplifier. This isn’t distortion it’s linear (i.e. relatively pleasant sounding) crosstalk. But any other significant new distortion products are cause for concern. The E11 shows the expected crosstalk at 300 hz while everything else is still below about –90 dB but there’s a new “forest” of IMD products visible in the spectrum that were not present in the baseline graph above. Each spike is summed or added together to form the total amount of distortion—something I would have to write a custom script to get the dScope to calculate while leaving out the 300 hz crosstalk. In short, this is still a potentially audible amount of distortion due to the shear number of distortion products. The second graph shows the much worst result from the Mini3 where there are several distortion products above the –80 dB threshold and very likely sum to an easily audible value. See Virtual Grounds & 3 Channel Amps and the original Mini3 review for more. The third graph shows the vastly better result from using a real ground in the O2’s conventional 2 channel design. Notice the “forest” is gone in the O2 graph with mainly just the expected signal-related harmonics of the 1 Khz signal visible:

FiiO E11 THD N 1 Vrms 300 hz Left 1 khz Right (shown) 15 Ohms Ref 1 Vrms BW=22 Khz

AMB X-Audio Mini3 THD N 1 Khz Left 300 hz Right 1 Volt RMS 15 Ohms (ref 1 V) comments

O2 AC 1 Khz & 300 hz (other channel) Interchannel IMD 1V 15 Ohms


CHANNEL BALANCE: For a relatively inexpensive amp, the E11 had decent channel balance being just slightly over the goal of 1 dB at -45 dB. You can also see from the gap between the blue lines there was also about 1 dB of error at –22 dB. This is similar to the Mini3’s worst case error of –1.1 dB at –28 dB. The O2 was better with a worst case error of 0.6 dB at –55 dB:

FiiO E11 Channel Balance Tracking -22 -35 -45 dBr (ref 2 Vrms)

AMB X-Audio Mini3 400mv In Vol= 9 0'Clock (worst case balance error) (ref ~400mV)


GAIN: At the low gain setting the E11 measured 8.2 dB (2.5X) of gain (no EQ) as can be seen below with 387 mV input and 1 volt of output. This is identical to the O2’s default low gain setting. The second graph shows the high gain setting at 12.9 dB (~4.5X). This means, at high gain, with 500 mV of input from an iPod LOD (line output), and many portable players, you’ll get 2.25 Vrms out. This is well shy of the 2.9 Vrms the E11 is capable of and may not be loud enough for many headphones like the HD600/650. For comparison, the O2 has 16 dB (6.5X) of default gain at the high setting, and the default fixed gain of the Mini3 is about 14 dB (5X). These graphs also show respectably low distortion at 1 volt output into 150 ohms. For more see: All About Gain

FiiO E11 Low Gain Volume=100% 150 Ohms (ref 1 Vrms)

FiiO E11 High Gain Volume=100% 150 Ohms (ref 1 Vrms)


EQ: Here’s the frequency response with the EQ switch set to “0” (EQ off yellow), “1” (white) and “2” (blue). Both EQ options create a peak centered at about 50 hz with a 3.5 dB maximum boost on setting “1” and a 6.8 dB peak on “2”. both EQ options also raise the overall volume about about 0.6 dB at all frequencies. There is significant boost above 80 hz which may result in some “boomy” bass with many headphones. I personally prefer bass boost down lower that’s mostly nil by 80 hz but some may still find the E11’s options useful:

FiiO E11 Frequency Response EQ=2 EQ=1 EQ=0 Top to Bottom


SQUARE WAVE PERFORMANCE: The high speed scope shot below shows the E11 into Sennheiser CX300 headphones with 0.01 uF of added capacitance to simulate more difficult headphones. As was observed with the same very fast AD8397 op amp in the Mini3, there’s some ringing and signs of instability. What’s different is the Mini3 show even more ringing without any added capacitance into the same headphones. So the E11 is at least better behaved than the Mini3. The thick “fuzzy” horizontal portion of the waveform is RF noise from the DC-DC converter. The second graph shows the O2 on the same test with the same load. Note the lack of ringing, and the lack of “fuzz” as the O2 doesn’t use a DC-DC converter and uses a 2 stage design with properly compensated op amps better suited to audio use:

FiiO E11 10 Khz Square Wave Response CX300   0.01uF 2Vp-p

o2 agilent 10 Khz Sennheiser CX300 plus cap 2v p-p

SLEW RATE: As shown below, the E11 slews 4.5 volts in 710 nS which is a 6.6 V/uS slew rate. This is many times what’s required for any audio signal the amp will ever encounter. It’s worth noting it’s a bit slower than the Mini3’s measured 10.7 V/uS which is further evidence FiiO wisely applied some additional compensation, RF input filtering, or otherwise decided not run the AD8397 “wide open” as AMB chose to. The O2 has a 3.6 V/uS slew rate which is still well in excess of what’s required any recorded music for the output voltage the amp is capable of:

FiiO E11 10 Khz Square Wave Rise Time Max Output 600 Ohms

DC-DC CONVERTER NOISE: Here’s a zoomed-in view of the “fuzz” seen on the square wave response above. The scope measurement shows it has a frequency of 96 Mhz and it’s likely the DC-DC converter which is probably designed to run somewhere around a nominal 100 Mhz. You can also see it’s about 15 mV peak-to-peak or about 5 mV RMS. That’s only –43 dBu. Obviously nobody can hear 100 Mhz but if it were in the audio band it would be plainly audible. It can also be troublesome for FCC and/or CE approval for RF emissions:FiiO E11 RF Noise From DC-DC Converter at 96 Mhz and 12 mV p-p


CLIPPING PERFORMANCE: The E11 clipped cleanly and symmetrically at a bit over 8 Vp-p into 600 ohms. This shows the internal DC-DC power supply is likely around 9 volts (accounting for the inability of the AD8397 to swing all the way to the rails). The 100 Mhz “fuzz” is plainly visible on the entire waveform:

FiiO E11 10 Khz Sine Wave Clipping 600 Ohms

THIRD CHANNEL VIRTUAL GROUND: The large number of distortion products created by just a single sine wave in each channel is something the E11 shares with the Mini3. In the E11 the Interchannel IMD is much lower in level but the total sum of all the distortion could still be audible—especially with the complex interactions of real stereo music fighting between the channels via the shared OPA690 ground channel. The E11 likely uses a single ground and perhaps a better ground layout (the Mini3 uses a poor ground PCB layout and isolates the input and output grounds) but it still suffers from the shared OPA690—an op amp not very well suited to the task. As explained at the start of the Tech Section, the E11 would have been much better as a two channel amp with a DC-DC converter that produced two supply rails instead of just one. See Virtual Grounds & 3 Channel Amps and the original Mini3 review for more.

TECH COMMENTS: The E11, in short, measured better than I thought it would although, just like the Min3, it failed to deliver its claimed power output. When driving low impedance headphones its true clipping point is a bit hard to determine likely due to either an undersized DC-DC converter and/or wimpy power supply capacitors. Even being generous and using the 1% THD points with only 15 minutes run time on a fully charged battery it it still fell significantly short of the claimed output. It’s best to use 16 ohm headphones that need no more than 1 Vrms and 32 ohms headphones that need no more than 1.7 Vrms. I’m also concerned about the very large number of Interchannel IMD distortion products created by the third channel as discussed in the paragraph above. It’s interesting the E11 performed significantly better in nearly every area than the Mini3 despite both using a 9 volt power supply, similar 3 channel design, and the same op amps. As mentioned earlier, it’s more evidence implementation is more important than the components used.


  1. Another interesting review :)

    On a side note, I found at least two errors:
    1) double "the"
    2) double "pleasant"

  2. Dear NwAvGuy,

    thanks a lot for this review, I know you didn't have a lot of interest in that amp (knowing beforehand it was a failed design), but I think it will give people a much better perspective on what is going on in the audio-fraudile scene.

    It is sad to see AMB not taking responsibility over his own designs. IMHO he should be sincere about the real performance of the Mini^3, otherwise he is losing fast all credibility.

    Even the best engineer makes big mistakes, we know audio design is a hobby for AMB, so just saying, "guys, I screwed up while measuring the Mini³" would honor him.


  3. I have a minor quibble :)

    The K701/K702 is 62 ohms and has 105dB/V sensitivity. Is is really going to clip with the E11? Should it really be lumped in with the DT880-600 and the planars?

    Otherwise, another great review.


  4. Too bad about the 3rd channel issues, still a pretty good product at the price though, I have one myself. FiiO did a good job but missed critical opportunities to create a stellar product.

  5. Once again great review!!

    Can't wait for the E10 review...

  6. Thanks all for the positive feedback! :)

    @DetlevCM, thanks for the corrections. Fixed.

    @Anon, I agree with your comments about AMB. I should point out he's apparently used the same measurement technique, which does not apply a proper load, to all of his amps--not just the Mini3. The crosstalk, distortion, and other measurements are equally suspect for the M3, beta22, etc.

    @odigg, You're half correct and I've revised the review slightly. If you look at Tyll's review of the K701 on InnerFidelity he measured 0.32 volts for 90 dB SPL. That's 3.2 volts for the guideline 110 dB SPL on peaks. Into 62 ohms the E11 can deliver only about 2.2 volts so it falls short. It will only hit 106.7 dB which, I agree, might be plenty for some people--but not to reproduce highly dynamic classical music at live levels. As an aside, there have been a couple K701/702 owners who have replaced their Mini3 with the O2 and report it's a much better match (one of whom is Shike in his reviews of the O2). The E11 only has a bit more output than the Mini3.

  7. Great review. I have a couple questions:

    1. The output impedance is nicely low on the E11, so I'm inclined to think that something about the design makes it easier for them to not feel like putting larger resistors on the output path. Is that what's going on here, or did they just pick amps that are already stable enough with low resistance?

    2. Did I miss something? Where are your rating cutoffs for the various categories? It SEEMS that this objective blog has a subjective rating system. I find the letter ratings very useful but a rating standards document would be much appreciated so if I had a particular measurement then I could establish what the letter rating should be. That would be awesome.

  8. Yeah, I think some people do some weird things when "measuring" active ground amps. However it is you hook it up, you've got to have realistic current levels going out of the left and right channels into a low headphones-like load (in the order of tens of mA I would think), and then the current of both L and R channels into the ground channel amp. This requires some kind of isolated ground on the measurement device input, so you don't short out the active ground channel? I'm going to guess that most consumer sound cards do not have such line inputs? Instrumentation is definitely not my strong suit.

    Anyhow, regarding the E11, if only the grounding/power scheme were altered and the thing could operating while charging, I think it would be a very interesting choice given the price and size.

  9. The THD+N vs Output graphs at low impedances are interesting, in comparison to the O2. The O2 at the knee the graph is very close to the voltage output at where THD+N reaches 1%, while the low knee on the E11 is way lower than maximum voltage at 1%.

    Measuring at the knee (where distortion is still low) would make the power output figures of the E11 even lower though I'm not sure if any low impedance headphones need more than ~1.75V.

  10. @akgk171 I suspect FiiO copied the Mini3's technique of using an output resistor but putting it inside the feedback loop. That reduces the output impedance because the op amp corrects for the drop across the resistor. The AD8397 isn't short circuit protected so just unplugging the headphones with the volume turned up might kill the op amp without the resistor. The stability issue is more tricky as the AD8397 is a very fast op amp and prone to problems. FiiO has done a better job with stability than AMB but it's still marginal into challenging loads.

    The article I did for InnerFidelity on measurements helps define my cutoffs/thresholds but, you're correct, I should be more explicit. I have adjusted a few categories, and also test methods, after researching and writing that article. If you haven't read it, the article can be found here:

    Headphone Amp Measurements

    It's important to note some measurements are not purely a number. IMD and Jitter are two examples. The nature of the spectrum can be more important than the raw number. So, in those cases, I have to interpret the spectrum and assign the letter grade accordingly. I try to be consistent in how I do that and explain any problem areas in the spectrum in the paragraph associated with that graph. But, obviously, someone else might disagree with my interpretation. The more technically inclined readers of this blog have been pretty good about spotting any errors I make and I try to quickly correct them.

  11. Hmm...

    Compared to the Mini3 its smaller, cheaper, and generally better in the areas that matter.

    Jumping on the fad bandwagon is a fail for the average consumer but getting better performance from the same parts must be a win for the marketing constrained engineers, right?

    Its sort of like a "Microsoft interview". Given these suboptimal parts and basic design make the best amp you can.

  12. @Maverickronin, the irony here is the opamps in the E11 and Mini3 are relatively "high-end" and over ten times the cost of those in the O2. So many would consider the O2's components "sub-optimal" yet it far outperforms both the E11 and Mini3.

    IMHO, FiiO's greatest strength is offering a lot of value for the money and their impressive industrial engineering (build quality and packaging). They're sort of like Hyundai cars used to be a few years ago before they started raising their prices. Hyundai used to give you lots of extras that often weren't even available on the equivalent Hondas and Toyotas at a price that was thousands of dollars less.

    FiiO is much the same offering things like bass EQ even on their $20 amps. And, for the price, FiiO has done a respectable job with the audio performance as well. In this case, however, I personally believe they would have been much better off starting with a clean sheet of paper rather than cloning a highly flawed design.

  13. Well, the OPA690 *should* at least be recognized as suboptimal. Its not even meant to be used in audio applications. The power rating on the AD8397 is pretty attractive but dealing with its lack of short circuit protection as well as excessive bandwidth and slew rate seem to be problematic.

    Of course, if people just judge components by their price they'll probably end up out of a lot of money they didn't really need to spend to get the performance they're after. By those standards the O2 must be crap, but I'm still loving mine. I have to thank you again for all the effort you put into the design of the O2. I enjoy mine every day.

  14. Once again a great and informative review.

    I was wondering if you had heard of the DIY amp called "the wire", which is a bit more expensive than the 02 but looks like it could have even better measurements. I think it would be useful if you could test it. Here's the link to the thread on DIY audio: www.diyaudio.com/forums/headphone-systems/179298-wire-ultra-high-performance-headphone-amplifier-pcbs-87.html it seems like it could even drive some hard to drive headphones like the LCD-2 or hifiman headphones, if I'm not mistaken.

    Anyway, thanks again for the review and keep up the good work.

  15. @maverick, good point about the OPA690. I agree it likely was a sales/marketing requirement by whowever thought cloning the Mini3 was a good idea.

    @anon, I've written quite a bit about The Wire but it's scattered around. The Wire is one of very few DIY amps tested on a proper audio analyzer, and for that reason alone, I have a lot of respect for it. And yes, it should outperform the O2 but it's not very suitable for battery powered portable use, and as you suggest, is more expensive. It's also a full surface mount design making it much more difficult to built. I'll put the O2 up against it in a blind ABX test with any headphones the O2 can comfortably drive. The Wire's main benefit is it's ability to better drive that last 2% of headphones--especially if higher supply voltages are used.

    I also have a lot of respect for the somewhat more reasonably priced National reference design using the same LME49600 output buffers. It's performance is also well documented by National and it was designed by some great guys there who are serious about high-end audio.

    @mikeaj, sorry I missed your comment earlier. Because AMB chose to split the input and output grounds, it's true you have to isolate the two for proper measurements. But if the amp is running from battery, or an ungrounded wall transformer, the DC virtual ground voltage doesn't matter. The dScope has full isolation of both its inputs and outputs from each other and from the line/PC (USB) ground.

    Load resistors are floating independent devices (as are headphones for that matter). They're in no way referenced to any ground themselves. So there's ZERO excuse not to properly load an amp during testing regardless of the amp's grounding scheme. AMB tried to imply otherwise in a forum post but I think Ti Kan was just doing some creative hand waving.

  16. On both Chrome, FF and IE8 there's a "An internal error occurred: 4B1F39A7D2668.AB414EB.6E55" on the contact page.

  17. @Ferongr, thanks. I got the same error. I switched the contact page to use an external link to a contact service host and that seems to be working. Please let me know if you have any further problems.

  18. Thank you very much for this - as always - wonderful article. I enjoy reading your articles very much: finally someone who looks behind some products. It really is a relief.

    Oh, do you plan to review the FIIO E6 at some point in the future? I received it today and I´m fascinated by it.

  19. Thank you very much for this - as always - wonderful article. I enjoy reading your articles very much: finally someone who looks behind some products. It really is a relief.

    Oh, do you plan to review the FIIO E6 at some point in the future? I received it today and I´m fascinated by it.

  20. @Marlene thanks for the encouragement :) I have a brand new E6 sitting here and I haven't had time to even unbox it yet. But yes, should find its way onto the test bench in the next few weeks. I'm anxious to see if it really does significantly improve on the E5 as some have claimed. I'm all for manufactures learning from feedback, reviews, etc. and improving their future products.

  21. Excellent review as always.

    Its always good to know whats really working.(Besides all the claims..) Now waiting for your Fiio E10 reviews. :)

  22. Great number of reviews this month! Fantastic! I wouldn't mind if Fiio copied your O2 design..wouldn't mind at all! More to the masses!

  23. FiiO should put the E15 plans to the dustbin, and wait till you finish the ODA+DAC project... then start to manufacture it for the masses. :)

  24. Are there any amps that are like E6 and as cheap, but can be used as a desktop one?

    The information available about E6 tell me that there is no option of not using the battery. I could use E6 while charging and it may stop charging when full, but the battery will be continually being charged if it is used at home most of the time.

    Any alternatives?

  25. @Duncan. You´re very welcome. I discovered Head-Fi roughly two years ago and I was aware after a few months that many reviews are based on misinformation or taste - and not on objective results. I especially enjoy your measurments, I always try to do them myself but 1. I´m not an engineer (so I´m lacking some background) 2. I can only use the dreaded RMAA. Only half a year ago I discovered impedance related frequency problmes or mismatches - and I´ve been listening to headphones for over 20 years now! Shame on me...

    To my ears the E6 comes close to being sonically invisible. I don´t think that this can be improved considerably with more expensive gear and if it can then only a bit and not like the huge steps usually described with much more expensive gear.

    BTW, I also write reviews (if you´re interested) over here:


    They are not objective of course although I try to include measurments whenever I can. I guess you´ll find some hairaising comments from me over there considering your objective perpective :)

    I´m eagerly awaiting your review of the E6.

    Yours, Marlene

  26. @kingpagem I'll be testing my E6, but the E5 can be left connected to power indefinitely and, unlike the E11, it works while power is connected. Li-Ion batteries require charge controllers that shut off the charging when they're full--otherwise they tend to catch on fire. So both the E5 and E6 should stop charging when the battery is full and can be used as a desktop amp with one catch.

    At least with the E5 if the power source is a USB port on your PC, and your PC shuts off power to the USB ports when it's off (or sleeping), the E5 will stay on and run its battery dead when the PC is off unless you remember to turn it off every time. I talk about that in my E5 review. The solution is to buy a $2 USB charger on eBay and run the amp from that 24/7.

  27. @kingpage:

    I can confirm that the E6 works while being connected to the USB port. It also shuts off the charging when the Li-Ion is fully charged, I´ve tried this several times. It starts charging again when you disconnect and afterwards connect it again (then it will only take a few minutes until it shuts off the charging again). And the charging stays off as long as it stays connected.
    So it should be possible to use it as a desktop amp (it can be used with normal Line-Outs and it gets incredibly loud with them, have to use the -3dB option with those). It can even power my HD-600 although it seems to lack true power for that (but it still gets loud as hell). Therefore I´m very curious about the measurments with a 300 Ohm load.

  28. thanks for another great review.. oh and fyi.. i think the e10 is available from amazon now.. it'll be great if you can also review it.. even though the amplification might be similar to the e11, perhaps the line out feature could be used as a great budget DAC..

  29. the e10 is now available at miccastore

  30. Yeah. I know it stops charging when full, but what I'm worried about is constant recharging when the battery drops, rather than draining power from the USB port directly. It takes power from the battery which in turn takes power from the USB port, but never directly. If used as a desktop, it wouldn't be good for the battery...

    I look forward to the review on the E10 DAC.

    I wonder if you would review some onboard sound cards or even some good discrete sound cards (perhaps the ones you already own).

  31. @kingpage Li-Ion batteries don't mind frequent charging. Hybrid cars are offering 5+ year warranties on their Li-Ion batteries where they're charged/discharged hundreds of times a day. It's best to store the batteries at 50% - 80% charge but that only modestly extends their lifespan.

    I hope to publish at least some brief reviews of internal on-board audio. But there are many chips and the implementation can make a huge difference--especially for noise. So the same audio chip might perform much worse in a Mac then in a HP for example.

    As for soundcards, I think they're a dying breed unless you're a serious PC gamer in which case check the gaming hardware sites for reviews. Most people are moving towards laptops, all-in-one PCs, small HTPC's, etc. and most of those cannot accommodate a soundcard. Stereophile tested the Asus Xonar and it seems to be one of the best soundcards but still has some issues and a high headphone output impedance.

    The inside of a PC is also a really harsh environment. At least motherboard designers can locate their on-board sound away from the worst of the electrical noise. But a soundcard might end up right next to a major source of electromagnetic noise (like a graphics card).

  32. It's impressive how Fiio made such a flawed design work as well as they did...

    I hope they consider making a surface-mount version of the O2 eventually, I'd be rather interested in seeing what they could do with that, their engineers clearly know what they're doing. I suspect it's the marketing department that's making them use flawed designs...if people believe in the claims made by AMB then they will be more inclined to believe in what Fiio has to say about it too.

  33. @wcecsharp It's got to be their marketing department. I've been following the O2 on the diyAudio forums (or is it fora?) and the few folks from Hong Kong just went ahead and switched out the 4556 OpAmps for the more popular ones and even noted how they felt they were more detailed. They know a good design... and they know their customers. Compromises all over the place.

    @NwAvGuy How would you measure detail resolution? We all know that better equipment reveals more, but how is that shown in measurements? I can't seem to make a correlation to anything I've seen other than cumulative noise measurements. Is that all there is? Is it even possible to have a consistent scale or rating system for such a thing?

  34. @akgk171, For an amplifier to mask detail it will generally have one or more measurable flaws. In other words, it will fail some of the criteria I established for the O2 and in my InnerFidelity Headphone Amp Measurements article.

    Once an amplifier meets certain criteria for flat frequency and phase response, low distortion, enough power, inaudible noise, etc, it becomes entirely transparent. So if you can't hear the amplifier in the signal chain, how can one amplifier be more "detailed" than another?

    If an amp rolls off the high frequencies more by more than 1 dB at 10 Khz it will generally sound less detailed. If the CCIF IMD distortion is sufficiently high (with distortion products significantly over -80 dB), detail can be "smeared" or masked by the distortion. If the amplifier has poor transient/square wave response (i.e. ringing, overshoot, borderline unstable, etc.) it may suffer from loss of detail or an artificial sense of detail. There's also some evidence if an amp has significant phase shift at 10 Khz it may sound less detailed.

    You say: "we all know that better equipment reveals more" and while that can apply to headphones and speakers, it generally doesn't apply to two amplifiers that pass the criteria mentioned above. Anyone who doesn't believe that, should check the Matrix Audio blind listening test link in the right hand column of this blog. A cheap Behringer amp and cheap Sony CD player had just as much "detail" and were just as "revealing" as $10,000 worth of high-end gear. And the relatively inexpensive O2 amp will have just as much detail as a $1000 Violectric amp.

    A lot of the perceived differences between electronics are often the result of sighted listening bias rather than actual differences. Throw a bed sheet over the gear, without changing anything else, and the differences disappear.

  35. One of the photos from your sansa fuze mini review showed that you have a Sansa E200 series player. I have a E250 myself. I'd like to know what is its output impedance and whether FR is flat or it has bass or treble roll off, if it's easy for you to check (without doing a full review).

    I'm not sure what's the right way to measure output impedance with a multimeter. Can I just take the resistance number as impedance? It seems my E250 has over 700ohm resistence, whereas my laptop headphone-out and my desktop amp have around 2ohm resistence. That's very strange.


  36. I do have a couple E200 series players. They roll off significantly in the bass. The Clip+ sounds, and measures, much better so I don't bother with the E200s anymore. I'm not sure about the output impedance. I believe it was at least a few ohms. No you can't measure it using the resistance range on a multimeter. You can measure it with a 60 hz test track and a resistor using the AC Volts range on your DMM. See my amp impedance article from February.

  37. I'm beginning to recognize that many sources will overdrive IEMs even at unity gain to the point where we're not high enough on even a good volume pot to feel comfortable with the channel balance. Are there plans to address that issue with an additional attenuated gain setting (i.e. 0.5x) or a write-up as to replacing the analog with a stepped digital volume control? Either would be great.

  38. In an analog amp, like the E11, O2 or ODA, it's seriously expensive to do a digitally stepped volume control without the volume control chip being, by far, the worst performing part of the amp. In a DAC, that function is often built into the DAC chip making it much more practical.

    The FiiO E5, E6 and E7 use a volume control built into the TPA6130 chip amp that's respectable but not great. But in those products the chip amp itself isn't very high-end so the volume control probably doesn't limit the performance much (being inside the chip the chip can't be measured without it).

    A 1X gain sewtting, which the O2 Amplifier offers, helps a lot with IEMs. Most amps always have gain even when you don't want it.

    The other options are DACT attenuators, relay switched precision resistor attenuators, and the flagship Cirrus and TI electronic volume chips which require a microprocessor and more. Any of those would easily cost more than the entire E11.

  39. Sorry. My last comment was actually meant for the ODA section. Don't know how it got here, though it is surprisingly valid here as well.

  40. Hi!

    Have my 770 Pro/250s connected to an E10 and the sound is very clean. Bass response should be enough for most folks. Still breaking them in as only had them for 2 days but they will probably be staying and sharing my desk with the Grado 225s which need a lot less driving of course.
    Look forward to your review of the E10 some time soon.

  41. Can E11 drive DT770 80ohm?

  42. @Anon, yes the E11 can drive the DT770 Pro 80s. Even my iPod Touch without an amp can do a decent job with the Pro 80 on typical pop music unless I'm looking to damage my hearing. See: More Power

  43. I have a E-MU 0202 USB sound card and a pair of AKG K 240 Mk II headphones.
    E-MU's headphone amp puts out 16mW and has a output impedance of 22 Ohm.
    K 240 have impedance of 55 Ohm. Everything sounds pretty good, flat (the way I like it) and AKG can be driven loud enough with 0202.

    Will I benefit from incorporating E11 into the system?

  44. @MacGriogair, I don't have K240 headphones, but in theory they may sound better from a low impedance source like the E11 or O2 amplifiers. For a second opinion on headphone output impedance, see: Sonic Advantages of Low Impedance Headphone Amps

  45. Oh wow. The frequency response of the E11 is pretty flat w/ a slight roll-off in the bass region while the E7 has a slight roll off on both ends. But the E7 is a better amp than the E11. A couple of questions:

    1. When designing the O2 amp, which was at the top of your list? A flat frequency response or the ability to drive all sorts of headphones and IEMs without hiss, clipping, etc. Or do they correlate?

    2. I did sighted tests (yea they're flawed) on both the E11 and E7, but I've always thought that the E11 sounds a lot darker/bassier than the E7. Is this my mind playing tricks on me, or is there another explanation?

    W/ the E11 and my BA iems, I hear a faint "beep/hiss", when there's no music playing but it disappears after a bit. It's definitely not the beep sound you get when wearing ear defenders. Weird stuff. Anyway, sweet articles. Keep us updated.

    1. Please see: Headphone Amp Measurements

      I explain the O2 design criteria in great detail in the first two O2 articles. The short answer is it's designed to be 100% transparent with all three of the criteria you list and to drive as many headphones as possible for anything close to the price.

      Your mind is probably playing tricks on you. The E7 might have some high frequency "harshness" due to all the ultrasonic crud in the output. That could make it seem "brighter" than the E11.

  46. Hi, after reading your review I finally got FiiO E11 and DT 770 Pro 80 ohm. Even after reading your review I still don't understand the Low/Hi Power switch of E11. I also have Sennheiser HD570. I have some questions:

    1) Regardless of power consumption, should E11 be set at Low or Hi Power for these 2 headphones? (i.e. does the power position affect sound quality and which position to use for both of my headphones?)

    2) Why is there a need for high gain switch when you can turn up the volume (i.e. I don't understand using 1 verses the other)


    1. It's simple. If the E11 will play loud enough on Low Power, then use that setting. If it won't, try the High Power setting. You won't get better sound quality, at the same volume, from high power. If anything the opposite might be true. See my All About Gain article if you want to know more about how gain is different than volume.

  47. Hi, great review.

    I was wondering, do you know when the input on the E11 clips? I'm fairly sure my sources have enough output, and they all have volume controls, so it's up to the Fiio's input if I'm going to end up with enough level at the end.


    1. Glad you liked the review. Because the E11 (from what I can tell) has the volume control at the input it doesn't suffer from input clipping. Or to put it another way, the output will clip before the input.

      The E11 is a bit low on gain, even in its high gain mode. Also, if you have power hungry headphones, you'll need to use the high voltage mode (the switch under the battery).

  48. Hi, thank for the informative review.
    I have phonak Audeo PFE IEM connected to a LG optimus 7. According to the review from GSMarena the heaphone output has very bad measurement wise when driving headphones and lot better when driving a external amp. Would you recommend the E11 as a good portable amp for this setup? I use it mostly on the move when bicycling so it has to battery powerd and small.The Optimus output allready seems to have a slight hiss to it and was wondering if this is because of the headphones. I have no further info about the dac from the phone, but I know it uses a WM9093ECS amp. Would appreciate any help and suggestions you might have.

    1. If your phone already gets loud enough, you're going to be forced to run the E11 in the low end of the volume control range. If that's the case, ideally you want an amp with 1X gain. It's hard to say if an amp will help the sound much or even at all depending on what weaknesses your phone has. But the E11, or even the E5/E6, would be a relatively inexpensive way to find out.

      Generally people use amps to drive headphones that won't get loud enough driven directly from the phone/player. That's rarely a problem with most IEMs.

  49. So you are saying that you cant use this amp as a desktop amp? or you mean it cant be used as a destop amp while it is charging? i want a amp that can be used at the desktop and still available for potable usewhile maintianing good sound quality, i have considered the e7 but have learnt that since fiio stop porducing them , the one im gonna buy are old ones and sometimes you get the bad ones that stop working waaaay too early , i cant afford the fiio e17 and the e10 is not for portable use. so could you recommend me an amp?

    1. The E11 will only run from battery. If you connect external power to the USB connector, it will not turn on. The E10 isn't an amp at all, just a headphone DAC. If your budget is really limited, and you're sure you even need an amp, you may want to consider the E5 or E6 both of which use the same amp chip that's in the E7. And both can be left powered all the time while using them.

  50. I have been offered a brand new paid of AKG Q701 at a great price - so am tempted! Most of my listening will be from an iPhone / iPod - is the E6 / E11 'gutsy' enough to power them?

    I wanted to buy an O2 but seems it would take at least 2-3 weeks to get one - if I like the headphones recon that would be next on my list - but would a E6 / E11 be worthwhile using in the short term?

    1. An E6 won't come close with the Q701. The E11 also falls short but would be better. It's up to you. If you another use for the E11 even after you get an O2 (or other higher power amp) it might be work in the short term. It's also possible you might be happy with just the E11.

    2. Think the E17 is similar output to the E11 so probably same issue? Think the O2 may be a better bet.

      I'm new to all this - with the E17 if you use it connected to an iPod (using a line out cable) the DAC would not be used and it would be effectively an 'amplifier' - can you still use the tone settings to boost bass (if required) or with a USB DAC are they only used when the source is connected digitally via USB?

      Basically guess I am asking is a E17 used as an amp for an iPhone / iPod (i.e. a non USB source) really going to be any better than an E11 - or are you paying another £50 for an OLED screen and DAC features you may never use.


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