Behringer DAC Modifications

INTRO: The little Behringer UCA202 DAC, for under $29, did surprisingly well in my review.  In all areas but one, the UCA202 is a respectable performer. The photo shows the much larger output capacitors (big black objects above the volume control) laying on their sides that are part of the modification.

PRO-SOUND DESIGN: Behringer likely designed the UCA202’s headphone jack for use with higher impedance studio headphones. While it can drive such phones fairly well, it’s not very compatible with typical consumer 16 – 32 ohm headphones.

WEAK HEADPHONE OUTPUT: The 50 ohm output impedance of the UCA202 limits the volume you can get with low impedance headphones and also causes substantial frequency response variations with certain phones due to how the impedance interaction (see the article on headphone/amp impedance).

The graph (click for larger) shows the frequency response of the original UCA202 with no load in yellow and, without changing the levels or volume, what happens when you plug in a pair of Ultimate Ears SuperFi 5 Pro headphones. The balanced armature design of the SuperFi’s create wide impedance swings that cause a total of 14 dB of frequency response deviation (+/- 7dB). The broad midrange boost and high frequency cut is very audible and undesirable.

THE CURE: I wondered if it’s possible to lower the output impedance and raise the output level? If so, the Behringer would be a bargain USB Headphone DAC.

SOME RESEARCH: Unlike similar “disposable” consumer electronics that are often glued or snapped together in a way you have to destroy them to open them up, the UCA202 comes right apart after removing two easily accessible screws. My first observation was the general purpose op amp used to drive the headphones can’t drive lower impedances very well. And there are 47 ohm resistors in series with its outputs (required by the op amp). These further raise the output impedance and limit the output power. I came up with a modification that requires replacing the op amp and four to six other parts.

THE EXPERIMENT: I warmed up the soldering iron and went after the UCA202’s PC board. I replaced the headphone op amp with a much higher quality pin compatible IC. The new amp can much better drive lower impedances at low distortion and generate much more output power from the same power supply voltage. A few other component changes were also required to optimize the new design. The photo shows the old IC removed and the other component locations circled.

THE RESULTS: The table below summarizes the measurements between the original UCA202 and the modified version. The main goals were met with more than 12 times more output power into 16 ohms and the output impedance dropped from 47 ohms to a much more reasonable 2.5 ohms. Using Ultimate Ear’s headphones, the frequency response improved from +/- 7 dB to a much more neutral +/- 1 dB. The other measurements were similar enough to be the same (better results are in bold):

 

 

 

 

 

 

 

LISTENING TEST: In the Listening Test the modified version scored a slightly better than the unmodified version’s headphone outputs. But I used headphones in the test with a relatively constant impedance so the frequency response changes were minimal. The listening level was also (barely) within the unmodified versions power capability. Changing the headphones, or raising the volume, would have yielded a much more obvious advantage for the modified version.

NOTE TO BEHRINGER: Behringer could, if they wanted, mass produce a similarly improved version of the UCA202 and the retail price would only need to be increased by less than $10. The changes I made would raise their wholesale component costs by less than $2. Unlike the current model, the new $35 – $39 USB DAC would meet the headphone needs of both professionals and consumers. It would suit a much wider range of headphone types and play significantly louder with any headphones with more accurate frequency response.

NOTE TO OTHER USB DAC MANUFACTURES: This experiment, along with the original UCA202 Review, show what can be done with an inexpensive USB headphone DAC. It’s my opinion if any company is going to charge several times more, and can’t at least equal this level of performance, they’re probably not offering a good value for their customers.

BOTTOM LINE: I was pleased with the above results. The main purpose of this rather academic experiment was to prove: It’s entirely possibly to design, manufacture, and sell through retailers, a high quality headphone DAC for $39 or less. The bad news is, as explained in the Tech Section below, it’s not practical for most to perform the modifications. And I’m not holding my breath Behringer will offer an improved version given their target market. And if you paid someone else to do it, the total cost (including the UCA202, labor, and parts) would probably spend around $100. And, at that price, there are likely some better options that don’t require modifications (i.e. the $99 FiiO E7, $99 Firestone Fireye2, etc.). As always, I welcome comments and feedback!

---

TECHNICAL SECTION (all the details):

WHAT DIDN’T CHANGE: Because only the headphone amp was changed, the performance of the DAC chip itself, and the line outputs should remain the same. I did some quick checks to verify the line output performance, DAC linearity, jitter, etc. were similar to my earlier review. They were.

DIFFERENT TEST LOAD USED: The original UCA202 did so poorly with my usual 15 ohm load, I used a 150 ohm load for most of the headphone output tests. But this makes comparing the UCA202’s headphone performance to other devices I’ve tested more difficult. So, for these tests (unless otherwise noted) I used my standard 15 ohm test. Note this is a much more challenging load, so in some ways, one can expect degraded performance from the previous 150 ohm measurements.

MAXIMUM OUTPUT: The original UCA202 had a hard time with my 15 ohm test load. It only managed 179 mV RMS at 1.3% THD+N which is 2 mW into 16 ohms. Here’s the plot:

And here’s the modified version doing more than twelve times better better at 635 mV which is over 25 mW into 16 ohms:

And because the new amp IC can swing closer to the supply rails, even with 150 ohms, it has an advantage. Here’s the original max output at 150 ohms of 668 mV or about 3 mW with the volume set as high as possible for < 1% THD+N:

Here’s the modified version into the same 150 ohm load with nearly 4 times as much power (11.3 mW) this is with volume control all the way up at 0 dBFS:

THD+N AT REFERENCE LEVEL: At the reference level of 400 mV RMS here’s the original driving the relatively easy 150 ohm load:

And here’s the modified version, same level, but into a much more challenging 15 ohm load. The distortion is virtually identical to the above measurement and likely determined more by the DAC chip and implementation than the new headphone amp (anything below 0.01% is widely considered inaudible anyway):

OUTPUT IMPEDANCE: Besides output level, the other big problem with the original UCA202 is the approximately 50 ohm output impedance. Here’s the modified version with no load referenced to the same 400 mV level used above. Knowing the no load (source) voltage is 467mV and the voltage with the 15 ohm load is 400 mV, the output impedance calculates out to 2.5 ohms which is a huge improvement and very close to my ideal of 2 ohms or less. I’m not sure yet why it’s not closer to 1 ohm but I suspect it’s at least partly the output capacitors I used (more on those later):

NOISE:  The A Weighted output noise previously measured –92.0 dBA while the modified version measures a nearly identical –91.4 dBA. A difference of 0.6 dB is not generally considered audible. The 47 ohm series resistor in the old design was attenuating the output noise with a 150 ohm load by about 2 dB. Considering the new amp doesn’t have such an attenuator, it’s noise performance is actually better. The new amp IC has better noise specs than the original so this is what you would expect. And the remaining noise appears to be from the DAC itself. So, for all practical purposes, the noise performance is the same:

SWEPT THD+N: The original UCA202 did well on this test and the modified version does even better despite driving a much more challenging load (15 ohms vs 150 ohms). The original is shown in red, the modified version in blue, and the NuForce uDAC-2 in green (note the vertical scale is expanded to a max of 0.1% compared to the NuForce uDAC-2 Review):

CHANNEL BALANCE: Channel balance is very load dependent. There are multiple reasons for this. More challenging loads cause the power supply (which is shared between both channels) to have more “load ripple” which is essentially the audio signal being superimposed on the power supply. Another reason is higher loads create higher currents and higher currents create more electromagnetic radiation which “cross couples” between the channels. This happens both inside IC’s shared between both channels (like the headphone amp IC is in the UCA202) and even between tracks and passive components on the circuit board. So I wasn’t surprised the UCA202 did much worse at 15 ohms compared to 150 ohms. If you compare the 150 ohm performance with the modified amp, however, you’ll see the result is unchanged. So the red line below isn’t a result of the modification, but mainly the much more challenging load. It’s not a great measurement at about –38 dB. But, especially for headphone use where a lot of people add far more “crossfeed” on purpose, it’s likely not a significant problem:

FREQUENCY RESPONSE: Because the UCA202 lacks a split power supply, or bridged amplifier design, it uses AC coupled amplifiers and outputs. So in both versions, the headphone amp requires an output capacitor in each channel to block DC from reaching the headphones. Into 50+ ohm loads a relatively small capacitor works fine. The original UCA202 uses 100 uF output caps. With a 15 ohm load, however, the –3 dB low frequency roll off changes dramatically. So I increased the size of the output capacitors to help compensate for this. The values I used create a low frequency roll of only about –0.2 dB at 20 hz even with this worst case load. There’s also a high frequency rise that can likely be easily fixed. The new amp IC is “faster” than the old one, and hence requires different feedback compensation. I don’t have the right small value surface mount capacitors on hand to tweak the feedback loop. So I left the original Behringer compensation and it’s causing about a 0.4 dB rise at 10 Khz as shown (the original UCA202 is shown in yellow):

And here’s a composite plot showing the frequency response of both versions with a variety of loads taken at the same relative level. The most interesting thing to compare is the blue plot of the original UCA202 driving Ultimate Ears SuperFi 5 Pro headphones versus the red plot showing the new version driving the same headphones. You can also see the massive drop in output and low frequency roll off with a 15 ohm resistive load in orange versus the modified version in green:

OTHER TWEAKS: As mentioned above, the feedback loop could use some attention. Optimizing the feedback should remove the slight 0.4 dB rise at the highest frequencies and might improve the sound. And I suspect using higher quality output capacitors might lower the output impedance still further. The problem here is finding better parts that will fit inside the original case. All my modifications fit completely inside (although I did have to cut a plastic post off the inside of the case). Better audio-grade output capacitors (and possibly adding quality bypass caps) may also improve the sound quality and further lower the output impedance. It might also be possible to improve the channel separation with some tweaks to the power supply lines feeding the new amp IC.

THE BAD NEWS: This is mostly an “academic experiment” as few have the tools or skills for the surface mount re-work these modifications require. As can be seen in the photo, surface mount components are incredibly small and not made for human soldering (machines do it all when the boards are made). The photo shows three surface mount resistors in their original paper “tape” next to a regular pencil. The entire parts are smaller than the tips on most soldering irons.

THE DETAILS (added 3/25): I’ve received several messages and emails wanting to know the details of the modification. My plan was to research the issues mentioned above and then publish the final component values. However, for those of you who are just curious, here are the details. I want to stress that anyone who doesn’t have enough surface mount soldering/re-work experience could easily damage their UCA202 beyond repair attempting these modifications. And I also have not explored the high frequency “peak”, tried to improve the channel separation, or explored the somewhat higher than expected output impedance. So for anyone attempting this modification, please be aware it is presented “as is”, has not been fully developed, and you may damage your UCA202!

I chose an Analog Devices AD8656 op amp. It’s designed for high quality audio use from a 5 volt power supply with very low distortion and noise. And, equally important, it has +/- 75 mA of peak current capability to better drive low impedance loads directly along with a “rail-to-rail” output increasing the power available for higher impedance headphones. For a high-end op amp it’s also relatively inexpensive in single piece quantities at $2.75 (I wanted to keep the total parts cost < $5). If cost is not an issue, I would consider the $5.22 AD8397 as it has +/- 310 mA of peak current capability and is used in many respected commercial products as the headphone amp although it’s not optimized for a single 5 volt power supply.

I replaced the series 47 ohm Behringer output resistors (circled in the photo earlier) with 1 ohm 0603 SMT resistors shown next to the pencil in the photo. And I replaced the 100 uF Behringer output capacitors (pads circled) with 2200 uF 6.3v capacitors you can see at the start of the article (laying on their sides). I had to cut one of the internal plastic posts from the case (also visible in the phone) to make room for the larger caps.

FINAL WORDS: All things considered, for a fairly simple “hack”, I was rather pleased with how this turned out. The modified Behringer UCA202 has enough power to satisfy most anyone (significantly more power than a Sansa Clip+ for example). And the output impedance drops from a very poor 50 ohms to only 2.5 ohms. It retains its previously low distortion, similar noise levels, and most other characteristics. The very slight ( +/- 0.3 dB total) frequency response variations are inaudible but I’d feel better about this modification with the feedback compensation adjusted for the new amp IC. But I have to order some parts to do that. If enough people are interested, I can take this project further?