Full digital class-D Amplifier: HiFimeDIY UD20

Ordered and really eager to try it…
This is quite interesting, instead of performing 2 format conversions (digital to analog and then analog to digital, not really the second is a conversion but you got it…) we have a USB receiver paired directly with a Class D amp…
Theoretically (but don’t really know how the amp stage is) it could surprise me… Hope to see how it performs, and hopefully take some measurements…
You’ll be the first to know :smiley:



HiFimeDIY UD20 USB Digital Power Amplifier
The HiFimeDIY UD20 is a true Digital PWM amplifier with no A/D conversion, no preamp or any analog signal path anywhere before the output stage. This makes a very direct link between the digital source and and the outputs going to the speakers, creating a very clear audiophile sound.

The UD20 amplifier uses the Savitech SA9023 USB receiver to connect to a USB host (computer, iPad etc). SA9023 works in adaptive USB mode and accepts all samples rates from 32Khz to 96Khz with 16/24bits. The digital i2S signal obtained is sent to the St Microelectronics’ STA335BW chip where PCM is converted to Pulse with modulation (PWM) signal and then amplified using DDX® power output stage, enabling it to drive speakers directly. This high quality conversion from PCM audio to DDX’s patented tri-state PWM switching waveform provides over 100 dB SNR and dynamic range.

This is quite different from how a regular class D or T amplifier with digital inputs works - they include a DAC which first convert the digital signal to analog which is then passed on to the amplifier section where it is converted back to PWM. The fully digital technology avoid these extra steps of D/A and A/D conversion.

The UD20 can output up to 2x20W at both 4 and 8ohm. It requires external DC power and is very flexible as it accepts any voltage between 5V and 24V. 12V is required to output 2x20W, and 24V is recommended to achieve lower distortion. It draws 120mA (@18V) running without load, and 23mA when in standby mode. It is very power efficient, up to 90% at full output (2x20W). You can buy it together with this power supply.

The amp is very minimalistic in is functions. There is a volume knob which will also power down the amp to standby when the volume control is set to minimum.
Banana types speaker plugs is used to reduce risk of shorts between positive and negative outputs. The negative outputs are not common and can not be connected together.

The UD20 uses USB Audio Class 1 native drivers for Windows, Mac and Linux, no extra drivers need to be installed. It will work with the same devices as our Sabre DAC support (SA9023 version) - which includes some iPads and some Android devices. More Android devices might work when using the USB Audio record pro software.


  • Fully digital amplification
  • DDX® power output stage
  • Up to 2 x 20W at 4 and 8 ohm
  • 100dB SNR and dynamic range
  • Accept sample rates 32khz, 44.1khz, 48khz, 88.2khz, 96khz and 16 and 24 bits
  • USB input (no analog inputs!!)
  • No drivers for Windows, Mac and Linux. Some iPad’s and Android devices also works.
  • Low power consumption 23mA when standby and 120mA when active (@18V)
  • High efficiency, up to 90% at 2x20W
  • Accepts DC power from 5 to 24V
  • Dimensions: 12.5cm x 9cm x 3.8cm including volume knob

Let’s see…

Love those kind of new hardware. Amps should continue to evolved, Class A are so 20th century :smiley:

This could potentially be excellent.

I have previously used class T amps and they could drive the most difficult speakers effortlessly, (just not very loud, my upgrade from them was to THX ultra certified amps).

I suspect that even if the DAC isn’t a really top notch design, they will still have some interesting qualities.

I await your report on what it sounds like, with anticipation.

I can see me getting one to play with.


Did you get your ud20? I’ve been super curious about this amp. I read that they added an asynchronous option too.

Thank you!

I recently received a HiFimeDIY UD20 and it worked without issue with Volumio and a BeagleBone Black. I have the asynchronous version of the amp, so I can’t vouch for the adaptive mode, but I can’t see how that would cause any issues.

Some things I noted:

The amp can be run at 24VDC, but there are 4x power supply 560uF filter caps rated at 25 VDC. That’s a little close for my liking, so I swapped them out for 4x 680uF 35VDC caps. The new caps have 1/4 the internal resistance, over twice the ripple current capability, and plenty of voltage headroom.

The STA335BW amp chip itself gets quite warm. Looking at the underside of the PCB, I saw that the heat spreader on the bottom of the IC wasn’t soldered to the copper on the PCB. After correcting that issue, things are better. I may look into getting a heatsink that I can attach to the top of the IC.

There is a 3.3V linear regulator in one corner of the PCB that runs @ 117C when powered with a 24V supply. That’s with the cover off the enclosure! A low noise buck switcher would have run much cooler but would have required careful thought to prevent excess EMI. A second option would have been to physically clamp the linear regulator to the enclosure and let the aluminum do the work. I’ll need to see what I can come up with to solve this problem.

NullDev, are you saying that using the ud20 with the 24 volt power supply it’s sold with could be a hazard?

Let’s just say that as an electrical engineer, I wouldn’t design it that way. A few pennies may have been saved using the 25V caps vs the 35V caps, but I’d easily spend that for the piece of mind and longevity of the amp. Definitely do NOT use a 24V linear unregulated supply with this amp.

For instance, I am currently using a second sourced 24V AC-DC switcher that happens to regulate to 24.24VDC. There is a Schottky reverse protection diode on the input, so this voltage drops slightly to 23.83VDC across the 4 supply caps. That gives a whopping 1.17 volts to allow for supply transients and regulation tolerances. In my mind, this is cutting it close and putting an unnecessary strain on the caps.

Will the caps fail? With an unregulated 24V supply, most likely. With a tightly regulated 24V supply, maybe not right away, but you certainly won’t get the full 5000 hour life rating out of them.

Thanks NullDev for this info. I guess there is too much for me to modify to get this working without issue. I just don’t have the courage to solder anything tiny like this. Getting raspberry pi to play without clicks and pops is fiddly enough. Did you let hifimediy know about these flaws?

The changes are quite easy to make as the caps are large thru-hole devices rather than the typical small surface mount ceramic caps. A soldering iron and solder are all that is required, so I wouldn’t let that keep you away.

HiFimeDIY and I have talked a bit about the amp design already. Unfortunately, I can’t tell you if there will be part alterations going forward or not.

I can say that once these nits are taken care of, the amp sounds quite nice and performs up to my expectations. :smiley:

I considered designing what is basically a power DAC - really what you describe. The problem I did not overcome (easily) was implementing negative feedback. The input is digital but the output (to the speakers) is analog. Without negative feedback reducing distortion etc you are at the mercy of particular load you attach. Any comments.

i’m interesting to the products of the HiFimeDIY.
i red this discussion but i don’t find nothing relevant the listening performs.

there is someone that have already tried this amplifier or the new one DDX320 with digital input ?

i would like to realize a total digital installation in my car, like this:

  • player HD that support web radio + spotify + HDD; i like raspberry PI (or platform like this that support volumio) + volumio.
  • DSP processor to realize filtering + eq + ambient correction, connected digitally (i2s if is realizable, or spdif / toslink) to the player; i’m thinking about the minidsp product (minishark) instead the commercial product like mosconi, audison and so on.
  • digital amplifier (power dac); i would like to use this type of amplifier instead the classic analogic amplifier, in order to avoid to use a DAC, digitally connected to the processor.

while for the player and the processor i think to have found the correct components, for the digital amplifier i’m searching a product with the same performance to a traditional good class b amplifier.
the most important characteristic that i search in the sound is clear and open & wide stage, with a sound no cold or metallic.

do you have any suggestion for other product other the hifimeDIY ?

thanks to all.

so how to regulate the volume ?

what speakers can you recommend for that kind (20Wx2) of amplifier?

I’m interested at the same things more or less. Do you have any suggestions for each category?
For what I read on the web, the DSP capabilities of product as miniDSP are extremely low and the computing power of Raspberry Pi 2 is enough to realize a software DSP equalizer and a room correction.
For what concern the amplifier, I’m looking for a class AB amplifier with integrated DAC and I2S bus.
Let me know if you have any news.

There is no A/D conversion in class D or T amplifiers.

And the performances will be good for power efficiency, but are not that exceptional otherwise.
And that is a simple project (even if from one of the most skilled analog circuit guru alive)

If anyone is interested in these new direct digital technology amps there are some starting to appear using the newer higher performance STA326 chip and which are discussed on DIY audio forum here http://www.diyaudio.com/forums/class-d/270424-full-digital-amplifier-chip-sta326.html

I’ve recently bought and tried the FX-Audio D802 and put a comment on the thread above about this. Now It isn’t perfect and I don’t say that you’ll want to replace your £2000 hi-fi amp with this little £70 box of tricks … but you actually might do. At any rate its worth a listen and the simplicity of direct usb/coax/optical input straight to the speakers with no DAC needed appeals.