Hifime UD20 digital power amplifier manual

Rev 1.1 April 2014 


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 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 – which 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 avoids these extra steps of D/A and A/D conversion. 

The UD20 can output up to 2x20W at 8ohm loads, and 2x10W at 4 ohm loads. 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 18V 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). 

Technical parameters: 

  • Amplification: Fully digital DDX® power output stage
  • Input: USB 1.1/2.0 12Mbps
  • Output: 2x20W @ 8 ohm, 2x10W @ 4 ohm. Minimum 4ohm loads 
  • SNR: 100dB
  • Supported sample rates: 32, 44.1, 48, 88.2, 96Khz. 16 and 24 bits
  • Power supply supported:DC 5-24 V
  • Power consumption: 23mA when standby and 120mA when active (18V)
  • Efficiency: Up to 90% (at max power)
  • Dimensions: 12.5x9x3.8cm including volume knob 


Volume Knob

There is a volume knob on the front panel that also puts the amp in stand by mode when set to minimum position. A green light to the left of the knob will show when the amp is on. The volume knob has a middle position when turned halfway up (12 o’clock).

The volume knob will adjust the volume from -60dB to +20dB. 

Power connection 

Connect DC plug to the amp before connecting power to the power supply. This is to avoid sparks and excess current flow as you connect the plug. 

USB Connection 

Savitech SA9023 is used as USB receiver. It works in adaptive or asynchronous USB mode (depending on version) transfer mode and uses native USB Audio drivers in Windows, MAC OS X and Linux. No drivers need to be installed. 

Speaker connection

The speaker outputs require normal banana type speaker plugs. It is important to make sure that you will not accidentally short the + and – outputs when connecting speaker cables. This can happen if some cabling from the plug connected to red output touches the plug connected to the black output. This can burn the output chip and requires repair and replacement of the chip not covered by warranty. It is ok to connect only one speaker to out speaker output and leave the other output unconnected (if you should have the need for this). 


Amp doesn’t turn on, the green light is not on 

– The amp will only be on when USB is connected. Check that volume control is not at minimum position (standby) and that the correct DC voltage is connected. 

Amp doesn’t show up in audio devices on the computer 

If green power light is not on (when volume knob is not at min position) then check DC power. Reboot computer.

If green light is on, then unplug and plug the USB cable back in to another USB port on computer. Reboot computer. 

No Sound 

Make sure it is selected as the output device in the system audio devices AND in the player program (iTunes etc.). Make sure that volume level on computer is set to full.

If it shows up and is selected then try disconnecting and reconnecting USB cable. Reboot computer. Check speaker cabling. 

Short dropouts when playing music. 

This problem was experienced when playing some audio files using 24V2.5A power supply, and solved with using 18V PSU.