PSU filter for USB soundcard/amp?


2014-07-11 4:21 pm
Hi all,

This is my first post. A short introduction: I'm an embedded systems engineer, familiar with programming and digital electronics. For 1 occasion however I'm diving into audio :)

I'm working on a project to build a mobile speaker box. It's based on a Raspberry Pi with an internet radio client, a 5V power system from a lithium cell (DC/DC) and a set of 4 ohm speakers from an old 5.1 speaker set.

For testing I've used an old dual LM4667 amplifier board I made in college years ago. It works given that I put 2 speakers in series to create 2 pairs of 8 ohm. The amp board is powered from the same 5V as the RPi, with no filters at the moment. The board has about 220uF of capacitance from ceramic & tantalum capacitors.

This setup poses some problems:

- There is audible sizzling noise from the LM4667 amplifier; even when powered from a linear bench power supply with no other connections (no input; no ground loops).
- There is audible cracking noise when connected to the RPi (especially during boot).
- The distortion at high volume isn't great. LM4667 can only drive 1.3W with 2% THD @ 1kHz.
- The RPi DAC isn't great either.

In an effort to resolve these problems, I figured it's not that hard for me to design a custom USB soundcard & speaker amplifier that does things well (or better).

I happen to have a STM32F4 discovery board with a small I2S headphone amplifier, and wrote firmware for it to appear as a USB audio device. It plays music quite nicely, can't audibly notice much difference between my Creative 5.1 desktop set (although it could be both are crusty:p).

So my part list so far:
- USB microcontroller: STM32F411
- Speaker amp: SSM2305; seems easy to use, class D because power efficient and say 1W/speaker of reasonably clean audio should do it. The speakers I got are about 80dB/W @ 1m sensitivity. 4x1W is also about the safe limit of my power supply combined with RPi.
- DAC: PCM1780; because I don't need an extraordinary DAC for said class D amp (amp = 98 dB SNR). Also; this came on top with "Sort by price":rolleyes:

The DAC and speaker amplifier need to be powered directly from 5V USB.

Now comes my question: what kind of power supply filters are typical for DAC's/speaker amplifiers powered by USB? The output amplifier has about -65dB PSSR; which from my experience with opamps doesn't sound that high.
Is it desirable to get any kind of attenuation at 100.. 20kHz via a PI-filter? If so; I seem to get exceptionally high passive components (like thousands of uF capacitance or almost mH's of inductance).

Note that I don't need the best of HiFi or something; I seek some common sense to make this set-up functional and enjoyable with any creeks and cracks.:D
what kind of power supply filters are typical for DAC's/speaker amplifiers powered by USB?
I always recommend the fitting of passive single pole RC low pass filters at the inputs to audio equipment.
Most will hear a low pass F-3dB @ 100kHz (RC = 1.5us)
Some go as high as 500kHz (RC = 0.3us) to avoid cutting off any of the audio band.
A 20kHz filter will be very audible !

The same rules cannot be applied to digital signals.
Digital equipment will almost certainly have filtering on EVERY connection into the equipment.


2014-07-11 4:21 pm
Yes I have fitted a low-pass RC filter at the output of my DAC. I have not determined what component values to use yet, I was first thinking about 10nF/470 ohm giving RC constant of about 4.7us. This could be a bit on the high side though.

The DAC and speaker amplifiers will be on the same board, powered from the same 5VDC USB power supply. I will add a ferrite bead and a few ceramic capacitors as is typical in any USB device, but I fear this is not enough for audio. Especially during silent moments in the music.. Additionally the RPi is powered from the same 2A power supply, which will draw current spikes from the 5V rail causing noise.

My question is basically what a typical USB powered DAC/speaker amp filter looks like. Is it typical to include a LC or RC low pass filter on the power rail that has -3dB point at like, 10 .. 20kHz, or even lower?
attaching the filter to the output is different from using it on the input.
The cables pick up interference and take that into the next receiver.
The filter should be at the input to the receiver and preferably right at the entry point into the receiver enclosure. This is usually not a practical location and so we move it to the receiver PCB. But that means the cable travelling across the gap from entry point to PCB also emits interference. This emitted emi from the unfiltered cable entry can affect the circuits inside the enclosure.
That is why I often suggest fitting two passive filters. A single low value capacitor (typically 22pF to 47pF, this can be increased to 1nF for speaker terminals) from Line to Chassis at the entry point and the full RC at the PCB.


Disabled Account
2010-01-17 9:20 pm
I expect most audible noise will be generated by the RASPI when powered by the same 5V supply. This noise is well within the audible band - so LC filtering could be really bulky.

To get some feeling for the way the RASPI poisons the supply rails, listen to your 5V-supply during RASPI operation by connecting your speaker to +5V and GND via a coupling cap of some 100uF. This is a very simple and revealing test.;)

I think you should decouple the DAC supply.
The power amp should provide enough supply rejection.
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You may find that a USB isolator is applicable, based on ADuM4160 IC. The isolator module I use includes a separate power input for the terminal USB device - for applications where that is an input device such as soundcard then I use a 12V battery for powering (to isolate from mains AC earth loops), and the isolator board has an onboard low noise regulator to reduce to 5VDC.