Re: Suggestions for audio noise mitigation?
From: Stefan Heinzmann (stefan_heinzmann_at_yahoo.com)
Date: 01/15/04
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Date: Thu, 15 Jan 2004 19:32:40 +0100
Lewin A.R.W. Edwards wrote:
> I have an appliance built around an SBC and an LCD monitor, all in a
> metal housing. We can't direct-drive the LCD, because of
> resolution/timing issues; we use analog VGA output to drive the LCD
> via an analog-digital board. The inside is a terrifying hive of EMI;
> to get UL compliance we have to seal every hole with conductive tape.
>
> The SBC's audio output doesn't deliver enough power to drive the
> internal speakers directly, so we have an internal audio amplifier
> based on the Philips TDA7053A (selected because it's very simple - one
> capacitor is about the sum total of the circuit). Now, we need to
> control speaker volume in software, but I didn't want the design
> complexity of I2C-controlled amplifiers or digital pots, so we just
> control the SBC's mixer output volume and the amplifier's
> characteristics are fixed.
>
> The amp is powered from the same 12V rail that powers the LCD
> analog-digital board. The SBC runs off a separate 5V rail.
>
> Problem is that high-contrast patterns on the LCD (e.g. the B&W
> stipple pattern XFree86 shows while starting :) cause a buzz in the
> amp output. This noise appears to be generated by the LCD controller
> board, not the SBC. So I'm working with a clean audio source; the
> noise is being picked up elsewhere.
>
> I've:
>
> * Decreased the input shunt resistors on the amp as far as practical
> while still maintaining a good volume range.
> * Moved the amp as far away as possible from everything else. This
> doesn't seem to make any noticeable difference.
> * Put a large bypass cap on the amp's power rails.
> * Disconnected the ground line on the input to the amp. This, plus my
> finger anywhere on the audio line, makes the unit into an AM radio
> tuned to WCBS New York.
> * Run the audio cable (from SBC to amp) through a fat ferrite bead.
> This was the best step I took so far.
> * Run the 12V line to the amp through another ferrite. This didn't
> make any noticeable difference.
>
> * For test purposes, disconnected the internal amp and connected
> external amplified speakers. No significant noise. Still no noise even
> when I disassemble the external speakers and put the PCB inside my
> housing. That circuit is complex and has an unlabeled IC in it, though
> - I don't want to try to copy it.
>
> The noise is still just outside acceptable despite my best efforts.
> So, I'm looking for other ideas on how to mitigate this noise. What
> else could I add to this circuit? Is there some better kind of audio
> power amp I could use?
>
> Any suggestions appreciated. This project is kind of an interference
> nightmare, I poke it gingerly with sticks...
First, have a look at your grounding arrangement. Make sure you keep the
different grounds separate. In particular, you should have a chassis
ground, which is separate from analog (audio) ground, which is separate
from digital ground. They should all be connected together, but in *one*
place only. This might be a convenient spot close to the power supply.
Connect all cable shields on external cable connections to chassis
ground, as close as possible near the cable entry into the chassis. Do
not connect it to signal ground! If you need a signal ground connection,
run a separate wire for it inside the cable. If this is done properly,
it keeps the radio stations out of your box.
Watch the layout of your amplifier PCB. Are there any wiring loops that
could pick up stray magnetic fields? Minimize the area of those loops.
Magnetic fields can not easily be shielded off (unless you use expensive
mumetal shields).
If the audio connection from the SBC to the Amp creates a ground loop,
try to break it. If that's impossible, put a differential amplifier in
front of your power amp (that's a simple OpAmp circuit). Connect the
ground from the SBC to the negative input of the diff-amp, not the
signal ground of the power amp. Alternatively use an input transformer.
Use a common mode choke on the input of the power amp. Use shielded
wiring from the SBC to the power amp. Connect the shield to chassis
ground on both ends.
Moral: You need to make sure noise currents of whatever origin *cannot*
flow in the audio amplifier's signal ground wiring, as this tends to
create noise voltages along the ground wiring which will be in series to
the input signal and hence get amplified with it. A common error is to
confuse signal ground and chassis ground. They serve different purposes
and need to be kept apart.
Good luck!
Cheers
Stefan
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