Re: Suggestions for audio noise mitigation?

From: Lewin A.R.W. Edwards (larwe_at_larwe.com)
Date: 01/15/04 

Date: 15 Jan 2004 09:56:20 -0800

Hi Tauno,

> play an AM radio. I'd put small capacitors directly across the amplifier
> inputs, with maybe some series resistance.

Check, I'll try this.

> There are separate signal and power ground pins on the chip. If the
> noise comes in due to a ground loop, it's tempting to separate them
> and take the signal ground along the input cable from the same source.

I'm not sure if this will achieve much. "Signal ground" on the SBC
goes straight to chassis ground, which is physically screwed to the
chassis. The LCD controller board's digital ground is also in direct
contact with this chassis ground. There is no isolation.

> To test for a ground loop, I'd disconnect the input and ground it to
> the signal ground, being careful not to disturb the DC level on the
> input.

? I'm not sure what you mean by the latter part of this sentence. I've
capacitor-coupled the inputs to the amp, so there should be no DC
component. There certainly is a significant DC component on the output
from the SBC.

> A second test would be leaving the circuit as it is now, but feed the
> power from a clean supply.

I tried this, and it makes some small difference but not a whole lot.

There is subtle trickery afoot here. If I touch a metal object to
random spots around the chassis, I can make the noise much worse (just
discovered that this morning).

> You do have both recommended bypass capacitors? Philips' data ***

Yep.

Thanks for the suggestions.