Re: questions on power supply for audio amp

From: Roger Lascelles (invalidl_at_invalid.invalid)
Date: 01/18/05 

Date: Tue, 18 Jan 2005 14:59:32 +1100

"alan" <no-longer-valid@yahoo.com> wrote in message
news:cshjur$41k$1@news.Stanford.EDU...
> I built a simple unregulated power supply for an audio amp. The usual:
> bridge rectifier with snubber caps and resistors on the diodes, goes
> into electrolytic caps with a resistor+capacitor snubber, ceramic caps
> at the power pin of the amplifier, etc. The problem is that there is
> some high frequency noise on the supplies that is getting thru the amp.
> Looking on a scope, I can see that there is a broad peak at ~10MHz and
> another at ~40 MHZ. I don't think these are due to capacitor ringing
> since they go away when I turn the power off while the caps are still
> charged.
>
> What's the best way to get rid of this noise? Should I do any filtering
> on the primary or secondary of the transformer? Or do I need some
> better bypassing at the filter caps? (e.g. finding a capacitor that has
> a minimal impedance at these frequencies)

Millions of audio amps operate with a simple rectifier and filter cap
without "RF on the powwer supply". Yes, you can get RF breakthrough if the
interference is strong enough. My initial response is "is this actually RF
interference?".

An oscilloscope on the power rails shows the rectifier sawtooth *plus* some
of the output signal - so RF on the power supply can mean either the RF is
coming in OR the amplifier is generating RF.

First off, I would look for amplifier instability. You need star
grounding - if you have a trace resistance common to power or loudspeaker
current and the input circuit, you will get instability. Does your CRO show
bursts of high frequency on peaks of the signal waveform ?

How large is the RF signal on the output with no signal ? How large on the
supply rail with no audio signal ?

If you live in the city and crank up your CRO, you will see RF in all sorts
of places, so 10mV of RF on the CRO may not mean much.

Is the amplifier quiet with no audio signal ? What does it sound like ?
What kind of diodes did you use for the power supply ?

An RF filter on the mains and speaker and audio input leads can be added -
but lets establish what is going on first.

Roger

If you see a signal other than your rectifier sawtooth on the supply rails,
there are TWO possibilities.

Unless you are running RF transmitters nearby, it is unlikely that RF noise
is being picked up

1) Your oscilloscope is picking up RF. and you don't really have a
problem. How big is the signal on the

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