questions on power supply for audio amp

already got star grounding. I don't think this noise is due to the amp coz that amp's (whatever you call it when the open loop gain goes to 1) is at 3-ish MHz. The fft also shows flat noise up to and past this point. I don't think it rings on a square wave either. This high freq noise also goes away when I turn off the power, i.e. the caps are still charged so the amp is still running, but the mains power is disconnected.

I think approx 5mV p-p.

There is only a slight hiss which I expect. I can also hear a quiet

60*n Hz buzz on the right channel, although I can't find that signal with the scope.

What does it sound like ?

One of those FWBR's that all come in one square package.

I don't think the scope is picking up RF coz this signal goes away when I turn the amp off.

At low frequencies (ie 50/60Hz), you should find that a .01 to .1uF capacitor placed across each input rectifier is sufficient to reduce rectifier artifacts. This reduces HF interference bursts that are synchronous with the rectification period only.

RL

I think the amp is stable, at least in the sense that it has no excess noise peaks below a few MHz. However, there was this one time when I tried to use film caps for a bypass, and it caused about 10-20mV of ringing at ~20 MHz (I think this was some LC combo with the power supply). I only did this to one channel, and when I listened to both channels, I thought that the one with the oscillations might have sounded a little bit worse. Maybe.

Anyway, I would like to get the RF fuzz out of my amp, if only so that I can measure clean signals on it.

There is good, and then there is better.

Use a mains line filter.

I read in sci.electronics.design that alan wrote (in ) about 'questions on power supply for audio amp', on Mon, 17 Jan 2005:

It is of absolutely no significance. Just forget it.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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

The mains wiring is an antenna. Also speaker wires. RF levels are high in city areas. You have computers and other switching power supplies. So you have low level RF crawling all over your circuit, especially visible when you use a CRO with its own ground. Perhaps breaking the mains connection disconnects your antenna.

You get used to the normal background levels for each location - for instance, here in the country I see no RF on my CRO - in the city I saw 5 mV pp quite often. AM stations are easily discernable by the modulation of the envelope.

It might be interesting to see if the RF persists when you disconnect the speaker leads.

If your amplifier is unstable, you can expect a volt or more of supersonic oscillation.

If your amp sounds good, you can enjoy it. Case closed.

Roger

Alan, let me put it this way.

You see RF on your CRO, because you introduce the CRO into the circuit of the amplifier, thus making a new circuit. Thus your CRO is not showing you what goes on inside your amplifier - it is showing the combined result of your amplifier with a great big lump of CRO metalwork hanging off it, and the CRO ground.

You would have to make a tiny battery powered CRO or a tiny light coupled CRO probe to see the true voltage between the nodes of your amplifier.

You have a measurement problem. We all have to live with it. Small signals on CROs are suspect. Short your probe to its ground and clip it onto another ground - you may see "RF", hum, all sorts of rubbish. Especially at RF, the CRO lead and CRO PCB inductance, mains lead inductance starts to cause voltage differences between different parts of the CRO. You can clip your probe to $10000 amplifiers and see the "RF".

Most likely the amplifier doesn't have RF between any points within its circuit.

This means you don't "have RF". It ain't broke.

Roger