Audio Amplifier Distortion Tracing

Hi all,

I'm using an old Advance AF generator (15hz-50khz) in an attempt to discover what kind of distortion my Technics hifi amplifier is producing so I can fix it. However, for some reason I'm getting terrible self-oscillation from the otherwise stable (though not-very-nice-sounding) amp that can't be stopped once it starts. I'm just wondering if I should be using some sort of buffer between the generator and the amp input? At the moment I'm just feeing it direct. It's the only cause I can think of. I've checked the generator by itself on a suitable scope and it's absolutely rock solid right across the range and the attenuators are fine so it can't be the gennie itself; must be something I'm doing wrong. What doesn't help much is the gennie doesn't state what its output impedance is. Not sure if that's relevant or not, though. Any suggestions? cheers, cd

Both channels affected? Stable power rails? Can you split at the preamp-poweramp stage?

Correction! It's the scope loading on the output that's making the amplifier unstable. Apologies...

Sounds like your problem is due to the fact that the amp is on the edge of oscillating. This can cause distortion, obviously.. The mere fact that you attach something to it is the trigger. Or maybe you don't have a proper test load on it, a test load should be a non inductive load the amp is designed for.

Maybe you need one of those resistor and cap in series across the output to force the impedance for the high frequencies down to a manageable impedance the amp likes?

Or, you could be having issues with the miller feed back compensation network, which is to help keep the amp from Os-Kill-lating! Not to confuse that with ovulating!

Jamie

Such is life. My amplifiers oscillate but my oscillators don't.

You can measure your generator's output impedance with a variable resistor. Measure open circuit voltage. Then connect the variable resistor and adjust for one half the voltage. Measure the resistor, which should be the same as your output impedance.

Your distortion might be related to the instability. An oscilloscope should have negligible effect on the extremely low output impedance of an audio power amplifier. The amplifier might be rated for an 8 Ohm speaker, but must have much lower output impedance to keep Voltage constant for a varying load impedance over the audio frequency range.

The first things I would check are the electrolytic capacitors. Assuming it is an older unit, several of the capacitors might have developed high equivalent series resistance (ESR). That includes power supply filters, audio bypass capacitors as well as a capacitor in the output stage that blocks DC from getting to your speakers.

An ESR meter is handy to have if you do much repair work. I have the AnaTek Blue ESR meter, built from a kit.

Fred

Thanks for the Thevenin reminder! I'm not using a resistive probe to measure so additional loading is scope's intrinsic 1Meg || 20pf. Still can't believe that little across 8 ohm can cause such mayhem, though. :-/

Ground problems?

You need to pull everything apart, and test items two at a time.

Until you connect the scope you are unable to observe the oscillations.

BTW what sort of frequency are you seeing?

... Phil

Might be! See my reply to Trevor.

Correct.

... Phil

Cant tell yet...

Good point.

No. It's combined. But there's a graphic equaliser inline that could also be to blame. The distortion I'm getting is identical to a poorly set-up GE. This dumb modern integration of everything makes it so tough to seperate possible component causes....

Nothing like a troll..

Jamie