oscillating audio amplifiers

ys

Plot the poles. An amplifier is a low pass filter -- figure the rolloff occurs well outside the audio range. Then look where the phase shift hits 180 degrees.

Always?

Are you familiar with the use of negative feedback, and how an amplifier needs to be designed to be stable?

No amplifier has infinite bandwidth. The response eventually starts to roll off, and phase shift increases with the rolloff. If the phase shift hits 180 degrees before the gain falls below unity (1), you have the conditions needed for oscillation -- /positive/ feedback (180 + 180), plus gain greater than one.

To get around this, we add a single "compensating" pole to the amplifier's transfer function (basically, a simple low-pass filter) several octaves below the amplifiers normal rolloff point. Because of its "distance" from the normal rolloff, it now dominates the response. Because a single pole can never have more than 90 degrees phase shift, the amplitude response will fall below unity (1) before the net phase shift (amplifier's native response

• added pole) reaches 180 degrees. No more oscillation -- assuming the pole has been well-placed.

Because negative feedback is used to reduce distortion and source impedance, and the reduction is proportional to gain reduction, the compensating pole has to be at least an octave above the audible band -- hence the tendency of oscillation or instability to occur at about that frequency.

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I suppose over the years about 5 such amplifiers, the fifth this week. All about 50 KHz , different makes , types and powers. I just would have expected more variety between say 30 and 80 KHz for amps with transistor fTs for audio use

Point taken. I'm surprised you've had a problem at all. You'd think the amps would have been better-designed. Your speakers might also be "pathological" loads.

"William Sommerwanker"

** Got absolutely nothing to do with an amplifier's inherent stability with NFB.

The Kook is talking about audio power amplifiers oscillating because of accidental POSITIVE feedback - ie capacitive coupling from the output to a non-inverting input.

Each case if different, but something like a 50 to 100pF cap from the speaker out to the non-inverting input will send almost ANY tube or transistor power amp into full power oscillation. ( A direct connection typically turns the same amp into a low frequency ( 1 to 5 Hz) square wave oscillator. )

The actual frequency of oscillation is determined by many things, but is typically about 50 kHz ( with stray coupling) because of the combination of input and output low pass filters, the amps slew rate and the available gain. While not a common event these days, I still see occasionally power amps damaged by HF oscillations. Bad wiring practices, like unshielded or unbalanced input cables running alongside speaker cables are the usual culprits.

Also, it is very easy to make a guitar amp oscillate at a high audio frequency - just put the guitar near the speakers and turn the gain and treble controls up. Capacitive coupling from the frame of the speakers injects HF signals straight into the pickups and any unshielded wiring in the guitar.

Some "combo" amps have the speaker frames connected to the amp's chassis to reduce this effect - though with tube amps it rarely damages anything.

... Phil

While at it, assuming the amp outputs, Tx, fuses , zobell components etc survive sustained 50KHz high level drive into speakers for a few seconds - how would bass speakers fare as there would be no airflow cooling ? I assume zobells would burn up to open cct promptly followed by the speakers.

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I don't know that that's what he /was/ talking about, but I understand what you're saying. I've never seen it in a consumer audio system, but I can imagine how it might occur on-stage.

By the way, supposedly correctly designed amplifiers /can/ become wildly unstable with "pathological" loads. I've seen it.

PS: If you need to derogate my name, please find some cleverer way to do it.

assume

It depends. What's the woofer's impedance at 50kHz? If it's high enough, there won't be enough current to overheat it.

" Some wanker wrote"

" Nutcase Kook "

** HE does not know what he is talking about. ** The loads that sometimes make SS amps go loop unstable are moderate values of capacitance - ie 10nF to 100nF at the speaker terminals. The resulting oscillation may well be at 1MHz or higher and at a level the slew rate allows.

Not at all likely to cause full power oscillation at 50kHz.

.... Phil

"Nutcase Kook"

** Only tweeters are at any risk. Bass and guitar speakers have lots of inductance that makes their nominal impedance value rise by a factor of 5 to 10 at 50kHz.

This means the current flow is greatly reduced and the resulting heat dissipation is but a small fraction of what you might wrongly suspect.

.... Phil