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Old 1st June 2011, 12:28 PM   #1
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Default How do you test your amp for stability / oscillation ?

Hi,

I'm talking about real tests, not simulations.
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Old 1st June 2011, 01:58 PM   #2
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I look for fuzz on the waveform during all the power and distortion tests, but ultimately I find you need to attach a real speaker, run all different waveforms at an uncomfortable level for both me and the speaker, and look for that same fuzz. I like a nice old analog scope with a tack sharp trace. Usually you don't need all that much bandwidth, but you do need to see very subtle anomalies. I don't have a standard load to simulate a speaker and IMO the values necessary to do this are impractical. Oddly, I don't find musical signals to be of much use in ferreting out problems. I do find it useful to test the system in it's installed configuration, with real cables and loads, rather than just on the bench.
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Old 1st June 2011, 03:40 PM   #3
lineup is offline lineup  Sweden
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Oscilloscope is understood. I get it.
To see waveforms.

But what can we do without scope??
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Old 1st June 2011, 03:49 PM   #4
DF96 is offline DF96  England
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Not a lot.

Overall loop oscillations are likely to be at inaudible frequencies, but they may induce distortion which you may hear.

RF oscillations within a stage may also cause distortion, and may be heard on a nearby radio receiver. Also look for changes in DC supply current when parts of the circuit are touched by your finger or screwdriver - beware of RF burns if it is an output stage!
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Old 1st June 2011, 04:18 PM   #5
HighTec is offline HighTec  Croatia
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Oscilloscope and a function generator is simply must have if you tend to do some serious business with electronics. Ok, digital storage oscilloscopes especially the quality ones are pretty expensive but analog oscilloscopes will still do the job very well. There are bunch of used scopes on ebay in good conditions for a fair price. Btw, I got mine Instek GFG-8219A function generator for 150 bucks on ebay. Almost new, slightly used

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Old 1st June 2011, 05:21 PM   #6
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If a function generator should not be present, you can take a simple triangle / square wave generator.

But an oscilloscope is mandatory! 20MHz is sufficient.

Test the amplifier with small signals, high signals and in overdrive.

Repeat the test with a 100nF capacitor in parallel with the load resistance (8ohm)

The amplifier must not start to oscillate.

Last edited by moschfet; 1st June 2011 at 05:24 PM.
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Old 1st June 2011, 10:29 PM   #7
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OK Lineup, I reveal old trick for person who has nothing but amplifier. Get yourself an old transistor radio with AM. You know, the small ones that take a 9V battery and have a 2' or so pull out antenna. Probably a collectible today! Tune it to a place with no stations at the lower end of the dial and put it near the amp. If the amp oscillates or generates RF in the power supply section, there's a good chance you'll hear it as a growl, tone or whistle in the AM radio. Those little radios make surprisingly good RF detectors and can be used as a "sniffer" probe.
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Old 1st June 2011, 10:37 PM   #8
a.wayne is offline a.wayne  United States
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Quote:
Originally Posted by HighTec View Post
Oscilloscope and a function generator is simply must have if you tend to do some serious business with electronics. Ok, digital storage oscilloscopes especially the quality ones are pretty expensive but analog oscilloscopes will still do the job very well. There are bunch of used scopes on ebay in good conditions for a fair price. Btw, I got mine Instek GFG-8219A function generator for 150 bucks on ebay. Almost new, slightly used

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OK, but is it any good ? .........
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Old 1st June 2011, 11:41 PM   #9
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If tweeters keep dying, I can suspect that something oscillates.
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Old 2nd June 2011, 06:13 AM   #10
HighTec is offline HighTec  Croatia
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Quote:
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OK, but is it any good ? .........
I'm not sure what do you mean. The quality of that function generator? Well, it works perfectly. No instability, clean output signal. I'm satisfied. Besides, this generator costs over 250 bucks in stores so I'll guess 150 bucks over ebay is a good bargain.
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