audible or noticeable symptoms of oscillation? - Page 2 - diyAudio
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Old 10th February 2006, 06:55 AM   #11
Eva is offline Eva  Spain
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Oscillation may cause offsets only when the circuit suffers from highly asymmetric slew rates. A simple diode rectifier may not detect mild oscillation at all because the voltage amplitude may be well below one diode drop.

Also, the most usual and most troublesome oscillation issues in power amplifiers appear only at certain output current and voltage levels, so it's required to feed some signal in the audio band to the circuit and play with its amplitude while monitoring the RF that is coming from the output through a high pass filter. Obviously, several load impedances have also to be tested. Most oscillation issues appear only near zero crossing or near clipping, but sometimes oscillation may appear only between these points.

The RC output zobel may be a very useful built in HP filter when the capacitor is connected to the output and the resistor to ground. In these circumstances, all the RF will be present across the zobel resistor without any bass signal disturbing the measurement.
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Old 10th February 2006, 08:56 AM   #12
clem_o is offline clem_o  Philippines
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Quote:
Originally posted by Eva


A simple diode rectifier may not detect mild oscillation at all because the voltage amplitude may be well below one diode drop.

Right - that's why an active rectifier (what I meant - one without any volttage drop) will work better. But Eva's right - often oscillations show up only when the circuit's actively driven; i.e. using a square wave to look for jaggedness and ring in the response. That means some kind of cancellation would be needed to remove the test signal, leaving the residual oscillations, if any.

Easier to buy an old analog oscilloscope! :-)


Cheers!
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Old 10th February 2006, 09:48 AM   #13
Eva is offline Eva  Spain
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Active rectifiers are usually op-amp based, so they are not likely to work properly with signals above 1Mhz.
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Old 10th February 2006, 03:37 PM   #14
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Default detection of oscillations

Thank you very much for all the tips; they are really interesting for me, because I`m doing my first steps in DIY -with a cuote of good luck, happily-, but I have no electronics background, nor experience in testing.

It appears that a good move will be the acquisition of an oscilloscope, and some reading and learning. It will be fun.

Besides that, the method described by Enzo is very interesting as experimentation, and a good way to excercise creative approaches to electronics.
AM radio is an interesting tip too, as well as the different sonic signatures of different kind of oscillations.
I believe that a mix of methods will be useful in casual situations; the serious approach afforded by the oscilloscope will be the icing of the cake.

Thanks to all of you again.
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Old 10th February 2006, 03:43 PM   #15
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Greetings from Norfolk

Just a thought - a 1N4000 series diode will loose performance somewhee around 1 MHZ I think - I would suggest that a signal diode be used (1N914 is OK to about 75 V rev. voltage) and will rectify better at around 1 MHz.
Also suggest that the smoothing capacitor be a ceramic one - not a 'wound plastic film' type - this may have a significant inductance in series with it due to its construction.

Good luck
Richard
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Old 11th February 2006, 12:04 AM   #16
clem_o is offline clem_o  Philippines
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Quote:
Originally posted by Eva
Active rectifiers are usually op-amp based, so they are not likely to work properly with signals above 1Mhz.
Hi Eva, of course you're right, but I was thinking of one of the hi-speed FET input op-amps, or an alternative active circuit to track the bias on the diode.

Hi gandalph - agreed, in914 or 1n4148, or in34...

Cheers!
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Old 11th February 2006, 03:15 AM   #17
Enzo is offline Enzo  United States
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Yes I suppose we should differentiate between instability and parasitic oscillation. I usually run into amps that will sit there cranking out the RF. I don't nearly as often see triggered oscillations except when the circuit is otherwise really screwed up too. I also deal with high powered stuff, so I worry about voltages. I suppose the diode Vrev is not an issue in a 30 watt amp.

As to not reaching the diode junction voltage, again I assumed that the amp was cranking out the HF. SO losing a half a volt from the signal is no big loss.
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Old 13th February 2006, 09:45 AM   #18
Eva is offline Eva  Spain
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For everybody that can get access to an oscilloscope:

Connect one end of a 100pf capacitor to the output of the amplifier, the other end to a 1K resistor, and the other end of the resistor to ground. This will form a nice 1Mhz high-pass filter. Connect one probe to the junction betwen the C and the R, and the other probe to the output of the amplifier. Set the trigger source to the low-pass filtered probe.

This setup allows to find out exactly when and where an amplifier oscillates. I've just used it to find and fix a 14Mhz oscillation with only 100mV amplitude that otherwise was very hard to analyse because it only appeared very briefly near zero crossing.

When everything is right, the filtered probe should show almost no signal at all (a few milivolts), even when the the amplifier is playing at full volume. Clipping may introduce some HF content on the output, but this is not a major issue.

Note that oscillation does introduce serious distortion because it impairs the error-correction properties of the global feedback loop while it happens.
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