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Old 21st August 2002, 09:25 PM   #1
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Default Amplifier oscilation

Today I was testing an 150W pioneer amp using a 5m cable , 1.4uH, 660pF, .066 ohm. I was applied a 1khz sinewave. Passing 20Vpp appears oscilation about 100Khz. Lowering applied signal, Below 20pp it dissapeared. i tryed a zobel network but didn't worked. What it might be?

The amp is a pioner VSX605S receiver.
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Old 23rd August 2002, 11:47 AM   #2
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I don't know that kind of amp, but is 20Vpp not a little bit high to connect to the input??? Probably the amp ran into oscillation because the input is too high, so the signal will clip; and square waves have lots of higher frequencies.


HB.
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Old 23rd August 2002, 03:59 PM   #3
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20Vpp at the output. The input was a 1KHz 1Vpp sinewave from a rohde-schwartz audio analizer.
It was caused probably by the output cable, I just want to know why and how to kill it.
The test was made under my professor's supervision.
I was thinking it had might be caused by an out of phase feedback, turning it from a negative feedback to a positive feedback.
Without the 5 meters cable it had run up to 120Vpp.
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Old 23rd August 2002, 07:12 PM   #4
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Put the "Zobel" (.1 uF + 5 ohms ) at the speaker end
of the cable also.
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Old 25th August 2002, 10:04 AM   #5
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Sometimes this happens with high frequency BJT's. A capacitor of some 470pF between collector and base of the power transistor might help to stabilize, but a use of an oscilloscope is a must.
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Old 25th August 2002, 03:07 PM   #6
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Default No problem.....

The use of instruments is not the problem, I have access to an digital osciloscope and an audio analizer (FFT, waveform, THD, etc.)
But even with it, as I'm only an student, I couldn't dicover, and I ascked for help becouse I think my professor is testing me!
The power BJTs are 2SA1302 and 2SC3286. They run in AB class.
I have to put this capacitor in the output transistor itself or in the darlington "driver"? Rod Eliott once told me that , he didn't know why, but the oscilation happens to appear only in the "negative" pair, and to use this miller cap in the driver, can I try this without damage of the receiver?
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Old 25th August 2002, 07:41 PM   #7
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It strongly depends on the circuit - sometimes it helps to put cap to the output transistor, sometimes to the driver. You can have a look at my www - I had the similar problem - see "push-pull class A follower", which was in fact operated in AB class. Rod Elliott is right, very often the osccillations appear only at one half of the waveform. I hope you will not destroy the output transistors by small caps, does the amp have an overcurrent protection? You may start with smaller value, say 100pF.
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Old 25th August 2002, 08:20 PM   #8
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It has protection relays, i was thinking about 100pF too!
Where is the link to your hp?
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Old 26th August 2002, 02:31 AM   #9
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An often ignored problem with regard to amplifier stability is the presence of a non-minimum phase, (RHP), zero generated by multiple signal paths from input to output.

An RHP zero has the magnitude response of an LHP zero, (i.e: it 'breaks up'), but the phase response of an LHP pole, (i.e: phase response 'breaks down', or tends to - 90 degrees in the limit).

This undesirable zero can be moved to the LHP by connecting a small resistor in series with the miller compensation capacitor(s), or moved to infinity by placing a capacitor in parallel with each of the often-used series resistors to the output stage.

This may, (OR NOT ) be the cause of your problems.....but give the later a try....a time constant of the order of 200nS for each parallel combination should suffice......
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Old 26th August 2002, 02:59 AM   #10
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Now that's a hell of a thorough answer - glad to see we have some properly trained EEs in the forum.

Quote:
Originally posted by mikek
An often ignored problem with regard to amplifier stability is the presence of a non-minimum phase, (RHP), zero generated by multiple signal paths from input to output.

An RHP zero has the magnitude response of an LHP zero, (i.e: it 'breaks up'), but the phase response of an LHP pole, (i.e: phase response 'breaks down', or tends to - 90 degrees in the limit).

This undesirable zero can be moved to the LHP by connecting a small resistor in series with the miller compensation capacitor(s), or moved to infinity by placing a capacitor in parallel with each of the often-used series resistors to the output stage.

This may, (OR NOT ) be the cause of your problems.....but give the later a try....a time constant of the order of 200nS for each parallel combination should suffice......
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