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ckwong99 17th November 2008 05:28 PM

Power amp outout stage self oscillation problem
Hi Folk,

I am much depress on my just finished class A 50W Power AMP. Because its front and mid stage mounting on the same PCB and checked no problem. However, connect it with output stage with 6 pairs of MOSFET (Hitachi J56 / K176) per channel then the AMP have small "giisss" noise (not from speaker but it happen it some component). I think it should be oscillation by it self.

Just want to know how to solve ?


CBS240 17th November 2008 05:31 PM

Got a scope????

You may need one......:dodgy:

ckwong99 18th November 2008 01:01 AM

I have a 100MHz scope and signal generator. How to find where or which thing incur the oscillation.

Just find some circuit such output stage FET drain-gate and gate-source parallel with a pico value of cap. What it the purpose of it ?

eyoung 18th November 2008 01:20 AM


Could be a cold solder joint making a noise. Maybe a schematic
and some idea of localization of noise(I have heard resistors ring).
go back to step one of the build and double check placement and polarity of parts and all solder joints. check the basics for DC offset and bias.Look at the output on a scope and trace it backwards .

keep us posted.

Regards, Elwood

ckwong99 18th November 2008 04:28 AM

Thanks your input and I will do this on week end.

unclejed613 18th November 2008 01:27 PM

output inductors "sing" as well. it's a magnetic effect when large currents flow through the inductor. if this is happening with no signal applied, it could be oscillation. output inductor "singing" is usually heard while driving a signal into a dummy load.

CBS240 18th November 2008 02:12 PM

Leave one channel measuring the oscillating output, then use the other to poke around certain areas of the circuit where such an oscillation might originate. If the oscillation is very fast frequency, the second scope probe may change it in some way via the internal components and can help point you toward the source of the problem.

I have found sometimes too small gate resistors with/or no gate Zobel and poor layout can cause a mosfet amp to become a RF transmitter.:devilr: I have set fire to the 10 Ohm output Zobel resistor through a small series 100nf cap a couple of times.(was using cheap carbon resistor:rolleyes: )

peranders 18th November 2008 02:23 PM

Do you have gate resistors on each mosfet? Value? NJormally it's the N-channel device that oscillation and also above a certain current level, easy seen with an oscilloscope. The oscillation is local, only the mosfet oscillates.

ckwong99 18th November 2008 05:07 PM

I do not use inductor at end of speaker output. It seems a resistor generate this sound. Most likely some of it is wire wound type.

I know Zobel is a type of filter. Not sure it will usually exist on the speaker output between + and ground. And Zobel is around 5 ohm in series with 0.1 u cap. But I am not sure why it exist and sometimes no need apply on some AMP.

I use 470 ohm in series with the gate of output FET. Suppose my AMP is self-oscillation then power on it again to trace the problem. Due to the fault finding can spend much time. Will it damage to the AMP ? How can I do ?

CBS240 19th November 2008 02:24 AM

Zobel is a filter that keeps RF from entering the feedback loop via the speaker wires and output node, assuming you take your feedback from the output. It is used, and sometimes with an output inductor || small resistor for stability. If your amp has a global feedback loop taken from the output you should at least have a Zobel filter at the output to help separate the noisy RF world from your little circuit.:)

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