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ALEPH 5 Oscilating 600KHz
ALEPH 5 Oscilating 600KHz
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Old 24th March 2020, 08:07 PM   #21
wayne is offline wayne  United States
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ALEPH 5 Oscilating 600KHz
It would not hurt at all to try 680 Ohm gate resistors on the outputs.
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Old 26th March 2020, 08:18 AM   #22
Extreme_Boky is offline Extreme_Boky
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The output MOSFET could be slightly damaged to cause that behaviour. I remember having the exact same symptoms, a long time ago, on few occasions, when I worked for Jands electronics (PA amplifier of some 2KW per channel, bipolar and/or MOSFET's). I also recall that you played a lot with one pair, two pairs... of output MOSFETs, so you did a fair bit of touching-the-legs activity.

If the oscillations go down with 680ohm gate resistors, then replace all MOSFET's and try again. If the oscillations disappear, go back to the original value of gate resistors.
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Old 26th March 2020, 08:49 PM   #23
Alex79 is offline Alex79
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Hello all,
with original values of 221R gate resistor and 680pF capacitor amplifier goes into oscillation. It is nicely visible on square wave, where is heavy overshooting and ringing.

Increasing gate resistor to 680R reduces overshooting as well as ringing, but it is still there and causing oscillation. If I increase 680pF capacitor, I have nice square wave. Due to increase in gate resistor, I probably do not need to increase the value of capacitor too much. Im still looking for right value, maybe 1nF would be enough. Waiting for ordered components to test.

And yes, the mosfets are not best quality. They are heavily used IRFP244 get from some old device, as obtain NOS 244 is almost mission impossible. I can change it for 240 but first I wanted to hear the original design

So seems, this is the way to fix my oscillation. Thank you all for help, really appreciate.
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Old 26th March 2020, 09:31 PM   #24
Zen Mod is online now Zen Mod  Serbia
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ALEPH 5 Oscilating 600KHz
after second look at your pcbs ...... LTP mosfets are on one end of long pcb , while LTP CCS mosfet is on other end of pcb

pcb made like that, with that logic - no wonder that bloody thing is oscillating

buy better pcbs
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Old 27th March 2020, 01:20 PM   #25
vitalica is offline vitalica  Europe
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ALEPH 5 Oscilating 600KHz
I agree with Zen Mod. When I tested BJX, in which on the long PCB, there are 10 pairs of MOSFET, I only mounted one pair for tests. For a long time I searched for the reason for the oscillation in the speaker. I got rid of the problem only after mounting all the MOSFET.
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Old 27th March 2020, 01:33 PM   #26
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different reason, most probably ..... edit: certainly

servo circuit optimized for greater number of outputs , so with just one pair in situ , servo having excessive gain for tiny xconductance of just one pair ; when all outputs are in place, servo gain and OS gain are in equilibrium

easy to conclude after, when you did that test
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Last edited by Zen Mod; 27th March 2020 at 01:36 PM. Reason: certainly
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