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Old 6th December 2003, 09:50 AM   #1
ABO is offline ABO  Netherlands
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Default oscillation problem

I'm having a problem with my power amp. It is a design from Elektor mag called IGBT power amp. Its schemetic can be found below:

http://web.tiscali.it/audiofanatic3/...20D101_201.jpg

The problem is that there is a burst of oscillation on the peaks (both positive and negative) of the output. This only happens when the output level exceeds 30W (I measured with a 5 ohm resistance). The oscillation is observed on the powers rails and also slightly on the output itself.

Does anybody have a clue as to resolve this problem?

Usually I only play at 1-2 W level, at which the oscillation is not present. Would oscillation at these higher levels affect sound quality at listening levels?

regards Abo
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Old 6th December 2003, 02:40 PM   #2
PMA is offline PMA  Europe
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No oscillations should occur in any amp, regardless the level.
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Old 6th December 2003, 02:55 PM   #3
sreten is offline sreten  United Kingdom
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cannot access the schematic
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Old 6th December 2003, 03:04 PM   #4
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I have experience the problem myself with 2SK135/2SJ50 where only K135 oscillated above a certian current value. This is caused by the behavior of the capacitances across the mosfet. The capacitances are very unlinear, a drawback of mosfets

You could increase the value of the gate resistors or insert ferrite beads. I suggest that you start to change the resistor values.
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Old 7th December 2003, 11:48 AM   #5
ABO is offline ABO  Netherlands
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I agree that there shouldn't be any oscillations whatsoever in amp amp. But that doesn't really answer my question.

Would oscillations at high power levels affect sound quality at lower levels at which the oscillation do not occur?

Strangely I don't get a hyperlink to the schematic to work. It can be found with a google search. Search for:

igbt monostato 140W.

This will yield 1 result. This is a bulleted list. The schematic is found by clicking the 10th bullet: 140 IGBT gt20D101/201.

Lex
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Old 7th December 2003, 03:45 PM   #6
PMA is offline PMA  Europe
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Yes, the oscillations do affect sound quality, even if they are only local, 30 MHz and hardly detectable, e.g.
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Old 7th December 2003, 05:16 PM   #7
sam9 is offline sam9  United States
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I've had a situation as described where oscillation seemed to "flare" a little bit on peaks. (Around 400kHz) I cured it by making a new PCB where: A: the out put section was better isolated from the small signal sections and B: taking pains to minimize parrallel traces and keep those that remained farther away from each other.

While I was working on the new PCB, I went ahead and installed the first set and used them untill the new ones were ready. Although it was reasuring to have a new set of PCBs without the oscillation, I can not honestly say that I could percieve a difference. Maybe if I had conducted very precise ABX tests I might have heard something - maybe not. I conclude while that as a matter of principle non oscillation is is to be preferred, very small amounts or parasitic oscilliation don't seem to do any audible harm. The key to this may be that it needs to be limited to frequencies where the the amps gain has begun ti roll off significantly.
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Old 7th December 2003, 05:32 PM   #8
Rudy is offline Rudy  Belgium
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there is an update on the design by elektor to prevent oscilating problems and actually the amp slightly got better alsow.

in dutch
-> Monteer parallel aan R31 een condensator van 27n ( aan de soldeerzijde )
-> Plaats tussen de collector van T8 en massa een weerstand van 20K
-> Plaats tussen de collector van T9 en massa een weerstand van 20K
-> vervang R20 door 1K8
-> vervang R17 en R18 door 390E
-> vervang R3 en R4 door 33E

Rudy / sry no time, if somebody would be so kind to translate it for the rest off the people here
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Old 7th December 2003, 06:42 PM   #9
PMA is offline PMA  Europe
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Maakt u nu een grapje?
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Old 7th December 2003, 07:26 PM   #10
ABO is offline ABO  Netherlands
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No he's not kiddin'!

I tried that very same mod. But it only made things worse. After changing the circuit, the amp was now really oscilating. I wasted a few evenings on that mod.
After I had changed everything back to how it was, I still had these parasitic oscillations. I think I need higher gate resistance. How much should I increase the values? Anyone?

Thanks
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