Can anybody help me? This amp oscillate at 10MHz? - diyAudio
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Old 5th March 2013, 05:01 AM   #1
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Default Can anybody help me? This amp oscillate at 10MHz?

Hello,
just in yesterday I have built and tested Lazy Cat’s latest symmetrical topology with variation of n-most only output. The amp has almost all aspects meet my expectation, except the S/R and oscillation.. I initially used 22pF Miller cap. And my first pair of IRF530 stayed for couple of minutes before it broke down silently, together with one driver B649AC. And then after I replaced the B649, I also increase the Miller cap to 50pF. Even though, another two pairs of IRF530 did not sustain for couple of seconds after power on. I finally increase the Miller cap to 120pF, this time the amp starts to sing. My PSU is +/-35v. It can give a 28Vp swing, which is not bad. However, when I view the waveform, I see an high frequency oscillation at around 10MHz. This oscillation appear only to the negative circle. Can anybody tell me how to fix this problem? As you see that the slew rate is already low, maybe increasing the Miller cap is not a good cure. Can anybody give me a hint?

PS. Oscilloscope images are attached. You can see in that the amplified triangular waveform there is an oscillation at about 10MHz.

Click the image to open in full size.

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Attached Images
File Type: gif JC2013VSSA.GIF (24.2 KB, 253 views)
File Type: jpg JC2013VSSAwithIRF530.jpg (631.5 KB, 258 views)
File Type: jpg SNC01911.jpg (226.4 KB, 261 views)
File Type: jpg SNC01920.jpg (204.6 KB, 259 views)
File Type: jpg SNC01926.jpg (212.3 KB, 260 views)
File Type: jpg SNC01919.jpg (209.0 KB, 261 views)

Last edited by jazzclassics; 5th March 2013 at 05:05 AM. Reason: put image in post.
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Old 5th March 2013, 05:08 AM   #2
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oops, still missed on image. Attach again.

This is the amplified triangular waveform with oscillation.
SNC01912.jpg
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Old 5th March 2013, 05:25 AM   #3
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First of all: Keep the wires to the output fets MUCH, MUCH shorter!
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Old 5th March 2013, 05:53 AM   #4
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the gate-stoppers (100R) should be as close to the gates as possible.
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Old 5th March 2013, 07:47 AM   #5
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Quote:
Originally Posted by peranders View Post
First of all: Keep the wires to the output fets MUCH, MUCH shorter!
I bet that is the reason. Whoever tested and designed it got much shorter wires.
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Old 5th March 2013, 10:19 AM   #6
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Quote:
Originally Posted by peranders View Post
First of all: Keep the wires to the output fets MUCH, MUCH shorter!
Thanks for your idea. Currently those wires are 13cm in length.
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Old 5th March 2013, 10:21 AM   #7
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Quote:
Originally Posted by djmiddelkoop View Post
the gate-stoppers (100R) should be as close to the gates as possible.
Thanks a lot for your tips. I will try it and report back later.
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Old 5th March 2013, 10:22 AM   #8
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Quote:
Originally Posted by djmiddelkoop View Post
the gate-stoppers (100R) should be as close to the gates as possible.
Thanks a lot for your tips. I will try it and report back later. .. Any more ideas?
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Old 5th March 2013, 10:25 AM   #9
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If still oscillation after you have attached gate resistors close to Gate,
then you can try 47pF for C3 and C5. (They are now 20pF.)
This will increase compensation and stability.
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Old 5th March 2013, 10:32 AM   #10
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I have seen such long connection wires as shown in the following SSA photo, but claimed to have no problem in hi-freq response neither issue with phase shift. Is there any other possibility with the amp that caused the oscillation?

SSA10.jpg

SSA1.JPG

SSA 2 MHz Phase Shift.JPG
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