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Old 3rd August 2014, 04:08 PM   #1
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Default Help needed - oscillation found in PA stage

Hi all,
A bass amp (Ampeg BA-115HPT, schema is included) from a friend of mine is having following issue: no output but only wierd/cracking noises. Assuming the amp has oscillation, I scoped the output and found square-wave-similar signal at about 2-30kHz/0.3Vrms.
The issue is later determined in the PA stage since I found oscillations (same freq, but triangle) at the output of the opamp NE5532. I tried replacing all the components around this opamp (include itself), also tried to power only the opamp by my +/-16VDC bench PSU, but the oscillation remained.
Later, I found that by removing the output MOSFETs (or their 0.33R Drain resistors), the oscillation dissapears. Also, if only a pair of output MOSFETs is removed then the oscillation frequency is halved, so the problem might relate to the input capacitance of these devices. But then again, replacing the MOSFETs (also the drivers MJE340/350) didnt solve the issue.
Since I'm no expert in SS field, now I'm stuck
Please let me know if you have any idea what's going on
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Old 6th August 2014, 05:10 PM   #2
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Default Help, please?

I'm still stuck, any help please?
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File Type: png Ampeg ba115hpt PAstage.png (91.0 KB, 43 views)
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Old 6th August 2014, 06:13 PM   #3
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This is a bit of a weird topology.

So there is no appreciable DC voltage on either U3:B output or amplifier output? Does an input signal (e.g. from CD Input) make it to the output of U3:A at least? Low-level oscillation per se does not totally prevent an amplifier from working, so you may actually have two problems.

So let's assume these check out OK. In your shoes, I would probably make sure that R42/R55 are on +/-16VA, verify that R39 / C26 are OK and properly connect to power ground, measure voltages across R41 and R43 (I would expect maybe 3 V each, and maybe 4 V over R58 / R67), and swap out C46 and C29 for good-quality new ones. I know it's very hard to check bias under oscillation, but is it no lower than specified at least?
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Old 6th August 2014, 07:42 PM   #4
PChi is offline PChi  United Kingdom
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In addition to the previous post.

My ideas are that I would expect the power amplifier on it's own to oscillate a higher frequency.
Could the oscillation be via the photo coupler from the output to the first stage?
You could disconnect D11 and D14 to cut the feedbak loop to isolate it.

The power amplifier itself may be very touchy about capacitance and layout. The operational amplifier gain is being boosted by the voltage gain of the output stage common source connected MOSFETs and then being connected back to U3B.
It may be worth isolating the power amplifier stage by lifting the end of R38 connected to U3-A and connecting it to 0 V and re-checking for oscillation.
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Old Today, 05:23 PM   #5
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Hi,
Today I checked again the amp (been busy lately).
I followed your suggestions and found that only lifting R38 (as PChi pointed out) can remove the oscillation. So can I assume oscillation starts somewhere around U3A?
I also try tinkering with the feedback RC pair C23//R51, looks like raising C23 can help taming the oscillation, but frequency decreases toward audio band as well. Attached is some visuals taken from my scope (channel 1/blue is amp output, channel 2/yellow is U3A output) with different values of C23.
Changing value of C35 on U3A feedback does similar thing, but not as effective though.
- 1st photo: original trace (C23 = 33pF)
- 2nd: adding another 30pF to C23
- 3rd: adding a 330pF to C23
- 4th: adding a 1nF to C23
- 5th: adding a 1nF to C35 of U3A, C23 is kept as original (33pF)
Any other suggestion?
Thanks,
Duong
Attached Images
File Type: jpg MAP002.jpg (63.6 KB, 11 views)
File Type: jpg MAP003.jpg (65.1 KB, 10 views)
File Type: jpg MAP004.jpg (69.1 KB, 10 views)
File Type: jpg MAP005.jpg (65.8 KB, 10 views)
File Type: jpg MAP006.jpg (66.2 KB, 3 views)

Last edited by hoangduongo; Today at 05:32 PM.
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