Stetsom EX 8k crackle in speakers

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I have another one of these Stetsom amps, this one has a crackle sound in the speakers. With music or without music. With RCA plugged in or without.

I have replaced the caps around the speaker terminals on the board, the driver ICs, and the output transistors and I have pulled the inductors from the board and checked them and unwound them a little and found no issues with them so I reinstalled them. I have pulled the rail caps and checked them out of circuit and they all pass. I currently only have two of them reinstalled in the board.

I noticed that sometimes, but not all the time, when I probe pin 1 of the tl072 that is driven by pin7 of the PIC16 IC that the triangle wave that is present will sometimes turn into a diamond shaped wave and the amp will soon after that pop loudly in the speaker and go into protection. If no speaker is connected when I do this the will give off a slightly audible buzz sound. When I first heard this noise I thought one of the plastic fan blades in the amp was hitting a wire but that wasn’t the case, but that’s what it sounded like.

I have good drive at the output transistors but I do notice an anomaly on the left side which feeds the negative speaker terminal. I don’t see a drive wave at the speaker terminals even though the amp will play music even though it has the crackle in it. I’ll post pictures of the drives of the two sides.

Do any of you have any ideas, because I’m fresh out.

Drive on the negative side with the anomaly. Probe is 10x. Scope is set to 5v per division at 5us.

It didn’t show up well in the photo but the flat line signal with the small spikes in it shift phase and are sometimes blurry for brief seconds on a regular cycle. Like the scope can’t lock in on the signal. But the side is steady and stable.


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Here is a picture of the audio and drive circuitry. The section circled with a highlighter is the drive circuit. The section to the right of that is the audio section. Basically a chain of tl072s and some pots. There is a push button switch at the bottom of the photo next to the speaker terminals that is labeled over clip. According to Stetsom literature it provides 15 percent power boost over clipping.

The drive circuit is the PIC16 then two 072s and a 393. After the 393 is another 072 that is connected to the overclip push button.


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Not all of the op-amps are drive circuit. The drive circuit is likely starts just a few ICs before the 4011.

Where do you see the start of the injection of a square wave from the DSP IC?

The square wave is likely converted to a triangle waveform and the triangle wave and the audio are fed into an op-amp that drives the 4011.

The 4k2 diagram on the type 5 page may give you an idea of what you have. If you can't see the noise on your scope, you may need an audio tracer to see where it's entering the signal chain.
This is pin 1 of the first 072 (U1) that is connected to pin 7 of the PIC16. I’m not sure if the PIC16 is concerned about noise.

U3 which is an 072 has some noise on the outputs very similar to the 393 but U1 has the most noise.

The crackle noise is present at the speaker regardless if audio is fed into the amp or not. I’ll check the other inputs. No audio is being fed into the amp at this time. Just so we’re on the same page.


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