MP-1: Improving ripple distortion in compressor.

[Kazinator]

This is not a new issue in my MP-1. On the SS clean channel, when steep compression is used, there can be noticeable distortion. It's most noticeable if you play a double stop (e.g. bluesy bend on G and B strings) with tone rolled down. It might be the ripple distortion that the data sheet for the NE572 talks about, which can be mitigated with the attack and release capacitors.

Twelve years ago I was looking into improving the SS sound, and I ended up swapping in a new SE572 from ON Semi. According to notes I wrote in 2012, I noted an improvement in sound quality: the SS clean no longer sounded "darker" compared to tube clean; when I set up a tube and SS clean with identical EQ and switched between them, they blended better.

I was really happy with the chip swap; just that bit of distortion was still there. I remember noticing that at that time, due to my attention being riveted on the SS clean, and probing the capabilities of the compressor.

Now, I'm looking at that compressor again and have noticed something peculiar in the schematics.

The 572 has two channels, and the MP-1 uses both. So there should be two attack caps, and two release caps. The schematic, though, shows only three caps! Nothing is shown for pin 14.

I strongly suspect that the pin 14 capacitor should be C25. Indeed, I cannot find a C25, which can be expected to be in that same schematic area.

For the first channel, the caps on pins 2 and 4 are labeled C22 and C22. Then for the second channel, which is shown in the schematic as if it were a separate device (separate box), there is a C24 on pin 12. Pin 14 does not appear. Then caps C26 and C27 are found in the compressor's input filter network.

Is this something known? Before I start taking a look at the device and following traces, has anyone spotted this before? Is there a 4.7 uF cap on pin 14, like (the schematic says) there is on pin 2? Or other value?

[Kazinator]

Aha! The component layout shows a C25 near U7.

In other words: