GSSL LF distortion mod.
« on: April 22, 2018, 01:21:56 PM »
Recently I have built a GSSL compressor.
It works fine, but I found the distortion, especially on low frequencies, a bit too high.
It is normal that during the compression of signals with a lot of LF content, distortion will occur at fast release settings.
This is because the release already sets in during the time that the waveform goes back to the zero crossing point, causing distortion.
Also I had the impression that sometimes I heard 'clicks' at the onset of compression, even with longer release times.
The modification I made to the circuit is very simple and cheap. What you need are 3 diodes and 1 capacitor.
An easy way to implement this, is to remove the wire jumper on the switch PCB (between the ratio and attack switch) and unsolder the wire connected to point 'D' of the switch PCB.
The diodes can then be tacked on the back of the switch PCB. The 0.22 uF capacitor can be soldered over the 3.3 M.ohms resistor on the main PCB.
The only disadvantage of this modification is that the release times will slightly change, although the difference isn't very big.
After the modification, the LF distortion is significantly reduced, even at fast release settings. Also the 'clicks' are gone.
Measured with fastest attack and release:

50 Hz, 10 dB compression, 10:1    Before: 3.95% THD, after modification 0.75% THD
100Hz, 10 dB compression, 10:1  Before: 2.32% THD, after modification 0.29% THD

Use at your own risk!
There is a solution for every problem!


Re: GSSL LF distortion mod.
« Reply #1 on: April 23, 2018, 12:15:19 AM »
Thanks for posting.

Would 0.1uf mess less with release times but still be effective in reducing LF distortion?


Re: GSSL LF distortion mod.
« Reply #2 on: April 23, 2018, 03:10:03 AM »
I found that the combination 0.47 uF (in the original release network) and 0.22 uF were the most effective.
Don't forget that there wil be a voltage drop over the diode(s), so that will compensate for the longer release time because of the extra 0.22 uF. But you would have to try what the influence of 0.1 uF would be.
Personally, I didn't have the feeling that the release times have changed after the modification. ('Short' is short enough for me!)
By the way: it is important that you use a 1N60 (or another 'low voltage drop' diode).
There is a solution for every problem!


Re: GSSL LF distortion mod.
« Reply #3 on: April 23, 2018, 06:57:22 AM »
... use a 1N60 (or another 'low voltage drop' diode)
Makes sense. I have a bunch of germaniums somewhere, specifically for sidechains. Will try this mod next time the unit sees the bench.
« Last Edit: April 23, 2018, 01:29:49 PM by Script »

Potato Cakes

Re: GSSL LF distortion mod.
« Reply #4 on: June 18, 2018, 11:53:12 PM »
Thanks for this info!

I have a GSSL that has been in various states over the years and I'm going to finish it up by implementing this.




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