RuudNL
Well-known member
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!
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!