Median bias voltage recommendation?

[huntest]

What is the recommended bias voltage setting for a stock build Median Compressor?

 

[micmac]

Hello huntest,

It's the same as with an Orange Squeezer. Q1 is used as a variable resistor for the compressor. The bias totally depends on the JFET you use. For it to work properly as a variable resistor, it needs to be put into the linear region, meaning it needs to be barely "on" when there's no audio signal. The more audio signal goes into it, the more "on" it will go.

If it's biased so it's off when it's idle, then it's no longer in the linear region. So that's the wrong bias then.

George Giblet provided a way to bias the Orange Squeezer, see this link. It's easy to follow.

Alternatively, if you know Vgs,off from your JFET, you could just measure the voltage between gate and source of the JFET and make sure it's barely on when idle (no audio signal). So for example, if your Vgs,off is -1,50V, then use the trim pot to adjust the voltage between gate and source to -1,49V while no audio signal goes to the JFET. Then it's just barely "on" and the Median is biased properly.

 

[marusz]

Hello everyone,
In the idle state without an audio signal, could the voltage between the gate and source of transistor Q1 depend on the position of the attack and release potentiometers? I measured the Vgs off for my JFET, but I'm having trouble setting it with potentiometer TR1. While I can adjust the voltage drop from 0 to 5 V directly on the TR1 itself, the Vgs of Q1 fluctuates randomly, depending on the TR1 rotation, but I can't reach the target Vgs off+0.01 V. What voltage should be at the gate of Q1 relative to ground in the idle state? Should it be fixed around Vref?
The rest of the circuit seems to be working properly. The input and output buffers, the blend control, the compression gain, the tone control, and the current source on Q2 seems to be OK. The section from the output of IC3.1 through the attack and release potentiometers to the input of Q1 is suspect. I don't get this part of the circuit and don't know what to expect there.
I have built the circuit on a perforated board. So I may have some wiring errors. Premanufactured PCB was too big for my enclosure and the shipping cost for my country was too high.