jubal81
Well-known member
I was looking at the Median schematic and I see a few changes that would make it perform a lot better and be easier to bias. Bonus: It would only take a couple more resistors and caps.
There's no reason you'd need matched JFETs or have to select ones with the VP and IDSS Wampler used. The 5457 isn't really ideal here, either. In this case, a FET with higher VP and IDSS would perform better as a variable resistor.
First job is fixing the feedback on the gate of the compressor JFET. Feeding back half the drain signal helps tamp down distortion, but taking it directly from the drain is not ideal. If you look at IC2.2, it's a 100% pointless buffer. Let's put it to work buffering the feedback.
Next, let's fix the JFET buffer on the source of that first JFET. We can add caps so their DC voltages don't interact, and getting rid of the trimmer on the source means we can use equal resistors for feeding back exactly half the signal.
Now, we can move that trimmer to directly bias the compressing JFET, which opens up using any JFET in there. I drew it up taking the voltage from VREF, and if you do it that way, you'd need a JFET with a VP less than 4.5V. I think ideally it'd be best to use a 5V regulator so the bias would never be impacted by input voltages and you could happily plug in an 18V supply.
That keeps the parts count almost the same. If you'e still getting distortion with humbuckers, you could add a switch to cut the signal in half going into the compression side and make it up on the mixer side.
Anyway, here's the original schematic and my drawing showing the changes.
There's no reason you'd need matched JFETs or have to select ones with the VP and IDSS Wampler used. The 5457 isn't really ideal here, either. In this case, a FET with higher VP and IDSS would perform better as a variable resistor.
First job is fixing the feedback on the gate of the compressor JFET. Feeding back half the drain signal helps tamp down distortion, but taking it directly from the drain is not ideal. If you look at IC2.2, it's a 100% pointless buffer. Let's put it to work buffering the feedback.
Next, let's fix the JFET buffer on the source of that first JFET. We can add caps so their DC voltages don't interact, and getting rid of the trimmer on the source means we can use equal resistors for feeding back exactly half the signal.
Now, we can move that trimmer to directly bias the compressing JFET, which opens up using any JFET in there. I drew it up taking the voltage from VREF, and if you do it that way, you'd need a JFET with a VP less than 4.5V. I think ideally it'd be best to use a 5V regulator so the bias would never be impacted by input voltages and you could happily plug in an 18V supply.
That keeps the parts count almost the same. If you'e still getting distortion with humbuckers, you could add a switch to cut the signal in half going into the compression side and make it up on the mixer side.
Anyway, here's the original schematic and my drawing showing the changes.
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