Too much gain out of this FET preamp, how do I reduce?

[exabrial]

This thing sounds pretty good! Except even with the gain pot turned all the way down, it's producing very hot audio. How do I modify the circuit to produce less gain? I was thinking I need to reduce the value of r7, but since this is a SMT circuit, I was wondering if I could add a resistor that bridges the output to save soldering... thoughts?

http://www.cafewalter.com/cafewalter/fetpre/graphics/pzp1-sch.pdf

[PBE6]

I assume that R9 on the output is being used as a tapering resistor, so it should probably stay in place. Try bypassing R7 with a jumper. When the pot is fully counterclockwise the output will be grounded, and when it's fully clockwise the output will be 100k/104.7k = 96% of the total gain stage output.

[snap]

the question is: you want less gain from the circuit, or less volume?

[Keppy]

I assume that R9 on the output is being used as a tapering resistor, so it should probably stay in place. Try bypassing R7 with a jumper. When the pot is fully counterclockwise the output will be grounded, and when it's fully clockwise the output will be 100k/104.7k = 96% of the total gain stage output.

This is probably the simplest thing to do, though R7 affects the taper as well. The control wasn't meant for a large range of volume, barely 10dB if my math is good. The schematic calls it a buffer, and it appears to be a follower, which wouldn't have significant voltage gain, though I'm not sure of the effect of the lower JFET. I'm surprised there's a problem with too much gain.

[exabrial]

Haha, I suppose I mean less Volume on the output. And yes, circuit is a buffer/follower, technically not a preamp.

I was quite surprised how hot the audio come off of it is too.

[exabrial]

I'll try the "short r7" method and see how it does. Thanks!

[PRR]

It doesn't have any voltage gain.

_I_ say rip-off R9, short R7. Gain will run smoothly from 0.95 to zero (no output). If you really want low gain, like 0.1, then make R8 an Audio taper.

> not sure of the effect of the lower JFET

Just a fancy replacement for a source resistor.

[exabrial]

Hey guys, thanks for the awesome advice. Shorting R7 worked perfectly. I can now set the pot about 30-40% and the guitar isn't overloading the downstream electronics.

Victory

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