mosfet mu-amp

[~arph]

Just a little question to see if I interpret this right.

Last night I built a mu-amp and subbed the JFet's with Mosfets (bs170's). To see what this will do to the sound. This works fine, but you get a crackling sound whenever notes or chords go through it. Not just on the attack or on a certain volume level.

Am I correct to assume this is because of the constantly changing bias voltage on the bottom mosfet?

[earthtonesaudio]

The bottom transistor of the "stock" mu-amp circuit has its drain fixed at 1/2 supply by the voltage follower action of the top transistor.  The gate of the bottom transistor is "self-biased" if it's a JFET by that single resistor to ground.  MOSFETs don't self-bias, so you'd need a separate bias voltage for the gate of the bottom transistor.  Or, just tap off the existing voltage divider and bias the bottom gate that way.

[~arph]

Yes, that makes sense, but does that explain the crackling I get?  I will give your suggestion a try and see what happens.

[earthtonesaudio]

Yes, that makes sense, but does that explain the crackling I get?  I will give your suggestion a try and see what happens.

I have no clue about the crackling.  Post your results, maybe we all can learn something from it.

[slacker]

This sort of arrangement might work better for mosfets than the minibooster style http://www.diystompboxes.com/smfforum/index.php?topic=52011.0

[brett]

Hi
what's probably happening with the crackling is that on larger voltage excursions on the gate the MOSFET is turning on (about +2.5V wrt the source).  With the JFET it is on all of the time due to the self-biasing.  For a less crackly sound, I'd replace the pull-down resistor on the gate with a red LED (or two), forward biased (negative towards ground, positive towards the gate), in series with a large value resistor (e.g. 100k to 470k).  Or check out the gate biasing on any mosfet booster.  That way, the MOSFET will be "on".

cheers