help with Mosfet booster mod for internal guitar use

[Johnny G]

hi, basically i want to build the mosfet booster and use it inside my guitar with a knob on the front to control the boost.

thing is id like to use the original pots from the guitar and they're all 500K pots. to get around this i came up with a different way of hooking up the gain pot in the mosfet booster.
instead of having it in paralel with the resistor from source to ground ive got it in series with the drain resistor. so that i wont only have a small amount of the pot turning distance before it goes to high gain ive changed the source and drain resistors to 50K

heres a picture for anybody who didnt follow that.




what i really wantto know is whether this circuit will work ok. i realise that by changing the pot for the gain id be changing the DC current flowing through the mosfet. would this have any serioues effect on it


many thanks for any help given??[/img]

[petemoore]

anyway
 maybe use a 5k trimpot on the mosfet booster board, to preset/adjust gain, then run  the output through your guitar's pot.
 Try a resistor across the outside lugs of your 500k pot to reduce it's value if you like,
 Also a small high end bypass cap [.0047uf?...<?] from wiper to signal input on the volume pot might be desirable.

[gez]

Not too sure why you've got a pot at the drain, or why you've used such large value for it and the source & drain resistors (will be more prone to loading and probably noisy).  If you have a look at the EA trem input you'll see a trimpot for the bias (replaces the divider), it's an easier way to bias these circuits (you can always wire in some resitors later once you've measured resistance on the pot).

To control gain at the input you can replace that 10M tied to the gate with a pot.  Outer lug to input cap, wiper to gate, other outer lug to centre of divider/wiper of trimpot if used for divider.

[Transmogrifox]

What you had there likely won't boost much unless you were driving a 10 to 20 Meg load.  Another problem is that the 10 Meg resistor works to generate a good deal of noise...added to that, a MOSFET does have a certain amount of gate current and I have found that 4.7 Meg is about the highest I'll go. I whipped up a drawing for you.  It's a good booster that's not very sensitive to different components or power supply, yet doesn't drain the battery badly. It makes a nice overdrive if driven by a buffer or low impedance load on maximum pot setting.  I haven't driven it without a buffer yet to see if it overdrives, but it will boost for sure.  You may want to tweak the 330k resistor if the voltage at the drain isn't between 4 and 5 volts. If used for an overdrive, the assymetry may be desirable, but for a booster you like to have as much clean headroom as possible.



http://www.geocities.com/transmogrifox/Mosbooster.JPG

I still haven't figured out how to get the image to appear, but the link will get you there. Just remember to copy and paste it to a different browser.

If you like that MOSFET booster you showed above, do what gez said.  Take that 500k pot out of the drain.  Use your 500k pot in place of the 10Meg resistor (this will reduce noise too), and connect your guitar signal through the wiper with a, say, .1uF cap, and you may want to add a cap between the wiper and input for fun.

[Johnny G]

brilliant. thanks for the help all of you. i should be able to mock up a PCBthisafternoon and get the thing working so i'll tell you how it all goes

cheers

JG

[petemoore]

I think I fell out of the Boat !!!

[Transmogrifox]

I think I fell out of the Boat !!!

Swimming is fun, too :lol: LOL