FET Buffered MOSFET Boost

[Bill Mountain]

I wanted to share with you guys a simple MOSFET boost I made for a friend.  I gave him a SHO clone I built and wasn't using and he really dug it.  I always prefer a dirtier boost pedal.  I have since been trying out different circuit ideas on him and this is the first one that he like'd as much or maybe more.

The idea was to basically keep the punch of a normal MOSFET boost but I wanted a dirtier sound.  The drain resistor is high because I started out messing with Ge transistors and stuck a BS170 in there on a whim and it worked out really well.

I added the buffer because, I'm assuming due to the biasing I chose for the MOSFET, the impedance dropped dramatically at high boost setting and clarity suffered.  This boost is for leads and such so a dark tone wouldn't suit my friend.  It was fine on bass though!  It was trivial to add the buffer on so it was worth losing some of the simplicity for better functionality.

I've been brainstorming some more tweaks but haven't settled on anything yet so I'd love some suggestions.  Maybe a defeatable EQ of some kind.

Thanks for looking!

Edit:  Change the Attachment from a PDF to a JPEG for ease of use.

[Gus]

input resistance is (1meg || 1meg || 10meg) under 500K
If you want that you could use a high beta Silicon NPN make the bottom resistor a 1.2meg

You should not need the protection diode at the mosfet the JFET should protect the mosfet.  You could use a Silicon NPN stage there

How does a MOSFET boost have more punch than another simple gain stage like a Silicon NPN?

[Bill Mountain]

input resistance is (1meg || 1meg || 10meg) under 500K
If you want that you could use a high beta Silicon NPN make the bottom resistor a 1.2meg

You should not need the protection diode at the mosfet the JFET should protect the mosfet.  You could use a Silicon NPN stage there

How does a MOSFET boost have more punch than another simple gain stage like a Silicon NPN?

I agree the impedance is low.  I calculated it was around 470k.  This seemed high enough for this circuit.  Thanks for the 1.2M tip I was going to sim it at some point but  guess I don't need to now!

Never said it had more punch than an NPN.  I was trying to match the sound and feel of a similar pedal but with more distortion.

I agree about the zener possibly being excessive.  RG preaches using one at all times.  I'm encouraged to hear you say I don't need it.

This circuit also worked well with a Ge NPN with no changes.  So I bet a high gain Si could sound really good too.

Thanks!

[PRR]

Being a JFET, the gate resistors could both be 10Meg. No modern JFET has enough gate leakage to be a problem with a node that high-Z.

Agree that 500K is generally ample.

[Gus]

I think a two silicon NPN circuit could sound good.
I would use a Jfet at above 1meg and up to 1gig

[Gus]

Bill a DC sim screenshot for some ideas.

I don't like JFETs and MOSFETs as much as BJT for certain circuits. JFETs can be all over the place and with MOSFETs you need to protect the gate insulation.
Two different input buffers one bootstrapped for higher input resistance the other a standard 4 resistor bias((hfe*(10k || the input loading of the 2nd stage) || (910k ||1.5meg) input resistance)
You can ground the 2nd stage emitter
Note the offset in the EF voltage dividers to take into account the Vbe drop that can matter at lower supply voltages like 9VDC
The bias in the 2nd stage can be predictable with higher beta transistors and I like to use high beta transistors for emitter followers
Set the 2nd stage gain for a passive bass/hot guitar humbucker first try

[Bill Mountain]

Thanks Gus!

Looks really neat and clean.  Could be a good input stage/boost to almost any kind of circuit.