Clean Boost - Transistor Biasing

[mikelambresi]

Hey everyone,

Lately I've been researching and experimenting with breadboarding clean boost circuits, and would be great to pick up your brains on the matter.
I'm trying to build a transistor based clean boost, to use mainly as an always-on clean enhancer.
My knowledge is still very basic on the matter, so while studying different circuits it raised a couple of questions.


- I'm curious: where do you guys bias a transistor on a boost circuit?
Do you bias at half voltage or by ear on the loudest point?

- If the gain of the circuit is set by RC / RE , on a simple fet boost circuit like this but with no bypass cap, would the combination of 10K/1K or 20K/2K have the same gain?

- What's your favourite configuration for a volume knob on a boost pedal and why: input, emitter, collector or output?

- On a common emitter circuit, which one is the cleanest and less colourful out of these 3: J201, 2n5457, BS170.

Thanks a lot in advance.

Cheers!

[antonis]

Just before Sir Mike comes with his MosFet booster proposal..

- "Loudness" has nothing to do with bias point..

- R_D/R_S (not R_C/R_E 'cause it's an FET..) ratio sets gain but R_S value sets Drain current..
(V_GS/R_S)

- Volume pots ALWAYS are set as in the circuit posted..
(on any other placement, they aren't VOL pots anymore - maybe Gain pots or Bias pots..)

- Try the one which pleases your ear..

[niektb]

Let me point you to this blog about the Fetzer Valve, should answer your questions I think 
http://www.runoffgroove.com/fetzervalve.html

In my PEAQ booster (you can find it around here on this forum) I use a mini-booster config which is self-biasing (the Rd resistor is replaced with another FET) and a Rs resistor tuned to give maximum gain. The volume pot I put in front of the JFET (R1 in your schematic) as the JFET can get quite dirty, not very 'clean' boost if you ask me

I used the 2N5457 because it has more input headroom (so it's less overdriven than the J201), it has however less gain. The BS170 can't be used in this topology! It is a MOSFET, not a JFET!

But maybe the Fetzer Valve Deluxe already satisfies your needs

[antonis]

Alternatively, you can try a BJT Booster (LPB1 style..) to take advantage of bias easiness & predictability with higher transconductance & better linearity alongside benefits.. 

[PRR]

> which one is the cleanest and less colourful

TL072