NPN BJT Stage switchable (Buffer/Booster)?

[KarenColumbo]

Hi, guys,

After much time and experimenting I'm about to finish my take on the Big Muff Pi.
Since I want a boost switch I stumbled upon a CW BMP version with a 5th stage in front.


Is it possible at all to make this switchable to unity gain? Maybe with a 3pdt stomp switch? A typical unity gain buffer like found on the WWW has three (four) major differences:

- R4 (biasing resistor?) missing or = R3
- R5 (load resistor?) missing
- obviously R2 missing
(- Emitter resitance R6 is much higher, 3k and more)

I could get rid of R2 and just put a voltage divider there. Won't twist this anyway.

R4, R5, R6 "could" be switched in and out of the system via a 3PDT. I guess it will pop like hell, won't it?

Just trying to keep the part count low - in this case it's not very sensible, is it?

[Rob Strand]

One thing that might not be obvious is you don't want a big "bang" when you switch from gain to buffer mode.
The other issue is you don't want to affect the behaviour of the circuit too much.  (For that reason I'm not going to present a whole new circuit.)

If the resistor R1 is coupled to the previous circuit via a cap (ie. no DC on R1) then probably
the best mod is to split R1 into a series combination of R1a= 22k and R1b = 1k. 
Then switch the Cap C1 between the top side of R1a and the join of R1a and R1b using a SPDT switch.

Another method would be to switch a resistor in series with C1.
I suspect 680k would be in the ball-park.  Normally I wouldn't put resistances in series
with the signal due to noise.  In this case however the signal on the input side is quite high
and the resistor R3 is a relatively low value so you shouldn't get problems with noise.

Yet another method is to switch a 390R in series with say 47uF across R5. You will need
about 220k across the switch.   The circuit will take 30secs to stabilize after *power up*
but after that it will work fine.

The switching in all three methods is fairly simple.   All should be "bang- free" when you switch the switch.

[KarenColumbo]

Thank you Rob - trying out those! It's damn hard to not get lost in something like the BMP. I already forgot it's basically a fuzz - I hope I don't mod it to death

Btw: The circuit I attached up there is the input stage - will be fed directly from a stompbox input device, e.g. guitar.

I found this snippet here: http://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part4.html - it's the 4th schematic from the top.

In the meantime I just switched directly from a "normal" input like in every unmodded BMP to the booster input. It works if I level the additional gain pot very gingerly, but it's loud as hell. And I have an additional phase reversal.

I tried to substitute the R2 pot with two resistor dividers - one 70/20, one 20/80. The difference was enough to call it a "boost", but the rest of the circuitry (4 gain stages, active Bass & Treble and parametric Mids) didn't seem to like too much attenuation here at the very input. The notes "die out".

Maybe I'm on a very wrong track here generally with the Booster idea?

I had another booster idea - switched emitter bypass caps at stages 2 and 3 in and out. Didn't make too much difference though - I guess the transistors in a Big Muff are pretty maxed out already ...

Will try out your suggestion with the R1 divider and cap switching!

[Rob Strand]

I found this snippet here: http://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part4.html - it's the 4th schematic from the top.

OK thanks.   I didn't realize the transistor stage you wanted to switch was the first stage.
The first two mods, attenuate the input, won't work so well for that stage.  It will be unnecessarily noisy.
That last one might work.

Keeping the extra stage in-circuit means it won't be exactly the same as a 4-stage muff.
Given that  maybe you could try switching 330k or so in series with the top part of the gain control.
The first stage will clip and might change the tone a bit but given it's not the exactly the same as the
the 4-stage muff it's worth a try.   You can trim the 330k to your liking.