increasing saturation with a front end boost on a Fuzz Face

[mordechai]

I did a search on this but the results didn't provide enough details, so I'd like your thoughts...

I have a friend who likes his germanium fuzz face (NPN), but would like it to saturate better.  He doesn't want a silicon because he likes the germanium tone, but also doesn't want something like a Tonebender MK II, which is too angry sounding for him.  Finally, he also wants to be able to control the additional saturation so he can go from "normal" fuzz face tone to "heavy" fuzz face tone, but still have it sound relatively uncompressed, the way a fuzz face should.

I thought of two...well, three, options:

1.  Increase the bypass cap on the fuzz pot, and use an additional pot to "blend" between the stock 22uF on one end and, say, a 100uF on the other.  The problem, I think, is that this might change the frequency response and have the sound get "fat" while also getting more saturated, and I don't think he wants that.

2.  Put an LPB-1 type boost in front of the circuit and let him control it with an external pot.  Maybe tweak the values of the in/out caps on the boost stage so that it keeps things fairly consistent tone-wise with the fuzz face stage.

3.  Put a SHO-type boost on the front end to do the same thing as option 2, and tweak the in/out caps the same way, or maybe just the output cap...taking it down to 4.7uF, maybe, to keep from too much bass hitting the fuzz stage.

His fuzz face circuit has fairly high gain transistors (120/300 in Q1 and Q2, relatively low leakeage), a 1UF input cap, and a 100KA volume pot.  No other changes from the stock circuit or its values.  Which of these options do you think will provide the best solution?

[Gus]

Look at the front end of the Hot Si it has a selected gain output resistance and input resistance.

Now any time you put a buffer/gain stage if front of a FF you can remove the interaction the FF has with the guitar/bass, cable

However you will note a gain stage like the BMP the first three stages have C to B feedback

Design a gain stage with the correct amount of gain and interaction with the guitar

[mordechai]

Well, I figured a boost stage might do that (interfere with the guitar volume control), which is why I thought going with something like a SHO might be the best option.  Whenever I've played through one going into a fuzz, it seems to still play well with the guitar controls.  But I think it would be helpful in this case to modify not only the in/out caps but also the 10M resistors, bringing them down to a lower value to A) decrease a bit of noise and B) decrease the "sparkle" so that the resulting signal is a bit more neutral going into the fuzz.  I also don't think this stage needs to push TOO hard going into a fuzz already at full throttle...so perhaps bring down the drain resistor going to the power rail from 5.1K to 4.7K...?

[Gus]

I don't think you understood my post.  First question why do want a SHO? in this application it make no sense to me.

Read up on setting the gain, input resistance, output resistance of different BJT transistor gain stages.

I gave you a hint study the gain stage that you see in the the first three stages of a BMP.  Understand how it works and then design/sim a gain stage that gives you the gain you want and the interaction with the guitar/bass cable you want

[Ben Lyman]

I made a FF with lpb-1 at the front, a tone knob and clipping diodes at the back. It took me 3 pages of stupid questions but it came out great:
http://www.diystompboxes.com/smfforum/index.php?topic=112558.msg1039113#msg1039113
Here's some pics and vids of it:
http://www.diystompboxes.com/smfforum/index.php?topic=112915.msg1043618;topicseen#msg1043618

[Gus]

Ben
I am trying to help give mordechai ideas for what I think they want.  Cut and paste is an OK way to learn building a fuzz.  Kind of hard to mess up a fuzz if it passes signal and distorts the waveforms a lot, it passes as a fuzz.

mordechai
I read your first post and posted what I would do if someone asked to build a boost for the FF for what your friend wants.

I would use a predictable Silicon NPN first stage.

In the first stage of the BMP the resistor from C to B is part of the DC biasing and it is also feedback.  The first stage of the BMP has a 30somethingK in series at the input if you think about this you should understand if you remove/reduce its value(I would try a 100 ohm resistor).  A PART of the gain is set by the feedback(C to B) resistor divided by the guitar LRC(s)

Now note FF type circuits have a range of feedback/part of the bias resistor values  100K is kind of common
So that is a hint for setting up the first boost stage(like the first stage in a BMP) to kind of interact the same way with the guitar that a FF does when the gain is not set to max

However this changes the output resistance so you might want to add a series output resistor between the boost and FF type circuit

It is going to be a compromise

A fast way to get there would be to use something like LT spice

FWIW adding a stage to the front of a FF like circuit is the reason for the values I selected for the first stage that you find in the Hot Si. I selected the gain I wanted the input resistance I wanted and the output resistance I wanted

[mordechai]

Gus, thank you.  I think I understand now what you mean: create a boost stage akin to the first stage of the BMP, but bring down the input resistor from 33K to much lower (I have some 150R's, so I'll try those), and use a 100K (instead of the 470K on the usual Ram's Head schematics) as a feedback resistor between B and C of Q1.  With this, the input impedance will be more conducive for sensitivity between the fuzz and the guitar volume control.  Do I have that right?  In terms of the output resistor between the boost stage and the front end of the fuzz...I am guessing a small-ish value resistor...less than 10K?  But I am having trouble finding your Hot Silicon schematic, and I'd like to consult it.  The search feature here is giving me far too many hits and it's difficult to sort through them.  Could you direct me to the schematic?

[Gus]

http://www.aronnelson.com/gallery/main.php/v/DougH/fuzz/hotsi/GS_Tonebender.gif.html

Doug adjusted and added a fourth stage to A Si fuzz circuit of mine and IIRC used a tone control Aron used in another fuzz

If you drop the feedback from 470K to 100K (BMP first stage)you will need to adjust the B to ground resistor So you might want try the stock 470K with a grounded emitter or even down 220K C to B and adjust the B to ground.

[mordechai]

Gus, I just want to make absolutely certain I understand...when you say "first stage" of the BMP, you mean the stage right after the 33K(ish) input resistor and before the first clipping stage, right?  The reason I ask is because some descriptions of the BMP "first" stage (i.e., the Q1 in a schematic) actually relates to what I see as the LAST stage...that is, the volume recovery stage following the tone stack.  Is THAT the stage you mean (since that is essentially an LPB) or do you mean the initial stage following the input resistor?

[Gus]

First stage is the input stage after the 33k.

Things to think about with a boost before a FF
Input resistance LPB type circuit
First stage C to B feedback with passive guitar bass interaction in BMP like first stage
Gain
output resistance

IMO no need for a JFET or small area MOSFET at the boost stage because you often don't need a high input resistance for a boost before a FF like circuit.
I distortions, fuzzes, overdrives I like to use NPN and PNP Silicon devices