Muff Fuzz gain pot

[estch71]

I built this version of the Muff Fuzz from a schematic and vero layout and wanted to put a gain knob in.  I'm guessing that it might go before the diodes?  I also switched out the output cap from 100nf to .47uf and changed the diodes from the 1N34A to 4148's.  Also, I built two of these and placed them together, taking out the volume pot from the first and running the output into the input of the second board.  Right now it has a great amount of distortion, which I like.  I just want to be able to dial back the gain from time to time.  I was also looking at installing a tone knob in it as well, most likely placing it inbetween the volume knob and output. 

Any ideas on proper gain pot placement?  Here's the schem -

[LucifersTrip]

what happens when the input resistor is increased?

[Gus]

Something I posted before at this forum  A mix of a muff fuzz and an ultra fuzz with a nice tone control
The circuit should give you a hint where to add a gain control

[estch71]

I haven't done that yet.  I just replaced the input and output caps with higher values (.22uf and .33uf).  

[joshcontact]

The 470k feedback resistor between the output of IC-A and the negative input controls the amount of signal that gets sent to the clipping circuit. Try replacing the 470k resistor with a 500k pot. Lug 1 would go to the negative input, lug 2 could connect from the output, and lug 3 would connect to the 10k coupling resistor. Methinks. 

[estch71]

Can't believe I missed that.  Thanks Josh.  I'll give it a whirl.

[joshcontact]

Course!

[estch71]

what happens when the input resistor is increased?

I'm not entirely sure what that would do but, I'm willing to try, just as long as it doesn't fry the trannies.  I could put a trim pot there to adjust the bias.  Maybe.

[Mark Hammer]

Under normal circumstances, the gain of an inverting op-amp is set by the ratio of the feedback resistance to the input resistance.  That works out to 4.7x for the first op-amp stage.  In the total absence of ANY feedback resistance in the second stage, who the hell knows what that would calculate out to?

That said, any clipping device will give you a degree of distortion based on the amplitude of the input signal.  So, if you consider the first op-amp stage AS the "input signal" to the second stage, then altering the value of the feedback resistor will serve to elicit greater and lesser amounts of clipping.  I don't know what it produces in stock form, so I'll suggest a fixed 220k resistor in series with a 500k pot (taper of your choice).  That will vary the gain of stage 1 from 2.2x to 7.2x, which should cover a wide enough range.  If not, then try a 100k fixed resistor in series with a 1M pot, for a 1x-11x range.

And, since I am a devout believer in initially holding back on harmonic content when feeding something that clips, I will suggest a 100pf cap in the feedback loop of stage 1.  That should warm it up a bit, and kill the fizz.  That, and Gus's suggested tone control should deliver a nice range of sounds.

[estch71]

This is how amateur I am.  I meant to post a pic of the transistor version and, instead, posted the IC version.  Here's the trannie version:

[Mark Hammer]

That's easier.  Here's what you can do.  I added a gain control and a simple treble-cut tone control.  Essentially, if you provide a lower-impedance path to ground from the emitter of Q2 for audio frequencies, you get more gain.  So, anything lower than 2k7 with a big-enough electrolytic cap will provide some gain.  You could use any value pot you want in parallel with R5, but you wouldn't really hear any change in gain until the emitter saw less than 2k7 into the cap. Following that logic, not very much of a 50k pot would be terribly useful.  I used a 5k pot because I couldn't get 2k pots in 2009. 

These days a better option would be to replace R5 (in the drawing) with a 5k pot.  One outside lug of that pot goes to the emitter of Q2, and the other outside lug goes to ground.  A 10-47uf cap would go from the wiper of that pot to ground.  The pot provides a constant 5k path from emitter to ground, but as you move the wiper, you change the portion of the pot resistance that lies between the emitter and cap.  So, if you turn it all the way up, you get gobs of gain, because there is zero ohms between the emitter and cap.

Since a direct connection of the cap can produce instability in the circuit, I'd suggest sticking a 100R resistor between the emitter and pot.  That way, there will never be less than 100 ohms between the emitter and cap.  Don't worry, you'll get LOTS of distortion, as well as being able to dial in something a little less agressive than a stock MUff Fuzz. 

[estch71]

Amazing.  I replaced the 2k7 with a 5k pot, as noted, and also replaced R4 with a 100k pot as well (wiper to board and bridged to lug 3 and lug 1 to board).  I didn't, however, place a 100 ohm resistor in between the emitter and the 5k pot.  What happens now is that the 5k acts like the Starve knob on the Fuzz Factory, which is kind of cool.  The 100k pot does dial back the gain, although it's muddy.  I'll try your schem and see what happens.  By the way, I'm using two of these in one enclosure (soon to be) which gives me more gain.  Both boards share the 100k volume pot.  I love this layout as it has a TON of possibilities, especially with replacing the input and output caps for higher values which adds more bass, more so on the output cap side.  I just have to watch the voltages on the trannies.

Thanks!

[Mark Hammer]

The EHX Double Muff packs a pair of these in one pedal, and is rather overlooked as far as EHX pedals go.  The circuit has them in series, with both using a 3k3 emitter resistor, instead of 2k7, for just a bit less gain from each.  However, the output of the first Muff Fuzz is used to drive the second, and its level control serves as a kind of "sensitivity" control for Muff #2.  Muff #1 also parallels the 100k feedback resistor from Q2 emitter to Q1 base with a 47pf cap.

Given the fact that the commercial pedal only has two pots (level 1, master output) and a single switch, it makes a pleasingly wide variety of tones.  Imagine what it could do if gain was variable, it included some sort of Wooly Mammoth style emitter-feedback pot, and some sort of tone control was included.......

[estch71]

I saw this.  Actually, I think my dad had one.  Mine are in series as well, although without the specs from the true DM.  I've got a bit more tweaking and debugging to do before I set it in stone, or aluminum. 

[LucifersTrip]

you also might consider putting a switch on that gain control. even at the "least gain" setting it's more  than the stock muff.

[estch71]

LT, are you talking about a switch in series with the pot or just a stand alone one?

[estch71]

That's easier.  Here's what you can do.  I added a gain control and a simple treble-cut tone control.  Essentially, if you provide a lower-impedance path to ground from the emitter of Q2 for audio frequencies, you get more gain.  So, anything lower than 2k7 with a big-enough electrolytic cap will provide some gain.  You could use any value pot you want in parallel with R5, but you wouldn't really hear any change in gain until the emitter saw less than 2k7 into the cap. Following that logic, not very much of a 50k pot would be terribly useful.  I used a 5k pot because I couldn't get 2k pots in 2009. 

These days a better option would be to replace R5 (in the drawing) with a 5k pot.  One outside lug of that pot goes to the emitter of Q2, and the other outside lug goes to ground.  A 10-47uf cap would go from the wiper of that pot to ground.  The pot provides a constant 5k path from emitter to ground, but as you move the wiper, you change the portion of the pot resistance that lies between the emitter and cap.  So, if you turn it all the way up, you get gobs of gain, because there is zero ohms between the emitter and cap.

Since a direct connection of the cap can produce instability in the circuit, I'd suggest sticking a 100R resistor between the emitter and pot.  That way, there will never be less than 100 ohms between the emitter and cap.  Don't worry, you'll get LOTS of distortion, as well as being able to dial in something a little less agressive than a stock MUff Fuzz. 

Built it and it sounds AMAZING!  Only problem is that it squeals when the 5k pot is all the way up, even with the 100 ohm resistor and the 10uf cap.  I also placed an 8k2 resistor across lugs 3 and 1 to bring it down to 3.10k.  I'm thinking of replacing the 100k resistor at R4 (R3?) for a 100k pot and leaving the 5k inside as a bias knob.  I had the 100k pot in there before as a gain knob.  I can't wait to etch this board and house it. 

[LucifersTrip]

are you talking about a switch in series with the pot or just a stand alone one?

a switch to disconnect the gain "assembly" leaving it stock. a simple spst will do. you can disconnect the cap from ground or the 100 ohm from the r4/5 junction

http://www.diystompboxes.com/smfforum/index.php?topic=77456.msg830467#msg830467

Only problem is that it squeals when the 5k pot is all the way up, even with the 100 ohm resistor and the 10uf cap.  

increase the 100 ohm until it doesn't

edit:
I checked mine...I have a 500 ohm trim pot I adjusted just to the exact spot the squeal vanished

[Mark Hammer]

edit:
I checked mine...I have a 500 ohm trim pot I adjusted just to the exact spot the squeal vanished

I second that strategy.  Alternatively, you use some sort of temporary variable resistance to identify the optimal "de-squeal" resistance, and then replace it in the final build with the closest suitable common value of fixed resistor.  So, if 343R comes up as the "magic" value, stick in 330R or 360R.

[estch71]

I'm going to test it with my multimeter tonight to see where the best resistance lies.  Thanks guys!