# (Trying to) Kill Hiss, Oscillation and RF in Vintage Fuzz Circuits (MK II, Supa, Fuzz Face, etc.)...



## Big Monk (Sep 27, 2021)

Conscious of not hijacking the MK I Tone Vendor thread, I thought it might be a good idea to write a quick post about my methods for killing sonic artifacts in Fuzz pedals.


Firstly, let me copy over my MK II/Supa comments over:



Big Monk said:


> For the MK II, I’ll usually start with a rough draft set of 100 pf, 82 pf and 82 pf for Q1, Q2 and Q3, respectively. It’s important to listen and mentally log the difference between this initial set and the sound with no caps installed.
> 
> With the Attack pot all the way up and volume at unity, increase the caps until the pedal is pretty quiet. Don’t try to find balance between noise and treble content. Kill the noise completely first.
> 
> ...





Matopotato said:


> I understand that the values differ between MKI and MKII, but would the principle be worth applying to MKI or is it so inherent already with MKI build that trying to silence it is a somewhat wasted effort? I mean I have some good tips on how to bring the noise down, bit perhaps I  should not hope on getting it as quiet as with your MKIIs?





Big Monk said:


> You could do the same thing. I’m just not as well versed in the circuit architecture of the MK I so I can’t say how changing frequency shaping components will affect the overall tone.
> 
> You can absolutely use what I described to clamp down on noise though. That is circuit independent.



The Fuzz Face can use roughly this template as well, but requires a few initial tweaks before proceeding to the base to collector caps. There are 2 issues that the Fuzz Face suffers from that the MK II/Supa (and the MK I, by extension) does not: RF Interference and High Frequency Oscillation.

The oscillation is really the first order of business in a Fuzz Face. I've seen many, many solutions proposed to handle this but the caveat is always that, "It may or may not work for you...".

One method I have found that works like gangbusters is to audition caps that bypass the feedback resistor. You'll want to start with something low like 220 pf and work your way up. Do not be alarmed if the caps get relatively HUGE. My recent Silicon Fuzz Face used a 4700 pF! Find a cap that gets rid of the majority of the oscillation without affecting the tone of the circuit too much.

With respect to RF, and also the last bit of the oscillation, resistance on the input can help. However, even a small reduction in signal from full up can start to neuter the Fuzz Face tone. 

I use a "Pre-Gain" pot on the input of all my MK/Supa and Fuzz Face units. Besides letting me reduce the gain of the circuit for different shades of distortion, turning it just ever so slightly back from full often exposes a "notch" that effectively kills RF and the last of the oscillation. Of course, what is happening here is that on full, the "Pre-Gain" offers no impedance to the front of the circuit, so reducing it slightly is like putting a very small fixed resistance in series with the input. In lieu of a "Pre-Gain" control, simply audition very small resistances, starting with say 47 ohm and work your way up until RF and Oscillation disappears but the tone of the circuit is not affected greatly. 

With that said, RF may be impossible to completely eliminate. I've tried all my methods and on a bad day i still get the local country station through my speakers. It's faint, but there.

Now you can attack hiss with the same method for the MK II/Supa, although start with lower values for the Fuzz Face than you did with the MK II/Supa. I would start with 82 pF for Q1 and 51 pf for Q2. You'll find that by bypassing the feedback resistor in the Fuzz Face, you need less capacitance across the base and collector of each transistor as opposed to the MK/II/Supa.


----------



## Big Monk (Sep 27, 2021)

A quick real world example:

I recently built a Silicon Fuzz Face on my Universal board. My benchmark was the "Time" solo from DSotM and the raging solos from "Echoes" off of Live at Pompeii.

I started out with a set of Amperex A115/A104 transistors with gains of 120/285, respectively. This had a nice sound but lacked the overall girth and sizzle that I was aiming for. From the outset, this set had major oscillation issues so i decided to get my transistors right before proceeding. I ended up using a BC109B/Amperex A104 combo with gains of 286/405, respectively.

First order of business was killing the majority of the oscillation so I could actually hear what it sou8nded like!

I started with a 470 pf ceramic across the 100k feedback resistor. I ended up tweaking that up to 4700 pf. Oscillation was mostly gone but the high end had been reduced. I still had some minor hiss so I put an 82 pf across B-C of Q1 and a 47 pf across B-C of Q2. That killed the rest of the hiss. 

I have a 500kA volume pot on this unit. In order to restore the right amount of high end, I immediately dropped the emitter cap from 22 uf to 10 uf. This helped greatly, although I did not want the pedal to fart out or get flabby when playing power chords so I also dropped the input cap from 2.2 uf to 1 uf. That did the trick.

Now here is where you have to use your judgement. I use a 100kA "Pre-Gain" on the input of this pedal. With the "Pre-Gain" all the way up, there is a slight trace of whistling oscillation and RF interference is more active. I will typically turn the "Pre-Gain" down so slightly it's almost not perceptible and listen for when this last bit goes away. For those using fixed resistors, tack in a 100k pot and find this "sweet spot" on the dial. Remove the pot and measure the small resistance that killed the rest of the oscillation/RF and place that value in series with the input.


----------



## BuddytheReow (Sep 27, 2021)

Dude, you should write a book on this!


----------



## Big Monk (Sep 27, 2021)

BuddytheReow said:


> Dude, you should write a book on this!



I'm just repeating things others have told me and passing it on!


----------



## Dan0h (Sep 27, 2021)

This is why I spend so many hours on this forum. There are so many awesome nuggets of wisdom. Thank you for sharing.


----------



## Feral Feline (Sep 28, 2021)

Thanks Big Monk for sharing the info aggregation — such amassed knowledge will surely reduce aggravation.


----------



## andare (Jun 22, 2022)

I'll have to experiment with these ideas on a Fuzzrite. Those things are ridiculously noisy with both transistors on and they get radio with the volume rolled off.

A 2n2 cap from input to ground got rid of the radio but not the noise. Bigger caps eliminated the noise, I went up to 100n, however they reduced the gain and high end too much and the circuit lost its trademark sound.


----------

