My Fuzz Box Schematic

[Rob's Radio-Active]

Hi, all,
I just wanted to post the diagram of an easy to build stomp box (or built-in to the preamp circuit) that makes a nice tone from clean to "creamy" distortion.
I just finished tweaking it the way I wanted it to operate.
cheers
Rob

[J M Fahey]

Cool
It certainly must sound good

Care to share some MP3/soundcloud/YT video?
Thanks.

[Rob's Radio-Active]

Cool
It certainly must sound good

Care to share some MP3/soundcloud/YT video?
Thanks.

Thanks!
And sure...that's a great idea! It will be maybe next week though at the earliest. The guy I was borrowing a guitar from took it home for now.
I could do it with a signal generator, I guess, but it's not the same.
Rob

[J M Fahey]

Ever seen a signal generator playing hot, steamy Blues?




Me neither

[Rob's Radio-Active]

Ever seen a signal generator playing hot, steamy Blues?




Me neither

Ha!

But anyway, here's as good as I can do for now:
Clean to distorted...The scope voltage settings are the same in both. The second part of the audio taper pot (A10K upper left), which is a stereo type volume control pot, lowers the op amp gain as the first part (at the output in series with the diode stack) swamps out the diode effect. That way, the volume doesn't change too much when dialing in the desired distortion level. There is a separate A1M volume control with on/off power switch at the output. Those controls are where the three shielded cables on the circuit board go.
cheers
Rob

[Rob's Radio-Active]

OK, I got a video with sound but need to figure out how to edit it.
This is actually not the same design, but a second and more elaborate version of the previous posting with more op amps and a Baxandall type bass and treble. It can be used as a "clean" signal booster (about 12dB) when using a long guitar cord or whatever. It also has an input level control, as well as the output volume control, so the op amps won't be overdriven by really hot pick-ups. The distortion is really sweet and can be shaped a lot by the tone controls.
cheers
Rob

[Steve A.]

Here are your two schematics shown in-line...



............



They look interesting!

Steve Ahola

[Rob's Radio-Active]

Thanks! They are pretty straightforward, I think. The first schematic in my very first post only has one op amp though. (That was the one I was comparing to.) They work about the same way, but adding the tone stack load made it need more gain to bring the output back up--and actually has a bit more output than the first iteration. I also switched to a lower noise op amp type. There is a "power on" LED not shown in the schematic. I will probably add a small relay to bypass the signal around everything when it's powered off. Right now there is no "play through" when the box is not on.
cheers
Rob

[Mark Hammer]

A few comments:
1) You'll need to explain the rationale for using a 1k and 1k2 in series when 2k2 is a standard value, and similarly using 8k2 and 27k I series when 36k is a fairly common value and you're evidently not aiming for 1% precision. I also don't know what advantage is held by including both the input-level attenuator and the dual-ganged gain/clipping adjustment pot. Simply put, the circuit doesn't need to be as complicated as it is.
2) The decision to add a gain-recovery stage after the Volume pot IS entirely appropriate since germanium clipping diodes, followed by a passive Baxandall tonestack is sure to severely constrain output level. Giving U2 a gain of just over 10x seems about right. But...
3) Other than the Treble pot, you don't have a shred of upper bandwidth trimming. Ideally, it should be possibly to go from a clean bright tone to whatever an overdrive pedal does, without having to rush back to the amp or pedal and dial back the treble. Personally, I find many such pedals actually have more upper bandwidth than they really need. The treble pot on the pedal should be for re-voicing the tone, rather than hiss-management. Consequently, it might be useful to include 100-120pf feedback caps in parallel with the 300k resistors for both U1a and U2. That value, in tandem with the feedback resistance, will give you a rolloff around 4.4khz (120pf) or 5.3khz (100pf), which will still leave plenty of upper midrange bite if you want, while not venturing too far into insect noises, should your amp Treble control be above 3 or so. That sort of rolloff should also help to tame any hiss that accumulates over the circuit, and yield a slightly smoother tone.

[Rob's Radio-Active]

Thanks for your input...
1) That's a 1.8K in series with the 1.2K.
2) I could have ordered a 36K, it's available, of course, but it's not that common. I don't stock them anyway.
3) The tone stack I have used offers a very wide availability of tone shaping. I like that. Hiss has not been a problem. I wouldn't use a treble control for that. You could never dime it without excessive hiss.
4) The gain control is not for turning it down, it is for leveling the output when the drive is increased or decreased-hence the separate input level control.
The output volume stays fairly level as drive is turned, it just becomes clipped. That's the purpose of the ganged pot. Otherwise, when the drive is turned down for a clean play-through, the output level rises tremendously.

cheers
Rob
Just to clean up loose ends, I did put a bypass relay on it:

[Mark Hammer]

Gotcha. Carry on!

[Rob's Radio-Active]

Made a couple minor changes to make the output more even when adjusting distortion level.
Finally done!
cheers
Rob