Another OpAmp Fuzz

[seedlings]

I wanted to come up with an opamp based fuzz, so the research began... watching tens of videos and sound clips.  I tried several different circuits mentioned here- some from manufacturers and some hand drawn, and then started the switchery.  Here's what I ended up liking quite a bit.  It looks pretty standard and boring... but if anyone has free time to breadboard one and offer feedback it would be quite helpful.  I didn't try the one TL072 I have in another pedal, just the LM833N noted.  Thanks!



CHAD

[Gus]

The schematic has an error.  I guess you did not build it yet?
Here is a hint what is the input impedance?

another hint
Why a 100k volume control when using an opamp?

[Bill Mountain]

The schematic has an error.  I guess you did not build it yet?
Here is a hint what is the input impedance?

another hint
Why a 100k volume control when using an opamp?

Infinity? 10k? 5k?

[seedlings]

The schematic has an error.  I guess you did not build it yet?
Here is a hint what is the input impedance?

another hint
Why a 100k volume control when using an opamp?

Hi Gus.  It woks fine on the breadboard right now.  Maybe you're saying there should be a 1M to ground before the input coupling cap, which would be there when put in a pedal.  I have no idea what the input impedance is, just playing with components to get something like the sound I'm after- and learn.  Maybe learning is the most exciting part.  I had a 10K pot on the output.  Then I decided to use that 10K pot for something else and put a 100K on, which seems pretty common on the schematics I've seen.  Remember this is opamp is setup non-inverting.

Since I'm in the infancy of learning pedal stuff, if you wouldn't mind, please let me know what I've done wrong.  Like I said, it does sound close to what I'm after.

Thanks!
CHAD

*edit: here are two circuits I breadboarded, which have 100k output pots
http://www.montagar.com/~patj/mufffuzz.gif
http://www.aronnelson.com/gallery/main.php/v/gus/sickbass.GIF.html

*edit *edit: I just realized you might be talking about the gain pot not the volume pot.  I used a 100k because it sounded basically the same as when it was removed from the circuit, which is how I came up with this setup.  I got the sound, then decided I did want gain so tried a 1M and the last 3/4 rotation was useless, so went to 100k.

[ORK]

Who cares about 5k input-impedance if it sounds right? Compare Fuzz Face! The circuit will introduce new highs to the muffled guitar.


Ooops, edit: that 10uF at the input to ground indeed kills the last millivolts!

[Earthscum]

Basically, what people are alluding to here is that you have no resistance between the reference voltage circuit and the input. The reason you use the electro from the Vr to ground is to sink any signal that may bleed through and back up into other parts of the circuit. In this case, you can just remove that 10u and you will be sitting with 5k input impedance, which may not be that bad... as mentioned, the FF has a pretty low input impedance, as well as the Bazz Fuss. So, I would either pull the 10u, or put a larg-ish resistor (10k, low imp., and say max of 220-330k... my pref, in this case, would probably be on the lower side... maybe 33k or something) in between the voltage divider and the input pin.

Overall pretty neat for a first design! 15R is pretty small... once you get more signal going to the input, you'll probably bump that up to 220R or something (just a guess). My DOD Bass Overdrive uses 100k pot with, what was supposed to be, a 1.2k,  but the factory flaw I discovered is they used Red Red Brown (200R) instead of Brown Red Red. It really screams at higher gains, basically same feedback clipper design with a clean blend and a totally useless tonestack. 

[Gus]

A sim of a simple opamp circuit based on one I built years ago(I did not use clipping diodes and had some different filtering).  The LT opamp used in the sim is in the stock download of LTspice.  I used a 353 the build does not use the pickup sim part. it is in the sim to show what happens with the interaction of the guitar and input Z of the distortion
Simple circuit with a RAT type tone control(adjustable lowpass) 100k audio(10k,90k at mid) works backward max Treble at CCW(less treble cut) max Bass at CW(most treble cut)
Look at the tone control at 10K graph, nice mid boost for a Fender type tone stack in the amp
I like 470k with a .01uf at the input for a pickup load with distortions to remove a little of the lows to keep the sound "clearer"
10K volume control(read the tech of the TS for a hint why)
C4 is a .47uf it is hard to read as displayed

Gain at max, R1 tone at 1ohm

Gain at max, R1 tone at 10k

Gain at max, R1 tone at 100k


Nice tuning of that simple circuit as shown.  You can adjust to taste but I would build it as drawn first and then tune it.

[seedlings]

DOH!  I see what you're saying now!  It isn't an error in the circuit, just in the drawing.  I DO have a 100K from the voltage divider to input.  Here's the corrected schematic:



CHAD