Debugging an unknown circuit

[cab42]

Recently, a friend asked me to look at a diy pedal that a friend of his son had build years ago for his son. He had no idea what it was and it did not work.

As far as I can see it was a switchable buffer into a fuzz of some kind with a voltage sag control.

The debugging part was fairly easy. A defect switch and a partly fried voltage sag pot. But I don't recognize the circuit and wonder if anyone else does. It has two parts, a buffer that can be (true) bypassed into a fuzz. They were on separate boards (perf) so I drew them individually.

Buffer part:

IC is one side of a TL072.




Fuzz part:

Note: The diode switching is a single dpdt switch, switching both diodes. Transistors are 2N2222.



I guess the voltage sag pot burned because of too much current. Also a bit strange that it is log and also that the volume pot is linear.

Anyone knows what it is?

[ElectricDruid]

This buffer part looks weird. The two 100Ks should be creating the Vref, the 1M should be connecting the input to the Vref, and the + and - op-amp inputs should be the other way around. Compare with this:



I don't know what the other bit is though, sorry. I'm not good on fuzzes.

[Rob Strand]

The fuzz is two Bazz Fuzz stages in cascade,

Go here and scroll down to the Buzz Box,
https://home-wrecker.com/bazz.html

The transistors get their base current via the diodes.    The collectors sit at about 1.3V and 2.3V (depends on LED color).

This buffer part looks weird. The two 100Ks should be creating the Vref, the 1M should be connecting the input to the Vref, and the + and - op-amp inputs should be the other way around. Compare with this:

Maybe a tracing or build issue.   It's going stop it working if it's a build issue.

[typo fixed.]

[PRR]

I guess the voltage sag pot burned because of too much current. Also a bit strange that it is log and also that the volume pot is linear.

The 'sag pot' is driving two 100k resistors. It is hard to see how a 9V battry could smoke pot through 50k.

The log-taper might make sense for extreme turn-down, tho you'd think it would be reverse log.

Is there any sense that this was musically useful or just an experiment?

[duck_arse]

that 47uF hanging off the inverting pin to ground is going to roll-off your high content. drastically.

[ElectricDruid]

that 47uF hanging off the inverting pin to ground is going to roll-off your high content. drastically.

Yeah, that's why I think (like Rob) that there's either a tracing error or a build error. That should be decoupling the virtual ground, not hanging off the op-amp input.
If you fix the Vref and put the op-amp inputs the right way around, the whole thing looks a lot more reasonable.

[Rob Strand]

Yeah, that's why I think (like Rob) that there's either a tracing error or a build error. That should be decoupling the virtual ground, not hanging off the op-amp input.
If you fix the Vref and put the op-amp inputs the right way around, the whole thing looks a lot more reasonable.

Most people would automatically correct the traced schematic to be what you have drawn with R1=R3+R4=1M typo.R1=1M, R3=R4=100k.

[cab42]

Thanks for the comments. And of course you were all correct. My trace of the buffer was wrong, sorry. I gave it another go, and it ended up pretty much as Tom's illustration.



I checked my first trace against Jack Ormans buffer article and he has a buffer with a resistor to ground and another to v+, so I just thought that it was OK but just odd that the resistors did not have the same value. So now i have learned this:

Most people would automatically correct the traced schematic to be what you have drawn with R1=R3+R4=1M.

And it is a Bazz Fuzz! Should have thought about that. I have even built one!

Regarding the sound: I don't find it very interesting, and the voltage sag pot really doesn't add much.

Again thanks for the comments!

[Rob Strand]

And it is a Bazz Fuzz! Should have thought about that. I have even built one!

Regarding the sound: I don't find it very interesting, and the voltage sag pot really doesn't add much.

Yes, it's probably not the best pedal to try out a sag pot.  The Bazz fuzz is fairly immune to supply voltage!   Perhaps a little less gain as you drop the supply voltage.   It you wanted that sag pot to do more maybe go for a 100k,  no idea what it would sound like.

Another common Bazz fuzz variation is to use a Schottky for one of the diodes.

[Rob Strand]

Regarding the sound: I don't find it very interesting, and the voltage sag pot really doesn't add much.

You could try this "variant":
https://www.diystompboxes.com/smfforum/index.php?topic=30794.0
https://www.diystompboxes.com/pedals/mtlsim.bmp

[cab42]

Thanks for the suggestion! I will try it on my breadboard