Op Amp Muff Fuzz overdrive mods?

[antonis]

Now, after hooking it up to a footswitch, I get a loud pop when engaging the effect. What would be the recommended way to remedy this?

You must ground C1's negative plate via a 1 - 2,2 M resistor...

[Mark Hammer]

It's giving me about the same amount of dirt as the 50k alone, so perhaps I overheated the LEDs.

1) I don't know that much.  I just talk a lot.  You reminded me that I still have to stick up a picture in the office I moved into mid-December.  I've had this picture in every office I've been in since maybe 1973.  It's a photo from the cover of the journal Science, of an old world monkey, with its hand perched on its chin looking for all the world like it is deep in thought about an important problem.  I put it on my office wall to remind me that there is a difference between looking like you're wise and thoughtful, and actually being so.  Keeps me intellectually honest.

2) The similar sound, whether the diodes+1M are there or not, behaves exactly as expected.  The gain of that stage depends on how much negative feedback there is from the output, which is always going to be set by the lowest-resistance path from output back to input.  Since 50k is a lot lower than 1M, the diodes+resistor are having negligible impact on the behaviour of the circuit.  What you are hearing is the headroom limitations of the chip itself.  Conceivably, if you jacked up the power supply to power the circuit with +/-15V, instead of a mere +9v, then it would not clip with a total gain of 25x. 

Keep in mind that your input signal is going to be maybe a couple hundred millivolts on peaks if you strum a chord hard (more if you have "overwound" pickups).  With a "floating ground" and a single-ended +9v supply, you are going to have usable headroom of maybe +/-3 to 3.5V.  Multiply 300mv by 25x, and clearly you can expect some clipping.

Indeed, we've seen all of this before.  I recall back in the early days of this forum, when building a Distortion+ or DOD250 was considered a big deal, many folks were stumped at how they still ended up getting distortion even though there were no diodes, or the diodes were lifted.  What folks were hearing was the headroom limitations of chips that could only "swing" so far on a +9v supply.  In effect, the circuits they were trying to clone used a sorted of double clipping, in which the headroom limits of the op-amp would be reached, and that signal would then be clipped again by the diodes to ground.  Note that, on the EHX Bassballs, their "fuzz" signal is the output of a 1458 op-amp that has severely exceeded its headroom limitations.  I've never tried it, but now you've mademe curious as to whether the "fuzz" disappears when the supply voltage is increased in that circuit.

[MuffinFlop]

Now, after hooking it up to a footswitch, I get a loud pop when engaging the effect. What would be the recommended way to remedy this?

You must ground C1's negative plate via a 1 - 2,2 M resistor...

Tried the 1M to ground, and it deadened the pop, but didn't completely get rid of it.

It's giving me about the same amount of dirt as the 50k alone, so perhaps I overheated the LEDs.

2) The similar sound, whether the diodes+1M are there or not, behaves exactly as expected.  The gain of that stage depends on how much negative feedback there is from the output, which is always going to be set by the lowest-resistance path from output back to input.  Since 50k is a lot lower than 1M, the diodes+resistor are having negligible impact on the behaviour of the circuit.  What you are hearing is the headroom limitations of the chip itself.  Conceivably, if you jacked up the power supply to power the circuit with +/-15V, instead of a mere +9v, then it would not clip with a total gain of 25x. 

Keep in mind that your input signal is going to be maybe a couple hundred millivolts on peaks if you strum a chord hard (more if you have "overwound" pickups).  With a "floating ground" and a single-ended +9v supply, you are going to have usable headroom of maybe +/-3 to 3.5V.  Multiply 300mv by 25x, and clearly you can expect some clipping.

I put the diodes/ 1M on a switch, and they definitely weren't making an audible difference. Running the diodes parrallel with the 1M gives me a bit more compression and grit.

I've also changed some things- replaced the 4n7 with a 2n2, changed R2 to 100k (gain of 1, right?), removed R7 and, swapped R5 with 1M. It's giving me an awesome trashy, lo-fi overdrive that responds well to picking dynamics. I think these are probably the values I'm gonna keep. Though, I have noticed that even with the volume maxed, I'm only able to get just barely to unity gain with the clean signal. I'm wondering if there's a way to get more total output, without resorting to using an outboard boost circuit.

So far the only thing I haven't tinkered with are the 680k that make up the voltage divider. What effect on the sound/ performance of the circuit will changing these values result in?

[Mark Hammer]

So far the only thing I haven't tinkered with are the 680k that make up the voltage divider. What effect on the sound/ performance of the circuit will changing these values result in?

They do two things.  First, they divide the supply voltage in half to derive a bias voltage.  But keep in mind that the 680k between V+ and Vref also limits the bias current going to the input.  I don't understand nearly enough about this, but my sense is that some op-amps "want" more bias current than others.  You'll see plenty of op-amp overdrives that use a 10k or 22k or 47k pair instead of the 680k we see here.  Heck, the MXR Distortion+ uses a 1M pair, and then runs another 1M resistor from the chip input to Vref.

I suspect that tinkering with the values of this resistive divider is unlikely to do much, or at least not nearly as much as playing with the value of input or feedback resistors.  That said, I can see where a person might wish to play with them.

Can somebody more knowledgeable point us to what spec to look for on an op-amp datasheet for guidance on how to select an optimal bias current value?

[antonis]

Tried the 1M to ground, and it deadened the pop, but didn't completely get rid of it.

Then you may try to ground the Input via the 3PDT switch..
(In to GND when bypassed...)

[MuffinFlop]

So far the only thing I haven't tinkered with are the 680k that make up the voltage divider. What effect on the sound/ performance of the circuit will changing these values result in?

They do two things.  First, they divide the supply voltage in half to derive a bias voltage.  But keep in mind that the 680k between V+ and Vref also limits the bias current going to the input.  I don't understand nearly enough about this, but my sense is that some op-amps "want" more bias current than others.  You'll see plenty of op-amp overdrives that use a 10k or 22k or 47k pair instead of the 680k we see here.  Heck, the MXR Distortion+ uses a 1M pair, and then runs another 1M resistor from the chip input to Vref.

I suspect that tinkering with the values of this resistive divider is unlikely to do much, or at least not nearly as much as playing with the value of input or feedback resistors.  That said, I can see where a person might wish to play with them.

Can somebody more knowledgeable point us to what spec to look for on an op-amp datasheet for guidance on how to select an optimal bias current value?

I'm curious how important matching the values are, and whether increasing the current bias will have any benefits. I've searched around but couldn't find much. Though, the op amp Big Muff uses a pair of 100k for the 4558, while the 741 has an 820k and 1M for around 4.9v...

Tried the 1M to ground, and it deadened the pop, but didn't completely get rid of it.

Then you may try to ground the Input via the 3PDT switch..
(In to GND when bypassed...)

This gives the same result- it only dulls the pop. Going to try this next

[MuffinFlop]

Found this-
http://ocw.mit.edu/courses/media-arts-and-sciences/mas-836-sensor-technologies-for-interactive-environments-spring-2011/readings/MITMAS_836S11_read02_bias.pdf

I'll have to re-read it tomorrow, but seems like a decent read on the subject of voltage dividers.

[antonis]

This gives the same result- it only dulls the pop. Going to try this next

Personally, I would first try to ground the Output also...
(even if there is a pull-down resistor via the Volume pot...)

[MuffinFlop]

This gives the same result- it only dulls the pop. Going to try this next

Personally, I would first try to ground the Output also...
(even if there is a pull-down resistor via the Volume pot...)

Would that be pre or post volume pot? Does it matter?

[antonis]

It doesn't matter - as long as it grounds the "outer" capacitors plate..

edit: I should propose one of the following switch wiring..