First Breadboard Test - MXR Distortion+ Mod - and it WORKS... now with schematic and soldered Frankenboard...

[Fingolfen]

I've been kicking around a modification of the MXR Distortion+ (1980) version where I added a BMP tone stack to it, but honestly I was having some problems. It worked, but it was very poppy. It already had an input pulldown resistor, and while adding an output pulldown (I tried 100K and then 10K) did fix the pop, it caused the pedal to lose volume. I tried putting the tone control before the volume pot, but that caused it to lose even more volume (even though my BMP tone stack has a gain recovery stage). So I decided to first rethink it a little and see if I could get reasonable tone control (and better volume) messing around with some of the Wampler mods. I decided to leave the extra switches off for now, and just go with the base circuit with the added tone control and relocated gain control...



As you can see from the photo above, our resident technical expert Steggi has been hard at work getting this thing together. I don't have my full prototyping board together yet, so I'm making do.



The good news is... the thing fired up beautifully first time... Steggi is a frikkin' genius. I did have to reverse the volume pot, but that's a minor detail The bad news is... the tone control is still very subtle at best. I did read that someone had put a larger capacitor in series with the tone pot and got better response. I'm going to try it, but I'd love to understand a bit better why that might work...

... I'm also thinking about trying out replacing that pot with the BMP tone stack... just to see what happens... but that's for tomorrow...

 

[BuddytheReow]

Take a look at the muff schematic tone section. There are no coupling caps or resistors right before it. The tone stack is passive so and stray power going into it won’t affect the signal. After tone control, yes, you’ll need a coupling cap. Try that.

 

[Fingolfen]

Just to be clear, when I was using the BMP Tone Stack in the circuit, it was the effects layout one with the added gain recovery stage. Is the 100nF capacitor after the gain transistor not acting as a coupling capacitor?

 

[Fingolfen]

Okay - so despite tweaking with different capacitors, the Wampler mod "Tone" control seems to do f*** all, so I went back to the BMP tone stack, but I still put it in roughly the same spot in the circuit. I haven't solved the "pop" but the volume is great, the distortion control is great, and the tone is... well, doing what that version of the tone stack should do... I may go with the flat mids variety as opposed to the '73 Ram's Head going forward though, because you can really hear the mids scoop as you move through the center of the knob...



(Steggi hard at work!)

So the rough circuit diagram so far (not showing the full 9V in and Vref conversion)



... now I just need to fix the "pop" and figure out how to draw this thing out...

 

[swyse]

If you used a dual op amp, you could use the second half for an active tone control and skip the make up stage. Like the one used on the end of this circuit.

https://www.freestompboxes.org/viewtopic.php?f=13&t=31259

 

[Fingolfen]

If you used a dual op amp, you could use the second half for an active tone control and skip the make up stage. Like the one used on the end of this circuit.

https://www.freestompboxes.org/viewtopic.php?f=13&t=31259

That's a cool idea, but I've been trying to keep the single op amp like in the original distortion+ for this one... at this point I'm really open to suggestions on how to kill the pop - do you think an output pulldown would work, and where's the best place for it?

 

[swyse]

That's a cool idea, but I've been trying to keep the single op amp like in the original distortion+ for this one... at this point I'm really open to suggestions on how to kill the pop - do you think an output pulldown would work, and where's the best place for it?

your schematic there doesn't have a 1meg on the input, and I would think your volume control at the end is doing a good job on that side so I'd try an input pulldown if you don't have one yet.

 

[Fingolfen]

your schematic there doesn't have a 1meg on the input, and I would think your volume control at the end is doing a good job on that side so I'd try an input pulldown if you don't have one yet.

It does have one already - the wampler mod didn't have it, but the drivestortion board already had it, and I built the 9V filters, resistors, and Vref as well as the pull down from that board before moving to the Wampler schematic... I need to do a full schematic at this point...

 

[Fingolfen]

Here's the circuit diagram (I think) so far. I've translated the BMP tone stack, the R1, R2, C1, and C2 can be changed of course depending on what variant or frequencies you want to enhance, etc.

(Edit - now with power stack attached)

 

[cdwillis]

I'd try removing C4 and replacing it with a jumper. That was the output cap before, but now you have the output cap after the muff recovery stage. The Barber Gain Changer has a modified muff type tone control after the first op amp and there's no cap there, just a 1k resistor before the tone stack.

I'd also try putting a small cap (under 50pf) between legs 2 and 6 of the op amp too to keep it from oscillating with the gain turned up.

Is the pop happening when you activate the switch on the test board?

 

[Fingolfen]

Unfortunately it still seems to be popping... I tried upping the input pulldown to a 2M2 - no effect.

I tried adding a 10K output pulldown - just sapped some of the volume, still popped... I may go up to a 100K, but I don't want to lose too much volume...

Thoughts????

 

[Fingolfen]

I'd try removing C4 and replacing it with a jumper. That was the output cap before, but now you have the output cap after the muff recovery stage. The Barber Gain Changer has a modified muff type tone control after the first op amp and there's no cap there, just a 1k resistor before the tone stack.

I'd also try putting a small cap (under 50pf) between legs 2 and 6 of the op amp too to keep it from oscillating with the gain turned up.

Is the pop happening when you activate the switch on the test board?

When I plug it in and it gets power - it has no real "switch" yet - even through the test board... I can even hear it when the effect is technically bypassed because of how I have the test rig set up...

 

[Feral Feline]

Unfortunately it still seems to be popping... I tried upping the input pulldown to a 2M2 - no effect.

I tried adding a 10K output pulldown - just sapped some of the volume, still popped... I may go up to a 100K, but I don't want to lose too much volume...

Thoughts????

Can you show on your schematic where you've placed the PDR on the output?

A 10k or 100k isn't enough, it seems, if a typical PDR on input is 1M or 2M2 — I would've thought you'd want a 1M or 2M2 PDR on the output as well. The high resistance is what's keeping ALL the signal from just bleeding off into ground, 10k is going to do nothing for the popping and as you've noted make your signal quieter — yet you're afraid of losing more volume by putting in 100k. If the PDR is placed correctly, the higher the resistance, the less volume-loss. No?

popping 3pdt site:forum.pedalpcb.com

 

[Fingolfen]

Can you show on your schematic where you've placed the PDR on the output?

A 10k or 100k isn't enough, it seems, if a typical PDR on input is 1M or 2M2 — I would've thought you'd want a 1M or 2M2 PDR on the output as well. The high resistance is what's keeping ALL the signal from just bleeding off into ground, 10k is going to do nothing for the popping and as you've noted make your signal quieter — yet you're afraid of losing more volume by putting in 100k. If the PDR is placed correctly, the higher the resistance, the less volume-loss. No?

popping 3pdt site:forum.pedalpcb.com

At this coming off of the "2" leg of the pot before I take it off of the board. I also tried it after but on the same rail. I haven't had a chance to play with the capacitor thoughts from above, though.

 

[Robert]

You shouldn't need a pulldown resistor on the output since the volume pot is DC coupled to the output jack.

Are you saying the pop is occurring when powering up / powering down the circuit, or when switching between active/bypass?

It's probably going to make some noise when power is applied/removed, there isn't a whole lot you can do (reasonably) about that.

 

[Fingolfen]

You shouldn't need a pulldown resistor on the output since the volume pot is DC coupled to the output jack.

Are you saying the pop is occurring when powering up / powering down the circuit, or when switching between active/bypass?

It's probably going to make some noise when power is applied/removed, there isn't a whole lot you can do (reasonably) about that.

Right now it is when power is applied / removed as I don't have an active / bypass switch rigged... it's pretty much on off. I'll have to double check, I don't think I'm getting an ancillary pop when I switch the tester from bypass to active, so I may be good??? (ish - *lol*) I should also rig a real "switch" and see what difference that makes.

 

[Feral Feline]

I think you're good.

Typically you're not going to be powering/un-powering your pedal in regular use. It's like "crackle ok", you've gotta expect some pop if you're powering/un-powering.

If you get it built up on vero or perf (and even boxed) and you STILL have a pop with a 3PDT or whatever... then I'll suggest checking out the threads on LED popping during bypass.

Yeah, that sounds right, coming off the Vol pot's #2 lug. As Robert and Swyse noted, most outbound pops are taken care of by the vol pot itself if it's last in the chain before bypass.

 

[Fingolfen]

I think you're good.

Typically you're not going to be powering/un-powering your pedal in regular use. It's like "crackle ok", you've gotta expect some pop if you're powering/un-powering.

If you get it built up on vero or perf (and even boxed) and you STILL have a pop with a 3PDT or whatever... then I'll suggest checking out the threads on LED popping during bypass.

Yeah, that sounds right, coming off the Vol pot's #2 lug. As Robert and Swyse noted, most outbound pops are taken care of by the vol pot itself if it's last in the chain before bypass.

I've got some boards I can mod and build / semi-box it enough to test... I have a next step!

 

[Fingolfen]

So... Steggi has been hard at work taking the breadboard prototype and converting it into something soldered together. So using an Effects Layouts Drivestortion board and a hand-etched BMP Tone Stack with gain recovery - the first actual soldered version of the pedal was assembled...



The first time around there was one problem. I'd reversed the potentiometer on my circuit diagram from the breadboard. So once I flipped the gain pot (and updated the circuit diagram below) - I gave it another whirl...



I still get a very slight pop the first time I turn it on, but nothing like I was getting before. I'm guessing it just has to do with the gain of the pedal more than anything else at this point.

 

[BuddytheReow]

The Dist+ is an excellent mod platform. I remember doing exactly this putting a BMP tone stack on it given the small parts count. You may also want to try altering R2 C3 for a different corner frequency or making C3 switchable for a “Fat” control. Diodes would be good mod too. You could also increase R4 for even more gain, but I think 2m would be the max before squealing. Great job nonetheless!