Problem with adding blend to Big Muff - maybe a phase issue?

[davidjnichols]

So I built a blend circuit using this scematic:  http://www.jmkpcbs.com/JMK_PCBs/DIY_Projects_files/Blend%20Pedal.pdf

The send of the blend goes to the input of a GGG Big Muff, the output of the Muff goes to the return.  The Muff circuit works well and the blend works EXCEPT when I have the blend pot turned all the way to the dirty side (all Big Muff, no clean), and I strum my guitar really hard, the Big Muff barely comes through and it's only clean!  I don't have that problem if I play very softly on the strings.  What the heck would cause that??

[davidjnichols]

I left out a key piece of information!  I used this exact same blend build on a Rat a few days ago and it worked really well.  I did not have the issues that I'm having with the Muff.  Maybe it's a phase issue between the Muff and Blend that didn't exist with the Rat?

[armdnrdy]

Which GGG Big Muff version did you build? There are several.

[davidjnichols]

Which GGG Big Muff version did you build? There are several.

Ah yes of course.  I built the "Triangle" version.  I would link the schematic and wiring diagram and stuff, but it appears that GGG is down for maintenance today.

[armdnrdy]

Down for you maybe!  

http://www.generalguitargadgets.com/pdf/ggg_bmp_tri_sc.pdf?phpMyAdmin=78482479fd7e7fc3768044a841b3e85a

Link for blender circuit:

http://www.jmkpcbs.com/JMK_PCBs/DIY_Projects_files/Blend%20Pedal.pdf

Just so we have all of the info:

How did you incorporate the blend circuit to the Big Muff?

[davidjnichols]

How did you incorporate the blend circuit to the Big Muff?

Input jack > 3PDT
3PDT > blend input
Blend send > Big Muff input
Big Muff output > blend return

I think that's what you're asking for?  I basically just stuck the entire Big Muff circuit in between the send and return of the blend circuit.

[armdnrdy]

Two things to try......and I'm guessing till someone else pops in,

If the TL072 is socketed, try replacing it with a non JFET type dual op amp, (4558, 1458 etc.) see if that helps the two circuits play nice together.

I think it's a impedance or bias issue.

Also, Take a resistance reading while you're moving the "blend" pot. See if there's anything going on at the end where the problem exists.

Just corrected some spelling errors.....It looked like Ricky Ricardo posted.

[davidjnichols]

Two things to try......and I'm guessing till someone else pops in,

If the TL072 is socketed, try replacing it with a non JFET type dual op amp, (4558, 1458 etc.) see if that helps the two circuits play nice together.

I think it's a impedance or bias issue.

Also, Take a resistance reading while you're moving the "blend" pot. See if there's anything going on at the end where the problem exists.

Just corrected some spelling errors.....It looked like Ricky Ricardo posted.

I swapped the opamp to a 4558 and still have the issue.  I tested resistance and the pot seems to be working fine, HOWEVER, I noticed that I accidentally installed a 20k pot instead of 10k and I don't have any 10k pots handy.  Would that cause the problem? 

[armdnrdy]

It could!

Try putting an 18 to 22K (whatever you have or can get...20K preferred) across lugs 1 and 3 of that pot. Then you will have a 10K pot.

If you don't have or can't get resistors in a timely fashion, (right now!) consider borrowing the pot from the RAT build.

[davidjnichols]

It could!

Try putting an 18 to 22K (whatever you have or can get...20K preferred) across lugs 1 and 3 of that pot. Then you will have a 10K pot.

If you don't have or can't get resistors in a timely fashion, (right now!) consider borrowing the pot from the RAT build.

Okay, I wired a 20k resistor from lug 1 to 3 of the pot, but it's still doing the same thing. 

[Seljer]

Try a different pot. Or to verify if it is the pot, swap around pins 1 and 3 and see if it does the opposite.

[armdnrdy]

Another thought:

Are you using active (hot) or passive pick ups?

[davidjnichols]

Another thought:

Are you using active (hot) or passive pick ups?

Passive.  Les Paul.

[davidjnichols]

Try a different pot. Or to verify if it is the pot, swap around pins 1 and 3 and see if it does the opposite.

Tried a different pot and still the same problem.

[armdnrdy]

Something else to try:

Wire the Big Muff directly to the input/output jacks to see if any problem exists.

I assume that you checked all of your component values to make sure everything is in place and oriented correctly, and trace side for questionable solder joints and solder bridges.

[WaveshapeIllusions]

Does it do this at all levels of the sustain pot? I've had a similar issue with a clean blended muff. I'm pretty sure it's something on the muff side in my case. It seems like a big dip and swell type effect that some compressors have. Perhaps it's the same thing?

[Keppy]

The Big Muff has a 100k volume pot. At low settings of the pot, there is a series resistance on the output, which adds to the resistance of the blend pot. This means that if the Muff volume is down at all, you can never get the clean all the way out. These controls are generally interactive as wired, if I understand your wiring.

If that's your problem (and not miswiring of some sort), there are two potential solutions (well, at least two).

1) Place the volume control AFTER the blend control
2) Buffer the return, preferably with the same IC, TL072, so the two signals have the same impedance at the blend pot.

[davidjnichols]

Thanks a lot for the help, guys.  This afternoon I'll take the Muff section out of the pedal to test it and make sure it's not something wrong with that circuit.  Then I'll try placing the volume after the blend.  If that doesn't work, I'll try the buffer suggestion.