Feedback on my first pedal/mod design.

[Spoonology]

Hi there, I'm in the process of designing a heavily modified Big Muff and was hoping to get some feedback and some advice if I've messed up anywhere,

Here's a link to the schematic - http://i.imgur.com/n74rfYI.png

I've included the following mods:

• Clean blend; I've placed the Big Muff inside a buffered loop to avoid feedback and impedance mismatching.
• Voltage sag using a pot and a resistor on the 9V rail
• Second clipping stage can be switched in and out. Because the clipping stages invert the signal I've added inverting buffers after each one so there's no phase cancellation when blending with the clean signal.
• Switchable diode types. I have a 2P4T rotary switch which I'll use to switch the diode array on both clipping stages.
• Separate bass and treble controls.

As far as I can see everything should be ok but I'd appreciate some feedback before I order parts! Cheers!

[Derringer]

holy cow ... i uttered a profanity when your schematic popped up after I clicked the link!

very ambitious dude, I like it!

I don't think you need/really want that 1M R24 going to ground from your power rail. Instead, on your power supply, have your incoming voltage hit a small valued resistor like 330R in series first, just before it hits your 100uF C1 filter cap to make a proper high pass filter. Ditch R24 and have your V+ go to the variable sag pot.

I don't think you'll need a 1M mixing pot either for your clean blend. 100K should work fine. You may run into an issue though blending the buffered clean with the unbuffered output of the muff into another stage. I've had trouble blending two signals like that into a gain stage ... the low impedance signal always seems to dominate and sounds weird. You may not have this problem though since you're not amplifying. But if you do, take that opamp U1B buffer, put it on the output of the muff and then just blend the buffered clean with the buffered muff. In that case, you could probably use a 10K mix pot.

Order your parts. These are all parts you'll want to have on hand for the future anyway so order a bunch of duplicates too. Breadboard everything first because I think you'll find that some of those diode options just don't make that big of a difference.

Good luck! And post back with results.

[Spoonology]

Thanks for getting back to me so quickly!

330R for the power filter seems a tad high; it gives a cutoff frequency of 4Hz and running it through a simulation suggested it would reduce the overall volume, I'll probably use 100R instead? And your suggestion for moving the buffer to the output of the muff makes a lot of sense now I think about it!

[Derringer]

less volume? hmm .. because of a tad less voltage maybe? You'll have plenty of volume on tap so I wouldn't worry about it.
But 100R works fine though too if you like.

Another thought, you may want to toss in an extra switch that removes the diodes from the circuit of the second gain stage.
No extra clipping there gives some more volume and a "bolder" tone.

[Ice-9]

Try it out with the diode selection and just pick the ones you like, quite a lot of the options available will make little difference. good work for you first mod/schematic.

[Spoonology]

I've made a few changes to the design:

• The original layout was blending the buffered clean signal with the unbuffered muff signal; I've changed it so that both signals coming to the blend pot are buffered.
• I've increased the opamp resistor values to 10k to avoid loading the power supply and transistor stages
• Added a decoupling cap between the clipping stages and the inverters stages as the bias of each will be different and will vary when the voltage sag is used
• Added a small value resistor to the power supply

Here's the modified schem - http://i.imgur.com/TIkAzMe.png

[Derringer]

breadboard it up, see how it sounds.
In just looking, your inverting buffers may still load down the gain stages with only 10K input impedance. But see what it sounds like first.

in the meantime, check out this page for muff ideas
http://www.bigmuffpage.com/Big_Muff_Pi_versions_schematics_part4.html

[Quackzed]

looks cool. only small suggestion i have is to swap the switch positions of the 1n914 diodes and the 1n34a's so that the switching will go from lowest thresholds to highest... ge's si's 2/1si's then leds. no different other than a bit more logical... 

[Bill Mountain]

I really like the separate bass and treble.   This is quite ambitious.   Good luck!

[GGBB]

Not sure you need U2A, and U2B could be done differently to maintain a more "pure" muff circuit. Get rid of U2A, and instead of switching out the second stage including the inverter U2B, switch between the second clipping stage and the U2B inverter. That way the muff circuit itself is the same when the second clipping stage is used, with no added op-amps between the last three stages. U2A could be repurposed to provide Vref.