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Input Cap Blend Pot Wiring?

Started by db72, May 17, 2010, 11:59:09 PM

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I've read a lot recently about input cap blend pots and know some boutique guys use them (Blackout Effectors Musket Fuzz for example) but I'm not sure how to implement them myself. I've added switches to go between two values before but haven't wrapped my head around adding a pot to accomplish something similar, especially to existing layouts.

Help anyone?

fine diei est quod est


Here are two ways to do it. The first one uses a series resistance with C2. When the blend knob is turned up, resistance decreases and the two caps work in parallel, increasing the overall capacitance. When it's all the way down, the resistance is at maximum. You need to use a good sized pot for this...maybe 500k.

The second one blends the signal between two parallel caps. Make C3 the larger value in this case...IIRC.

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Sorry for the stupid question but...What is the purpose of the input cap blend pot wiring?  I thought the input cap was more or less to eliminate non instrument freq's?  Thanks for the help.
Contract PCB designer


There are some instances where it's helpful. It can act as a control for bass frequencies. For example, in a fuzz, it can give you more control over how much bass you allow to go into the circuit. Another example is the Timmy. The set up is similar, except that the control comes off the inverted pin of the first op-amp, allowing you to alter the point at which the frequency roll-off of the clipping occurs.


Do you have this attachment laying around somewhere.. im interested in using this on a Fuzz Face like the Creepy Fingers pedals as a "bottom end" and would love to see your attached explanation.



I have used this setup on fuzzes and a rangemaster with great results. The first time I had seen this was on Joe Gagan's easyface fuzz. I didn't use the pregain on the schematic but it works well to vary the input gain when going between single coil and humbucker guitars.  For the blender, you do need a pretty big variance between the two parallel caps to get decent changes when turning the pot. His choice of a 10nF and 3.3uF caps with the 100KB pot works well.