This one's more of a utility circuit, it combines two signals by blending the output and uses an gain recovery amp to reduce loss of volume. It's the expansion of a schematic included in an older Geofex PDF about panning circuits for use with guitar effects (1). The inputs are buffered to help with variations in output impedance from the circuits being blended. There was an extra opamp left on the IC so I buffered the power (2) and used a TL074 IC. Right now this is designed as a standalone circuit but can easily be integrated into a circuit that internally blends signals, like the MXR Blue Box (3)...actually, more on the Blue Box some other time.
Fig 1, schematic
1. The TL074 can be substituted with two TL072s. If you don't want the power buffered, you can do a TL072/TL071 combination as well.
2. The non-inverting amp (IC2_C in the schematic) has a gain of 11 dB (4), but can be adjusted according to your taste. There's a loss of volume after the B10K pot and this is what got my setup to unity volume.
3. This circuit isn't supposed to color the sound too much and there are easy changes for mods. C1 and C3 have a big effect by filtering respective input, increasing the value will eventually dampen the sound while going lower (˜100pF) could result in more ground hiss. The R9-C12 R-C filter (R13-C13 as well) after the buffer can also be adjusted. R22 also has an impact on the output filtering and can be modified.
BREADBOARD BUILD
It took some work but I got this circuit to fit on a 400pt breadboard. Both inputs work (can hear signal in output) when a guitar preamp is connected and the B10K pans nicely between the two sides.
Fig. 2, breadboard build
PERFBOARD LAYOUT
It took three versions to get this laid out on perfboard but I have not verified it yet. The Volume pot can be wired on-board to help you secure the circuit in the pedal build. I'll update this project when I get the build up and going.
Fig. 3, perfboard layout and B.O.M.
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