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I'll give the long explanation, excuse me if this is overkill/patronizing. This is the output stage of the pedal. The eponymously-named-dry signal is being blended with wet signal (Delay Out) before being sent to output jack. The electrical principle of superposition is how you understand...

Found the issue. There was a dual channel dip switch modifying a portion of the wet path and a portion of the dry path. Crosstalk between wet and dry was happening inside the DIP switch.

Yeah you primarily do filter design yes? That would be very well suited for sim?

Yeah I recently realized I could do DC transfer functions with stepped DC sources which then reveal input output transfer curve. Really cool for visualizing the softest overdrive so to speak. OP has a point regarding when one is choosing between 2 different caps like 2n2 vs 2n7. That stuff...

Yes and no, over time you learn to associate gain plots with timbres. And properties like the abruptness of clipping can be viewed on waveforms and through FFTs. Breadboards are undeniable evidence of how something sounds but sims can give a lot of general insight whereas the fine tuning can...

Yeah the low passing trick is frequency dependant. You lose out put as LFO goes higher and lose sine wave shape as LFO goes lower.

Integrator opamps take square waves and make them triangle. Lowpassing square wave approximates sine. Hard clipping sine approximates square wave. Electric druid makes an LFO IC PDIP8.

The output impedance of a vacuum tube is dynamic and changes with instantaneous voltage across plate. Thus, the damping factor (think voltage division between load/SPKR and source/TUBE) varies. The XFMR has a turns ratio that multiplies the impedance seen from the TUBE by the SPKR and divides...

The beautiful thing about audio electronics is the blending of sound engineering/mixing into an otherwise objective field. We forget how much of the field is using your ears and having an understanding of the circuit architecture as a signal chain; same as adding EQ and clipping in the DAW...

Looking further into TL972, I don't see immediate cause for concern....NVM current out looks weak

A lot more expensive on digikey.......

Analog devices OP275 looks great. JFET input, <$2 per device, 6nV/sqrt Hz noise, output swing within ~1.5V of rails, up to 70mA output current.

I always include 1n914s to rails anyway.

Good idea. I'm pivoting to SMD for most new stuff, already have some 16xxs. Great opamps.

Yeah I saw the supply voltage might be tied to GBW so it could be used lower with little consequence to audio band but still... It's not recommended.

If you upgrade, get a high voltage one so you can do tubes if you want!

THe LF353 looks ok but suggests minimum 10V supply. Seems like going lower mainly affects GBW product.

So recently I noticed that the TI TL072 opamps have doubled their previous noise specs from 18nV/sqrt Hz to 37nV / sqrt Hz. Not so good anymore. I'm interesting in alternative PDIP style opamps that exceed the noise specs of the current TL072 for future use. I am aware of NE5532s but they are...

A true RMS multimeter will get you much of the way there For troubleshooting if you don't already have one.

Some useful back pocket theory/equations Ohms law Kirchoffs voltage and current laws F_-3db = 1/(2*pi*r*c) Impedance of capacitor goes to infinity as frequency decreases, goes to 0 as frequency increases Inductor impedance is opposite of capacitor