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Circuit Analysis Question


Hello everyone, I have a 'what does this chunk do' question for the analog folks.

So my friend wants a BD2 guitar pedal clone. No problem, there's sure to be something online, and there is! Two circuits are shown, one with a dual opamp where half is used to buffer the virtual ground at 4.5V (9V supply) and another without the buffer (probably doesn't make too much difference). I could use their designs, but I'm always after some layout practice so I figure I'll put the schematic in and make my own. I've got both TL071s and TL072s in DIP, but I'd like to try a double sided SMD board, so I go with the dual opamp version because I already have TL072s in SO8. If I wind up needing to I can use the DIP version to make life easier and cut down on vias. Excellent!

I manage to lay this out in KiCad, but it's painful because there seems to be no way to flip the board over, so I'm working through my topside layout. I've tried hiding layers but the ratsnest is all or nothing and the hidden components are still selectable and dragable. What a pain! To top it all off, I get the 'crash on 3D render' bug when I've never had it before. Ok, no sweat, I'll just learn gEDA and do it there, where board flipping and hiding is a tab key or mouse click away.

Much strife while I learn gEDA. This is no small task for a linux/programming/EDA noob, but I'm up and running.

So I begin laying out the board again. It's easier the second time as I've had some practice now. Then I notice something about the circuit. Capacitatively coupled to the inverting input of the second half of the opamp is a confusing block of circuitry that I don't quite understand, and may be unnecessary (everything to the right of the X on the schematic below). It looks like it's either there to help establish or stabilize a virtual ground, or perhaps act as an active filter block, I can't tell which. It's not uncommon for these small guitar pedals to have modestly complex filtering sprinkled all over, but it seems odd to use a transistor in that spot to do so, with the opamp right there.

My guess is that it's part of a virtual ground circuit on the original pedal and in the cloning process it's purpose wasn't understood, so it was left in the schematic and another method of creating stable bias was added (the opamp buffer section). I figured I'd see if anyone was sure before I go try to simulate it. I don't quite have the parts to breadboard it (56nF caps, sheesh), or I'd go with the old build and check.

What do you guys think, am I even close?

The first circuit is the virtual ground. Very common technique for stuff that runs on a 9V battery.

The other circuit is a Sallen-Key high pass filter. Often constructed with an op-amp, but bipolar or fets are also used. The output goes nowhere. It looks like the change in input impedance over frequency is used to influence the feedback loop of op-amp buffer.

Sorry I took so long to get back.

I understand a bit better now. I was hung up trying to sort out what a filter was doing with no output connected. I'll keep banging on it, and assume it's important to the circuit's function.

Thanks for the info. :)

Wire up a switch where the X is marked in the schematic above. See how it sounds both ways.

FX pedals have lots of interesting and strange circuits :)


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