My first pedal build. A buffer/boost/pan stereo pedal.

[BlueGrot]

Hi! I've never built a pedal before but I have rudimentary skills in electronics and I do a decent job of soldering stuff. I've ordered a lot of the components and will be prototyping it with a vero board in the coming weeks. Here's the schematic of a mono line which I've made by just plotting in a op amp circuit I found with an LPB-1 clone, so it's not my own construction. Does it make sense? The plan is to make two of these circuits and send them to a stereo pan knob.



As the pedal will be used to drive two amps it would be great to integrate some sort of ground lift and phase invert. Eventually an expression control circuit for volume/pan would be sweet as well.

Edit: R4 has the wrong value. It's supposed to be 390R.

[PRR]

Welcome.

The LPB is just a gain stage. Ambitious for one transistor; but here you have 39 transistors (in U1) and you sure-as-heck can get a gain equal to the LPB just with the opamp.

Also easily adjustable: I'm sure the LPB's gain will be excessive (uncontrolled clipping) for some hot sources.

Your U1 bias network feeds half the crap on the power supply direct to the input (somewhat diverted by the source). With battery this may be fine. Plug-in supplies may have negligible or gross 50/60/100/120Hz crap plus whatever leaks from other things they power. A Vref divider and decoupler is usually a wise plan.

[BlueGrot]

You are totally right, I'll simplify things and make a circuit using the U1. Thanks for the advice on the vref divider and decoupler, I'll keep that in mind when doing similar things.

[antonis]

150pF caps are too low values for 100k base resistor & volume pot..
(they create a high-pass filter at 10kHz cut-off frequency..)

IMHO, you should raise their value to 100nF and, maybe, C2 up to 1μF in case of severe loading the output..
(I don't know your amp's input impedance which, at least, will be the half of their individual one..)

Also, you've made an "indentical" Q1 bias for 1mA Collector current, but you've used a "weird" resistor value (830k) and you've set Emitter voltage lower than one Vbe voltage drop..
(leading to temperature instability of Q point because of Vbe temperature variation - which maybe should be acceptable for a distortion circuit but not for a "clean" booster)

IMHO, it's wise to leave as much headroom as possible but it's wiser to have an Emitter voltage not lower than 1V..
(depending always on power supply value, of course..)

[PRR]

> Emitter voltage lower than one Vbe voltage drop.. (leading to temperature instability of Q point because of Vbe temperature variation

I think those values are correctly researched/plagiarized from a Well-Loved design, the LPB.

Which DID go through several value changes (360, 390) over the years as Mike got different batches of transistors, or got feedback from players in extreme heat/cold.

And the LPB says "Linear" but is not really clean, not what it is for.

[antonis]

I think those values are correctly researched/plagiarized from a Well-Loved design, the LPB.

No intention to insult, debase or understate LPB-1 or any other Well (or Bad) Loved design..

My comments refered to BlueGrot's circuit in a mean of "rules of thumb for good design practice"...

:beer: :beer:   

[Transmogrifox]

:beer: :beer:   

Maybe this is what you had in mind (if only I could get a 'pint' glass bigger than my head):

Cheers!