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Passive Components in Guitar Pedal Input Stage
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Audioguru:
I see on the larger schematic that the bias resistor for the transistor is fed from a half-the-supply-voltage divider so it should work with low distortion.
floobydust:

--- Quote from: Hero999 on August 17, 2018, 02:15:10 pm ---... R4 and C2 provide the bootstrapping function, which uses positive feedback to increase the input impedance.
--- End quote ---

Bootstrapping with positive feedback I would not use, a guitar has a complex output impedance and there would be a (tone/pickup) potentiometer setting that causes oscillation.

Old tube guitar amps were a 1MEG input impedance and here 500k would be fine. Lower just shifts the guitar's passive tone control up.

You must have ESD protection diodes at the input jack. I repair a lot of gear that got zapped from the 1/4" phone jack tip.
Old Printer:

--- Quote from: Hero999 on August 17, 2018, 02:15:10 pm ---R3 is required for a DC return path through the transistor. Without it, there will be no current and the transistor won't work.

R4 and C2 provide the bootstrapping function, which uses positive feedback to increase the input impedance. See links below:
https://www.petervis.com/Radios/bootstrapping/transistor-bootstrapping.html
http://www.learnabout-electronics.org/Amplifiers/amplifiers43.php

--- End quote ---

Also thanks for the links. I have been investigating guitar effects and pedals as a practical use for the electronics knowledge I am gaining from this hobby. I really enjoy learning electronics, but without a practical outlet to use that knowledge it gets boring at times. Also thanks eev_carl for startiing the thread.
Zero999:

--- Quote from: floobydust on August 17, 2018, 06:34:39 pm ---
--- Quote from: Hero999 on August 17, 2018, 02:15:10 pm ---... R4 and C2 provide the bootstrapping function, which uses positive feedback to increase the input impedance.
--- End quote ---

Bootstrapping with positive feedback I would not use, a guitar has a complex output impedance and there would be a (tone/pickup) potentiometer setting that causes oscillation.

Old tube guitar amps were a 1MEG input impedance and here 500k would be fine. Lower just shifts the guitar's passive tone control up.

You must have ESD protection diodes at the input jack. I repair a lot of gear that got zapped from the 1/4" phone jack tip.

--- End quote ---
Why do you think it would oscillate? At low frequencies it just looks like a 1M resistor, in series with an AC coupling capacitor. At higher frequencies, the impedance does drop, but it remains capacitive, just with a lower resistance and the Q would far too low to oscillate with anything, even at 100kHz.

I've simulated the difference in impedance between, the bootstrapped amplifier and a 1M resistor and AC coupling capacitor. There isn't much difference between the two, especially below a couple of kHz. The red is the amplifier's input impedance and the green, is the impedance of a 10nF capacitor  (yes, I realised 4.7nF was a bit small), in series with a 1M resistor.

The only potential problem is bad design. If C2 is too small and C1 overly large, the bootstrapping wouldn't work and the input impedance would become R1|R2+R4, but I've mitigated that by making C2 big and C1 small.
floobydust:
As a signal source, a guitar has cable capacitance, pickup inductance and RC tone controls. Many variables that are unique to each instrument and setup. Add that to the Spice model and I think it would be risky. I've never seen bootstrapping used on a guitar preamp front-end. Sure it could work. But across permutations of guitars and settings, not my style.

I think OP would be better off using the JFET input stage instead of this emitter-follower.
Electrosmash has this J201 front-end with ESD protection diodes. You get some low-noise voltage gain instead of zero voltage gain with the emitter-follower.
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