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Passive Components in Guitar Pedal Input Stage
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eev_carl:
Hi,

I've been reading a lot of schematics on guitar pedal circuits and had a few questions about the input stages that I'm seeing.  One such schematic is attached below.  Most of the schematics have the following components. I'm assuming the function; please correct if it's wrong.

1. An AC coupling cap (C1)
2. A direct coupling resistor (R1)
3. A pull-up resistor (R2) - biasing?
4. A large path-to-ground resistor (not used by this particular schematic, but on others to prevent a "pop" on power off)

I've seen many different combinations of 1+2 and was wondering how to select the values.  I'm most interested in bass frequencies for my application.

The full schematic of the attachment is here at Electrosmash: https://www.electrosmash.com/tube-screamer-analysis

Thanks,
Carl
Zero999:
Yes, that's correct.

The problem with that circuit is its bias point is dependant on the transistors Hfe, which is extremely variable. The impedance seen at the base is equal to the emitter resistor, multiplied by the transistor's Hfe and forms the lower half of the potential divider. The solution is to add a potential divider, with a lower impedance, than the emitter resistor multiplied by Hfe, to the input stage. This will reduce the input impedance considerably, but bootstrapping can be used to increase it.
eev_carl:
Is the input stage using both direct coupling and AC coupling?  If so, are they trying to mitigate the downsides of each by selecting middle-of-the-road values?
JS:
No, ac coupled as you don't care about DC. The resistor there would limit the signal to the base of the transistor, but in this case not so much as it's pretty small compared to the surrounding resistors, including the emmiter resistor reflected at the base of the transsistor.

JS

Zero999:
Here's an example of a buffer with a high input impedance. It uses a low impedance potential divider to bias the output to near half the supply voltage and bootstrapping to increase its impedance to around 1M.

LTSpice Simulation
https://www.eevblog.com/forum/projects/resistor-and-pot-in-parallel-not-working-as-i-expected/msg1401083/#msg1401083
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