Electronic – Guitar pedal input stage buffer

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In the below schematic for the Tube Screamer guitar pedal input buffer, there is a 1k resistor (R1) after the coupling capacitor (C1). What is the purpose of this resistor? It has no effect on the DC biasing of the transistor, and only appears to degrade the signal.

Tube Screamer Input stage

Best Answer

Its effect to the signal is negligible because there's hundreds of kOhms loading (R2 in parallel with the output circuit seen through the emitter follower). But it can very usefully limit some static discharging pulses which in some unfortunate cases could affect directly the transistor.

Imagine the guitar cable is already connected to the Screamer, but disconnected at the guitar end. The tip of the plug can touch anything. R1 can absorb a substantial part of available static charge.

All harmful unwanted input isn't static, often there's available 50...100% of the mains AC voltage coupled capacitively inside of equipment. It can come from your amp or some external device - no, matter does it come from the output or the input, closing the current path is the harmful thing when the signal wire makes the contact first.

One can laugh and tell that 230VAC mains voltage through 20nF capacitor could cause max 2mA AC peak current through the transistor. True, if it's 50Hz sinusoidal. But it isn't. It can be 320V step input. Static discharges can be 10...100x higher step inputs. Fast enough pulses can create, before they spread to whole semiconductor junction area, high local current densities which cause irreversible changes.