[2607] Low Input Impedance Fuzz

Date: August 16th, 2019 | Comments : none | Categories: DIY.

Low input impedance is not really the issue making a fuzz incompatible with a buffer.

A fuzz face has a lot of gain…. VERY high. It is basically a crude inverting op amp with a 100k resistor in the negative feedback loop. There is no resistor in series with the audio input that would otherwise reduce the gain. The simplified gain equation is Rf/Ri where Rf is the feedback resistance (100k) and Ri is the input resistance. When connected directly to a guitar, the combined resistance and impedance of the pickups are in series with the input (acting as Ri) and will reduce the gain, thereby preventing the fuzz from going bonkers.

If you drive the fuzz with a buffer, the output resistance/impedance of the buffer is very low so the result is that the fuzz circuit runs at max gain with oscillations and other mayhem often as the result. Buffered input = max gain = bonkers, and guitar pickup = lower gain = fuzz heaven 🙂

The Rangemaster does not have this problem because the available gain from the Ge transistor is much lower than that available from a FF.

The Big Muff does not have this problem because each gain stage has a series resistor to reduce their gain… 39k on the input stage, for example.

 

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