• Clean boost

The op amp can be one of several. LM351, LM356, and TL081 work well and have low current draw if being powered by a battery. The gain is set by using (R1+R2)/R2. Keep R2 around 10K. For example, for a gain of 3, R1=20K and R2=10K.

Low frequency rolloff is set by R2 and C2 using f=1/(2piR2C2). A guitar's lowest note is around 80Hz, so to roll off the frequencies below this, C2 is 0.2 µf and R2 is 10K. C3 also rolls off low frequencies, but it reacts to the load the device is hooked to. Make it around 1µf.

R1 and C1 roll off the high frequencies, and keep the amp from oscillating. You can hear up to about 20,000Hz, so if R1 is 20K, C1 is 400pf (using f=1/(2piR1C1).

You need to power this from a bipolar supply that is greater than the input signal. If you run it off batteries, see the circuit in the Zoom 9002 buffer that will let you simulate a biploar supply from a battery.

You can disable the boost by using a SPST stomp switch to short C1 and R1. It is not true bypass, but the circuit then acts like a buffer and will reduce the effects of long cable runs to your amp. If you use a DPDT switch you could add an LED to indicate if the circuit was engaged or not.