Guitar Preamp with Tone Controls

Description

An electric guitar pre-amp with tone controls using dual supplies. The guitar pickup is high impedance, therefore good quality screened cable must be used between guitar pickup and the preamp.

Notes

The guitar input is via C1, R1 sets the input impedance of the first op-amp, U1, to 10Meg to avoid any shunting effects. The gain of stage U1 is set by the ratio R2 and R3, C3 controls the high frequency roll-off of the stage.

Tone Controls

The amplified signal is now fed via C4 and control VR1 which should be a logarithmic type potentiometer and acts as a gain control. The signal now passes through a passive Baxendall tone control network, VR2 controlling bass and VR2 controlling treble. VR2 and VR3 are linear type potentiometers.

As the tone controls are passive, they introduce loss. This is compensated for U2 which is another non-inverting amplifier. With the tone controls set to mid position the response is shown by the blue trace. Overall gain is about 24dB, if more gain is required then the values of R9 and R10 can be changed.

Noise Response

With gain set at maximum and tone controls "flat" i.e. centre position, the noise response is shown below.

Signal to Noise Ratio

With controls set as above the following plot shows how the signal to noise ratio varies with frequency.

Alternative op-amps may be used such as the NE5534, LF351 etc.