Guitar or Bass Midrange Control

Fully sweepable midrange frequencies from 160Hz to 1KHz

Portable unit - 9V Battery supply

Circuit diagram:

Guitar or Bass Midrange Control


P1______________10K  Linear Potentiometer
P2______________22K  Log. Potentiometer
                     (twin concentric-spindle dual gang)

R1_____________220K  1/4W Resistor
R2,R3,R4,R11____10K  1/4W Resistors
R5_____________100K  1/4W Resistor
R6_______________1K  1/4W Resistor
R7_______________9K1 1/4W Resistor (or two 18K resistors wired in parallel)
R8_____________100R  1/4W Resistor
R9_____________470R  1/4W Resistor
R10______________1K2 1/4W Resistor
R12,R15,R17,R18_10K  1/4W Resistors
R13,R19,R20______4K7 1/4W Resistors
R14,R16__________3K3 1/4W Resistors

C1,C2,C5,C9____100nF  63V Polyester Capacitors
C3,C4__________150pF  63V Ceramic Capacitors
C6,C10,C11______47µF  25V Electrolytic Capacitors
C7,C8___________47nF  63V Polyester Capacitors

IC1,IC2,IC3____TL062  Low current BIFET Dual Op-Amps

J1,J2__________6.3mm  Mono Jack sockets

SW1,SW2,SW3______SPST Toggle or slider switches

B1_______________9V PP3 Battery

Clip for PP3 Battery


Contrary to most guitar or bass midrange controls, this unit features a parametric, or tunable equalizer type. It consists of a single boost/cut element, but its center frequency can be varied continuously over a wide range, and the Q can also be switched to two values by means of SW1, so that either a broad (Q = 3) or narrow (Q = 10) frequency band can be equalized. The frequency range is limited to 160Hz - 1kHz, a crucial band for guitar and bass tone manipulation.

The unit should be inserted between guitar or bass and amplifier: a 9V battery will power the entire circuit for a long time, since the total current consumption is only 3mA.

Circuit description:

The core of the circuit is a so-called "State-variable" bandpass filter. A low-impedance source is provided by IC1A for the boost/cut control P1 and its associated amplifier IC1B. The input amplifier for the filter is IC2A, and IC2B, IC3A and IC3B comprise the filter itself.
The center frequency is determined by the ganged variable resistors P2A-P2B and by capacitors C7 and C8. The boost/cut range is determined by the gain of IC2A, and the attenuator at the input of IC2B. There is a gain of two from the output of the filter network (at IC3A) to the output of the circuit (at IC1B), and a two-fold attenuation at the center frequency through the filter itself, so the overall gain through the filter path at the center frequency is given by the gain of IC2A which is 10, allowing a maximum boost and cut just over ±20dB.
The "Flat" switch (SW2) provides a flat frequency response at unity gain.

Acknowledgements: such an elegant approach to a "Tunable audio equalizer" is due to Martin Thomas, in his article bearing this title, published on Wireless World, September 1978, Page 58.


Technical data:

100mV RMS input for 1V RMS output (P1 set fully clockwise)
±20dB @ 160Hz to 1kHz
Q factor:
Switchable to 3 or 10
Total harmonic distortion @ 300Hz and 1.2V RMS out:
0.003% (Q = 3 and 10, P1 set fully clockwise)
Total harmonic distortion @ 1KHz and 1.2V RMS out:
0.005% (Q = 3 and 10, P1 set fully clockwise)