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.
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.
- C2, C5 and C9 should be mounted as close as possible to the supply pins of IC1, IC2 and IC3 respectively.
- 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)