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  • Bass Guitar Parametric Multi-Band Compressor

  • Created: Jul 15, 2017

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Summary

A bass compressor, which belongs to the group of dynamic effects, is one of the most regularly used gadgets by bassists. Its compression effect makes the bass sound smooth and punchy. This reference design features a parametric multi-band compressor. It uses a state-variable filter to split the low and high frequency components of the signal coming from the bass guitar. The compressor utilizes the TL084 device, which supports the active filters and voltage followers used in the circuit. The TL084 is a device, which has four JFET-input operational amplifiers that operate with a voltage supply ranging from +/-3.5V up to +/-18V, consuming 1.4mA per channel.


This parametric multi-band compressor is a purely analog compressor, which has a 3PDT switch that can be toggled to use the effect or just bypass the signal. The compressor uses a state-variable filter to split the signal into low and high. This state-variable filter is handled by op amps U1A-U1D. The dual gang 100kΩ potentiometer RV1 controls the frequency at where the signal is split into low and high. The signal can be split from 44Hz to 3.3kHz. Once the signal is split, the low signal and the high signal are sent to the low compression circuit and high compression circuit, respectively. The low compression circuit is handled by op amps U2A and U2B, while the high compression circuit is handled by op amps U2C and U2D. The compression circuits use VTL5C2 vactrols (ROC1 and ROC2) to optically compress the low and high signals. This is done by optically varying the gain of the op amps U2A and U2C through the light-dependent resistors inside the vactrols that were used as feedback resistors. The 500kΩ potentiometers RV4 and RV5 controls the level of compression applied to the high and low signals, respectively. The volume of the compression circuits’ outputs is controlled by potentiometers RV2 and RV3. The compressed low and high signals are combined at op amp U3B.


The stereo ¼” phone jack connector J1 receives the input signal from the bass guitar, while the compressor’s output is taken from the mono jack J2. The circuit can be powered by a 9VDC power supply inserted at DC adaptor J3 or by a 9V battery connected at the battery holder BT1. The supply for the circuit is a single type, while the op amps operate with a dual-split type power supply. In order for the op amps to operate with a single supply, a virtual ground, which is equal to half of the supply (4.5V), is generated through the voltage-divider network composed of R21 and R22. The 4.5V generated is applied to the non-inverting terminal of the op amps.

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