DIY - Professional Grade Analog 1-channel 4-Band Audio Equalizer



  • single channel (mono) 4-band equalizer
  • non-inverting from input to output
  • DC-restoring output
  • low noise design
  • low parts count
  • Frequencies:  80Hz / ±15dB; fixed, 150...2.4kHz/±15dB; sweepable, 600...10kHz/±15dB; sweepable, 15kHz/±15dB; fixed
  • input level (0dB; 0.774Vrms)
  • input resistance: > 7k ohm, must be driven from a low impedance source (< 600 ohm)
  • outputput: impedance < 75 ohm, minimum load >= 5 kohm  (using TL074) 

    Schematic Diagram (including part indications)

Brief explanation

On the right is a picture of the first (and only) prototype built. It used a quad TL084 op-amp instead of the two separate dual op-amps that is indeed was  a compact solution. I build this prototype built around 1984. The layout is not VERY clean but it worked fine. The empty socket is just an empty socket, it has no other function than for probing different nodes in the circuit.

It seems also that a more recent project needed a knob... :) Please note that the double 100 kilo-ohm pots should be "reverse log" types, so that the swept frequency increases when turning clockwise, and that the frequency scale is properly progressive.

For a stereo equalizer you would need four-ganged versions. Such potentiometers are nowadays quite hard to find! If you cannot find a reverse-log version - then you have two choices;  use normal log and live with the fact that turning the pot clockwise decreases the sweep frequency - OR - build your own log pot out of a double linear 1 Meg pot where you shunt each glide to its other end using a 110kohm resistor. A good substitute, but not as good as a real reverse log type.

Unfortunately I never made a PCB for this excellent tone control. And this was the only piece I built. Therefore I can only provide here the schematic and the picture of the prototype. It is very well soundning and can dramatically enhance your guitar or bass.

The op-amp's voltage and current noise specification must be good. There are better types around nowadays compared to the ones in the schematic (they are actually not that bad either). The operating voltage should be as high as possible (±17V recommended and depends on the maximum allowed for the op-amp). This is to guarantee maximum headroom. Remember that the boosting of certain frequencies might be more than +30 dB due to the overlapping frequency ranges. 

Good high frequency bypassing must be followed. Solder at least a 100nF bypass capacitor from each power supply to ground, as close as possible to the op-amp supply pins as feasible. The resistance levels are fairly low to keep the resistor (thermal) noise level as small as possible.  Be careful not to cross the input and output signal wire,s especially if not shielded. Otherwise you have built  an oscillator (the circuit is non-inverting and surely oscillated easily if there is any feedback from output to input)! Use separate screened cables for the signals throughout! Below is the schematic. Enjoy.


How to wire the pot correctly