This is a simple guitar and bass compressor. It's only possible because of the performance of the NE5532 opamp, which can drive the low impedance of the rectifier without any sign of distortion. VR1 controls the limiting threshold, and the output level from U1B is set at about 3V RMS (sinewave) or ±6V peak with programme material. The gain of the first stage can be increased if needed, by reducing the value of R4.
With the values
shown, the limiting threshold is about 1.5V RMS when VR1 is at minimum
resistance. At maximum resistance, any input signal above 150mV will
cause limiting. Normal operating input level is 1V, but that can be
modified by changing the gain of the first stage. For example, if U1A has
a gain of 10, the maximum input level will be limited to about 850mV because
the opamp will clip with anything higher. The optocoupler can be a 'proper'
Vactrol VTL-5C4 (for example) or similar, or you can make it
yourself. This circuit has detailed instructions for making your own
LED/ LDR optocouplers. The indicator LED (LP1) must be a high brightness type
(as good as you can get), because the current needed for limiting is very low
and you won't even see the LED come on if you use a 'normal' low brightness
LED. I doubt that a simpler limiter can be made that works as well as this
one. Note that the power supplies are not shown, but the connections are
identical to those shown in Figure 2. I do not recommend supply voltages
of less than ±12V, and ±15V is preferred.
