To handle the great dynamic range of the Shakuhachi I needed a compressor for my Shakuhachi 2 Synth project. Because a limiter is not that different I added this feature as well. This comes in handy with my Vocoder project also. The structure used here is derived from "Small Signal Audio Design" by Douglas Self p682ff. The audio signal did not flow through a VCA as in many other implementations. Instead the compression or limitation is done by subtracting the audio signal at the output summing node according to the control voltage derived from the audio signal.
Specs and features
- Switch compress or limit
- Switch Compression/Limit rate 50% or 90%
- Compression/Limit rate adjustable 0--max
- Runs on +/-15V and +/-12V (with minor resistor changes)
- Power consumption below 15mA each rail
- Schematic Compressor / Limiter
- Download docs
The audio signal flows unaffected through IC1A/B. When the compressor - limiter kicks in the inverted signal is added (=subtracted) at the summing node of IC1A. The signal level to subtract is regulated through a Sims VCA. The CV generation for the VCA is pretty standard. Linear for the compressor and exponential for the limiter.
Precision full wave rectifier with filter to generate the control voltage for the VCA from the audio signal.
- Second harmonic: Adjust TR1 for lowest second harmonic. Easiest way use a THD meter. Other way with sine input and HPF by oscilloscope / ear.
- Exponential CV starting point: Set potentiometer P4B to ground. Audio input sine 1kHz 10Vpp. Remove SW1 connection. Measure signal at mid point from SW1. Adjust TR1 so that the audio signal is minimized.