Motivation
This is the revised version of my Limiter/Compressor. First built for my Shakuhachi to Synth project to handle the great dynamic range of the Shakuhachi. Here I left out the limiter and added a make up amplifier. The structure used is derived from "Small Signal Audio Design", second edition by Douglas Self p682ff. The audio signal did not flow through a VCA as in many other implementations. Instead the compression is done by subtracting the audio signal at the output summing node according to the control voltage derived from the audio signal. The compression rate and the make up gain is adjusted by hand or/and optionally with foot pedals. The foot pedals are an additional option particularly made for wind players. It works without this option in your setup as well.
Specs and features
- Compression rate and gain adjustable by hand or/and foot pedals
- Audio path not affected when no compression is used
- Runs on +/-12V and +/-15V (with minor resistor value changes for best performance)
- Power consumption below 20mA each rail
Implementation
Schematic
Compressor with optional pedal steering schematic: Main board
Compressor with optional pedal steering schematic: Control board
Description:
When the ratio is set to zero and the gain to one the input signal passes through the circuitry unaffected (IC2C, IC2A IC6OTA1, IC6OTA2, IC2D). When the compression rate is turned up a DC voltage is derived from the input signal wit a precision full wave rectifier and some filtering (IC1A, IC1B, IC1C, IC1D). This voltage is used to open the VCA in the side chain (IC3OTA1, IC3OTA2, IC2B). The signal from the side chain is then subtracted from the main signal (R13, IC2A). The now compressed signal is then potentially amplified (IC6OTA1, IC6OTA2)
Calibration
- Second harmonic: Adjust TR1/TR2 for lowest second harmonic. Easiest way use a THD meter. Other way with sine input and HPF by oscilloscope / ear.
Building hints
- If you don't use the pedal connectors you need to replace the switches in the sockets with bridges at the PCB connectores.
- The LM13700 differ in quality from part to part. You might want to select the IC for lowest DC output swing.
Special parts
- None
Download
Compressor main board documentation downloadCompressor main board Gerber files download
Compressor *.fpd file download
Compressor pedal connection documentation download
Compressor pedal connection Gerber files download
Compressor pedal connection *.fpd file download
Compressor with connector: Front view
Compressor: Populated PCB
