CP4500 Stereo Bus compressor

CP4500 Stereo Bus compressor for the '500' Lunchbox - Kit

The CP4500 is a Stereo bus compressor based on the design that was originally used in a legendary British console, adapted to the '500' format of API*. It is famous for adding punch and glue to the mixes.

The CP4500 adds a few features to the original such as an equalizer in the side chain with three high-pass frequencies and a high boost section which enhances the spectral balance of the output. It also adds a Mix knob that allows mixing blending dry and wet signal for parallel compression.

Very faithful to the original design,
Four paralleled THAT VCA's (plus one in the side chain) per channel,
True stereo side chain,
Stepped Ratio, Attack and Release,
Side chain EQ with 3 high-pass frequencies and one high boost filter,
Wet/Dry mix control for parallel compression,
Ultra-low distortion and noise audio op-amps,
Analog gain reduction meter,
High grade electronic components throughout,
High grade double sided PCB with plated holes,
High precision aluminum knobs,
Available assembled or as a full kit (everything included).


1 RATIO Rotary switch that sets the compression ratio to 2, 4 or 10.

2 ATTACK Rotary switch that sets the attack time from 0.1 to 30 milliseconds.

3 HP Inserts a 50, 100 or 200 Hz high-pass filter in the side-chain. Produces a Fat sound and eliminates bass driven pumping.

4 RELEASE Rotary switch that sets the release time from 0.05 to 1.2 second. The AUTO position is a program dependent automatic release time.

5 Meter Analog meter displays the dB gain reduction.

6 IN Compressor bypass (hardware by relay).

7 THRESHOLD Potentiometer that sets the input level at which compression begins.

8 HF LIFT Inserts a high boost filter in the side chain. Combined with the High-Pass it evens the audio energy in low and high bands and gives a very natural sounding to the compressed output.

9 MAKEUP Potentiometer that restores the signal level after being compressed (make up gain).

10 MIX Potentiometer that mixes the direct unprocessed signal with the compresses signal (0 to 100%) for parallel compression.

Frequency response of the side chain filters

What's in the kit?
Absolutely everything!

Passive components (resistors, capacitors, inductors),
Active components (diodes, transistors, LED's, IC's),
Front panel elements (switches, potentiometers, analog meter, knobs),
Connectors, relays,
Front panel and side panel,
All hardware.
You bring the solder (use good quality solder, 1mm diameter max) and the Lunchbox.

Difficulty level
The CP4500 kit is a medium difficulty kit. Nothing difficult but a rather large number of components.

The setup is simple but requires using a good quality AC voltmeter.


Available as an assembled Module and as a DIY project.

This Module is currently available.

$448 Price in €
2 HP
submitted Oct 14th 2019, 13:21 by Kivig