12413 Module Building Guide

12413 compressor module is our version of zener compressor based on the vintage British compressor that was built for the Abbey Road studios. It was extensively used on The Beatles records and has a distinctive and recognizable character.

One interesting feature of this compressor is the so-called "Hold" control. It's rather hard to describe how it works, but it's very intuitive once you get the actual unit at hand.

The main reason these compressors are not very popular in the DIY world is the lack of zener selection guide. A lot of problems are reported with the units being non-unity gain or not compressing properly. We spent a lot of time testing hundreds of zeners and figuring out which ones work best. As a result, we were able to produce a target zener IR curve and selection guidelines for choosing correct zeners for this circuit. We also worked out the trimming procedure for the "adjust on test" resistors and provided convenient test points that make it quick and easy to set the unit up.

Our unit is true to the original. We preserved the internal gain structure of the compressor and this made it usable for a wide range of audio material. Unlike Chandler unit, that uses Carnhill microphone transformer on the input, we opted for a proper line input iron from Cinemag. Don't get us wrong, we do not mean that Chandlers choice is bad or incorrect. We merely state that it's not what was used in the original compressor.

We managed to pack this zener compressor in our standard 1/4 1U rack space, and added some useful features:

  • Additional attack and release settings (as well as the "Slow" button).
  • High-pass filter with variable cut-off frequency in the sidechain path.
  • Both "hot" and "cold" bypass located conveniently on a single toggle switch.

PLEASE NOTE: complete building Guide will be available after purchasing of 12413 module PCB set.

Here is the rough estimate of costs of the 12413 zener compressor module:

Difficulty      
Electronic parts (Mouser, US pricing) 250$
Audio transformers 97$
Front panel, PCBs, mounting brackets 70$
TOTAL (shipping not included): 417$

Approximate build time - 8 hours.

GroupDIY support thread here.