The compressor uses zeners as variable resistors. Changing the amount of current that passes through the zener changes its impedance (this is a well known fact). If you put a zener in the circuit which uses resistance as its gain set element, you can then easily change the gain by just applying a spreset amount of current through the zener. Sounds simple, right? Well, actually it's easier said than done.
The main problem is - you guessed it - the zener itself! And this explains why these compressors are not very common among DIY community. Very few builders are prepared to sift through dozens of available zener models to find the right one for this circuit. You probably heard about common problems of these DIY 12413 - low signal at the output or very loud noize (or both). These problems are the indication of the not suitable zeners in the circuit.
We have tested close to hundred different zeners (including original NOS HS2051) and found a few species that behaved very well in the circuit (that is, had low noise, low distortion and an IR curve that is close to the original parts). We also determined the critical parameters for precise matching of these zeners.
Finally, we are proud to announce that the 12413 zener compressor module is ready for a public release!
Here are the features of our 12413 compressor module:
- Two pairs of matched zeners included.
- Based on original EMI 12413 schematic
- Original Hold control
- Additional attack and release options (including "Slow" button)
- Both "cold" and "hot" bypass
- Switchable sidechain high-pass filter
- LED indication of output level and gain reduction
- Link to allow stereo operation of two units