IGS Audio ZEN is a dual-channel stereo Zener diode compressor designed by Igor Sobczyk — a Polish interpretation of the diode-based compression circuit used in Abbey Road Studios during the Beatles era. The signal path runs through 14 bipolar transistors per channel and 8 hand-matched Zener diodes controlled by a perfect feedback loop. Custom IGS transformers on input (10 kΩ) and output (75 Ω), combined with an internal linear power supply, complete the design in a 2U chassis with fully stepped controls on Elma switches.
Zener Compression — Variable Ratio and Limiter Mode
Zener diodes don't behave like VCA or optical elements — the breakdown threshold is physically fixed, so ratio changes automatically with input level and Input Gain setting. Light drive produces soft, musical compression; deeper Input Gain increases gain reduction and sharpens the knee. This means Input Gain is simultaneously a ratio control, exactly as in the original EMI designs. Switching to Limit mode sharpens the knee further and increases transient control, moving the response toward hard limiting while retaining the Zener tonal character. Attack range of 8–100 ms with a ×4 multiplier covers up to 400 ms; release of 10–2000 ms with ×4 reaches up to 8 seconds. That spans from fast transient control to extremely slow, invisible level riding. The sidechain HPF (60/90/120/240 Hz) separates low-frequency content from compression control, preventing pumping on bass-heavy material.
AMP Mode, Wet/Dry and M/S Processing
Setting the sidechain to AMP at minimum input passes signal through the amplifier and compression circuit without triggering actual gain reduction — the result is THD saturation adding transformer harmonics without any dynamic effect. A direct way to add density and color to a bus or track with zero compression. The OpAmp M/S encoder/decoder enables independent compression of Mid and Side channels — a precise mastering tool for controlling center density and stereo width separately. The Wet/Dry control (0–100% in 4 dB steps) implements parallel compression without an external mixer. Hard-wire bypass per channel removes the circuit from the signal path entirely when disengaged. VU meters display gain reduction independently for L and R.
KEY FEATURES
- Dual-channel Zener diode compressor — 14 bipolar transistors per channel, 8 hand-matched Zener diodes
- Feedback Zener compression — variable ratio dependent on Input Gain setting
- Comp and Limit modes (switchable)
- AMP mode: THD saturation without compression
- Input Gain: 1–24 dB (stepped, Elma switch)
- Output Gain: 1–24 dB (stepped, Elma switch)
- Attack: 8–100 ms, ×4 multiplier (up to 400 ms)
- Release: 10–2000 ms, ×4 multiplier (up to 8 s)
- Sidechain HPF: 60 / 90 / 120 / 240 Hz (switchable)
- Stereo Link: yes
- M/S Processing: OpAmp encoder/decoder
- Wet/Dry Mixer: 0–100%, 4 dB steps
- Hard-wire bypass: yes (per channel)
- All controls stepped — 24-position Elma switches
- Custom IGS transformers on inputs and outputs
- VU meters: gain reduction L/R
- Internal linear power supply, 115/230 V AC selectable
- Format: 2U rack; made in Poland
TECHNICAL SPECIFICATIONS
- Type: stereo Zener diode compressor, feedback topology, 2U rack
- Input impedance: 10 kΩ (balanced XLR, IGS Custom transformer)
- Output impedance: 75 Ω (balanced XLR, IGS Custom transformer)
- Operating level: +4 dBu
- Max input level: +22 dBu
- Max output level: +26 dBu
- Dynamic range: >100 dB
- Noise level: –99 dBu
- SNR: >90 dB
- THD+N: >86 dB
- IMD (SMPTE): >90 dB
- Crosstalk: 80 dB at 1 kHz
- Max gain: 36 dB
- Max gain reduction: –35 dB
- Attack: 8–100 ms (×4 = 32–400 ms)
- Release: 10–2000 ms (×4 = 40–8000 ms)
- Frequency response: 20 Hz – 20 kHz, ±0.1 dB
- Power: internal linear PSU, 115/230 V AC, <30 W consumption
- Shipping weight: approx. 9 kg
- Made in: Poland
