Variable Mutual Conductance Compressor

Transformerless Tube Compressor



Schematic

4D SOUNDSCAPE

Sidechain schematic

4D SOUNDSCAPE

Power Supply schematic

4D SOUNDSCAPE

Gain Reduction Meter schematic

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Compression Curves and THD @ 1kHz

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Output noise at max gain (19 dB), 0 dBFS = +20 dBu

4D SOUNDSCAPE

Specifications:


Input impedance ≈ 12k ohms, balanced line-to-line, *


THD < 0.25%, +20 dBu, 20-20 kHz, unity gain, no compression, **


S/N = 80 dB re +4 dBu, 22 kHz BW, unity gain, ***


Crosstalk = -60 dB, 20kHz, +4 dBu, channel-to-channel


Frequency response = 20 - 20 kHz, ±0.5 dB


Maximum gain = 19 dB, ****


Output impedance < 300 ohms, balanced line-to-line


* INA2137 Internal resistors are ratio matched but have ±25% absolute value

** Will heavily depend on the tubes used

*** Should be better than 80 dB unless the tubes are very noisy

**** Will depend on the tubes used and plate resistor value used



Further reading:



Graeme - Applications of Operational Amplifiers 3rd generation techniques. Pages 52-53 (pdf pages 70-71)


Audio line receiver impedance balancing using a 2nd diff amp


Compression and Limiting, Elements of sound recording by John G. Frayne and Halley Wolfe.


Säätöputkikompressorit (in Finnish)


Contact

ghrdotfi@gmail.com