The Amtec Model 852 is a single-channel variable-mu tube compressor inspired by old Altec unit and similiar historic designs.
It shares the working principle of varying bias of the first tube stage and then amplifying the compressed signal in another tube
stage in a fully balanced push-pull internal signal path, but with numerous refinements.
There are modifications to the classic circuitry that increase the range of time constants, enhance stability of the unit and add
new features.

The 852 is a feedback-type compressor, but the sidechain rectifier is not driven directly by the output stage as in classic units.
Instead, two high-voltage FET buffers isolate the signal path from the rectifier and allow for faster attack times.

It has continuously variable input and output controls, wide-range attack and release controls and the threshold control that
lets to set the internal level at which the compression starts to take place over the range of 20dB and also changes the
compression ratio from about 2:1 at low threshold settings to about 8:1 at high settings.

Apart from these typical controls, the 852 also has a knob whose function is switchable between two different settings -
Distortion and Lag.
The Distortion function allows to add harmonics accompanying the compression action.
It uses the sidechain part of the circuit - this distortion is basically the compression with no time constants. At fully counter-
clockwise position of the knob the distortion is non active with all the compression action depending only on attack/release
controls.
The distortion achieved that way is completely independent from the distortion that occurs when overdriving the input of the
unit.

The second function is the LAG function which is a unique secondary attack control. It's a continuously variable delay and filter
of the control voltage that allows the compression action to be shifted in time (lagged).
It lets to obtain very long attack times that enormously increase punch or can be used for special effects.
This function at 0 position is also effectively out of circuit.

The compression can react to all frequencies (FLAT) or can be set to lose sensitivity below 90Hz or 180Hz. It can be also
disabled in the line amp mode, but some gentle compression is still available in this mode at low threshold setting.

The unit has a hard-wired bypass and can be linked with another unit.
The input and output are transformer balanced and fully floating.
Tape Op magazine (in English):  
https://tapeop.com/reviews/gear/122/model-852-variable-mu-compressor/

Bonedo website (in German):
https://www.bonedo.de/artikel/einzelansicht/amtec-audio-model-852-test.html
Gain:                               +35,5dB
Input impedance:         20K ohms   (transformer balanced)
Output impedance:      below 150 ohms   (transformer balanced)
Recommended load impedance:    from 600 ohms to infinity
Frequency response:    flat within 0.5dB from 20Hz to 20kHz
Max output level (1% THD):     +22dBu
Compression ratio:    soft-knee, from 2:1 to 8:1 (depending on the Threshold setting)
Tubes:   (2) PCC85
Power requirements:   230V/50Hz or 115V/60Hz (factory option)
Power consumption:    30VA