Behringer MDX-8000 Combinator

I just picked up one of these so I figured I'd put some info online:

It is a multiband (4 band) compressor/limiter.

High Level Audio Path:

Bands:
The four bands are fixed. The crossover frequencies between bands are 180Hz, 750Hz, and 3.5kHz. The filter slopes are 24db/octave.

Equalization:
Each of the four bands has an input LEVEL control knob. This knob can be used to equalize the bands against each other. The adjustment range is +/- 6dB.

Compression:
Compression can be enabled or disabled for each band using the corresponding IN/OUT buttons.
Each band is split into two controls paths. One goes through a fast power integrator (compressor) and the other goes through a slow power integrator (leveller).
There is a PROCESS BALANCE knob that determines how much of each integrated signal type is passed to the VCA.  The fast signal compresses peaks like a fast attack compressor would, the slow signal compressor has a longer release and "levels" passages of sound. A PROCESS BALANCE knob lets you mix the two different controlling signals.
There is a global (applied to 4 bands) THRESHOLD and global RATIO knob that is applied to all 4 bands.
Compression Ratio ranges from 1:1 to 6:1.
There is a single OUTPUT level control knob that is also global and affects all 4 bands together. It is post-compressor and pre-Limiter (all of these precede the band summing mixer); so the OUTPUT knob will load the four Band Limiter's inputs.

Limiting:
Each band has its own limiter that follows its compressor section and the Output Level control. The four Band Limiters are controlled by a single "PEAK CEILING" Threshold knob to set the limit. A DENSITY switch reduces the limiter's release time (window) to allow for more content to rise closer to the ceiling. If the limiter is overloaded, it will reduce the signal to prevent severe clipping distortion.

Summing:
The 4 bands are summed after the compression and limiting sections.

Programmable Clipper:
I'm not 100% certain on this sub section. It is post-summing-mixer and affects the total signal. Perhaps it is tied to the clip protection feature of the Limiter.

Input/Output Metering:
The rightmost IN/OUT BAR METERS monitor the total input and output powers. The PROCESS IN/OUT button acts as a bypass switch. When the unit is active, the meters show the output. When the unit is bypassed, the meters monitor the input (which is also the output as the unit has a hard bypass relay).

Compression & Limiting Metering:
The other GAIN REDUCTION bar meters monitor the amount of attenuation. The source to be metered is determined by the 4 SET buttons. Each band has its own SET (select) button.  With no SET buttons pressed, the meters show the total attenuation applied to the total signal.  If a SET button is pressed, the meters then monitor the attenuation for that band only. If more than one SET button is pressed, the meter simply monitors the lowest frequency band and ignores all others. NOTE: Set buttons have no effect on sound (which seems to confuse some users).

Pre-Emphasis
The rear panel has two Pre-Emphasis buttons that seem to set Pre-Emphasis to be 0, 25ms, 50ms, 75ms (I'm not sure on the 0). Pre-Emphasis is an FM broadcast term where the spectral output has an equalized slope/tilt with the high frequencies stronger than the low. The FM receiver has a de-emphasis circuit with the inversed slope/tilt.  The R/C time constant of the simple filter slope is referenced as 25ms, 50ms, etc. This concept is similar to RIAA phonograph filtering and tape noise reduction. Note: The MDX-8000 block diagram shows both pre-emphasis and de-emphasis ciruits so perhaps there is more to this than easily inferred.

Background:
A blurb from SOS magazine: RME head of development, Matthias Carstens was the head product designer for Behringer. At Behringer, he designed the Edison, the Supressor, the Ultramatch and his personal favourite, the Combinator, a four-way compressor/limiter with more than 1000 (1600) components inside.

From the great Uli Behringer on a gear website:
"Around 1990 we launched the Combinator MDX8000, a 4-band mastering compressor/limiter/leveler.
The Combinator contained close to 2,000 (1,600) parts and the unit was so complex that it forced us to make some true step changes. In order to design the complex print circuit board, I had to acquire a PCB design software called Topcad which was a fantastic tool, however PC’s in those days were so slow and monitors so low-res that it took me ages to complete the design. I like to take the opportunity and give credit to Matthias Carstens who worked with me on the circuit design. Matthias is a brilliant engineer and later set off to found the reputable company RME.
We subsequently also had to purchase our first auto insertion machine as the sheer amount of components would not allow for a manual process anymore. I still remember that the test and alignment procedure took over 1.5 hours and made us purchase our first Audio Precision System One test system.
I am very proud that to this day the Combinator is still highly sought after and has become a classic in mastering studios and broadcast stations."

Documentation:
Owner's Manual

Some photos from online:

Toroidal transformer, IC sockets, trimmers galore, a thing of beauty!
Are those through-holes for transformers near the back panel jacks?