We managed to get hold of an AES paper on the limiter.
It has a few nifty tricks in it:
It is a fet limiter, but has quite an unusual configuration using 2 fets & an op amp, which sort of self trims the 2 fets somehow. They call it a basic fet reciprocal control system.
The SAD1024 has 2 delay lines in it, & this design feeds antiphase signals into each to reduce distortion.
Many of the components date this unit to probably the late 70's. It is full of 741's & 1458's. Before most of you recoil in horror it actually sounds quite nice, & has a respectable (for the day) -70dB noise at output, less than 0.25% distortion up to the max -20dB compression.
For those of you who have AES membership the AES paper is dated `76 & is titled "A feed forward controlled delay line limiter" by CJ Evans & J Dawson. Apparently J. Dawson helped start up the ARCAM hifi company in Cambridge, UK & still works there.
