This is the Orban Section of
The Overload Prevention System - 1972
Bob Orban recalls:
The "overload prevention system" was, in a broad sense, a precursor to the Optimod 8000. I built it in about 1972-73 for a college friend who had bought a class A FM, which was known at the time as KPEN (Los Altos, CA, 97.7).
I had done an earlier FET multiple-time-constant limiter project while doing my master's degree at Stanford (broadband only). The OPS was based on this broadband front end followed by a pre-emphasized clipper. It had very low measured distortion (unlike the Audimaxes and Volumaxes of the time), so it was quite transparent-sounding on the air.
The big limitation was the pre-emphasized clipper, which prevented the unit from getting too loud without HF distortion. On the other hand, it retained the brightness of the program material despite the 75us preemphasis curve.
Of course, since it had no overshoot compensation, it was at the mercy of the 9th order elliptic lowpass filters in the Collins stereo generator it was driving. In fact, the extreme overshoots induced by the pre-emphasized clipper's waveforms was one thing that led us to investigate the whole filter overshoot issue, and which led eventually to the 8000's overshoot compensated clipper/filter.
The 8000, of course, added an HF limiter, overshoot-compensated clipper/filter, and a stereo generator to the system. But the 8000's broadband compressor/limiter was closely based on the one in the OPS.
Chris Holt comments:
The station Bob mentions (KPEN) was the station I was contracting with at the time. I had seen this apparently home-built limiter sitting in a pile of stuff at the transmitter site, and was amused at the crude "ORBAN" name on the front. I thought it was a joke - someone built a limiter in their garage and stuck the Orban name on it.
When I was cleaning the place up, rather than throw it out I brought it home, thinking that at least I could use the chassis and/or parts for something. It sat in my garage for a couple of years, and it wasn't until I heard Bob talking about the OPS that the light went on. That evening, I dug it out, opened it up, and found this had indeed been built by "Orban".
Got it cleaned up, brought it into the office, hooked it up and it still worked, aside from some very dirty pots!
Bob did seem pleased to see it again. It now (well, at least when I was last there) sits in a rack with every other model of Orban processor made; a little museum of Orban history.