A good working example of well-protected design is the RSD 800b one of which drove four 18" scoops for more than fifteen years without complaint, apart from the many shorted units that the optimistic hoped would work (amplifier was 'retired' when venue closed).
What was considered important by the author was the damping on the output current-limiting inputs. It was felt that the allowance of permissible transients produced a 'cleaner sound' compared to similar amps that would clip sooner.
Quasi-complementary designs such as this, ease output device choice and availability, given the commonality in both halves of the output stage (2N3773 = 150W, 16A, 160V, 700kHz, hfe 15-60). In high power installations little thought need be given to the transfer characteristic which can become markedly assymetrical at low signal levels, then contributing to distortion. High power output stages utilising 'slower' output devices like these are best reserved for bass and midrange use since higher frequencies can suffer degradation.
A fair number of manufacturers have followed this pattern, a smaller Phase Linear 400 being an almost exact copy, but poorer quality build, of this circuit. Are there any suggestions for links with regards the evolution of these designs?
Note (RSD) use of low-noise types (ME4102) for input pair and bias, and high-voltage types for drivers (300V RCA411s) and elsewhere. Both input and output are DC coupled, feedback gives -3dB at 5.89Hz. Other poles include 589Hz, 28k4Hz, 72kHz and 1M89Hz. The protection uses 33.8Hz, 13k2Hz and 1M25Hz.
Provision for DC offsetting is provided on the PCBs (and on the Phase Linear!) but no components are fitted. A high-quality multi-pin connector joined both the PCBs used; one for the driver and output devices, and the other for the input and level shifting amp. The 56R resistor between signal and chassis earths reduces the possibility of earth loops induced by input wiring.
The mains transformers for this size of power supply are common, the secondary (55-0-55) suiting industrial isolation uses. A single bridge rectifier fed two 10mF smoothers. 8A fuses were used for each amplifier's supply rails and the mains in. A mains fan gave continuous airflow over the output devices and associated 'fins'. Perforated mesh was used for the top and bottom panels. Finish - black with green detail, 19" rack case, XLR connectors.
There is no doubt that this RSD's ability to run 'all day, every day' into any load, at any level, with any user and for so long without ever blowing a fuse, was largely due to the protection employed and the available airflow over it's components.
Synthnerd who used to "service these for a Studiomaster main dealer..." says "Output stage wipeouts were common...", which makes it more remarkable that this one survived. The punishment was cruel and merciless and when 'better' designs died, this one always lived. Driving four 18" scoops, the smaller Phase Linear driving the mid-range and a Comberton 12-2 mixer, this must rank amongst one of the world's most powerful aphrodisiacs seen, being no doubt responsible for innumerable pregnancies, and much else besides.
Christian Zimmermann has kindly written about a clone he's renovated, views of which are shown below (note BIG elcos! - Siemens 37m6F, 75V - don't wire these in backwards).
I just want to thank you for the value-able info and schematics that you provide on your website. With this I manged to repair an outdated "RSD 800b- Do it yourself- clone" that was left over from the PA-System that my father used in his hard-rock days. Of cause inoperative for more than 20 years. Who had build this AMP is not known. You can watch the pictures I attached if you like. With the given info that it is an RSD 800b clone I came to your web-site. The driver boards are direct clones from the RSD 800b boards. Also your info that the RSD 800b and Phase Linear 400 with PL14A Driver Board use the same circuit was another clue to fix this AMP- puzzle. I changed the 2N3773 Output Transistors to ON-Semi MJ15024 types and it works fine now. :)
So thanks again and keep up the good work.
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