Rotel RB-850 hifi power amplifier
The Rotel RB-850 power amplifier appeared in the 1980s and offered excellent value for money at just £130 A dual mono 50W/channel, on test it was shown to produce over 67W into 8 ohm speakers, 97W into 4 ohm and nearly 130W into 2 ohm speakers.
These amplifiers offer bridging by simply flipping a switch on the back panel. Bridged operation combines the "left" and "right" amplifiers into a single amplifier. One is fed with an inverted signal. You connect your speakers between the two red outlets from the amp, ignoring the black outlets and have double the voltage output. It is hard on the amplifier as effectively each amp is seeing only half the load impedence of the speaker being driven, so it is called upon to produce more current for the same voltage. For this reason, it requires a robust amplifier design.
I bought two of these amplifiers and initially used one each for left and right driving Monitor Audio R100 speakers. I also tried splitting the crossover of the R100s and "passive bi-amping". Later, I changed my speakers for the wonderful Quad Electrostatic ESL57s. While electrostatics are known to be a difficult load due to an impedance curve which varies from a short at DC through a peak of 40 ohms or so around 100 Hz and falling back to under 1 ohm at 20 kHz. The "small rotel" is unfazed and makes a very good job of driving these speakers. As it is a good idea not to apply too much voltage to Quads, I used just a single amp in conventional left/right form. While the Rotels have been replaced by Quad amplifiers for the electrostatics, one of them is still in use to drive a home made subwoofer in bridged mode.
The Quads are not the only difficult load I've driven with these speakers. I've found the amplifier to be pretty much bullet-proof with any load. A few years ago I had a problem with a TV which would not degauss. I hooked one of rotel amps to its degaussing coil sent tones through at decreasing volume until the set was degaussed. It's a pretty extreme load, a reactive coil and a short at DC, but the Rotel didn't mind at all.
Apparently lots of effortless power !
The amplifier is out of use at the moment because I had the chance to swap it for the Quad amps designed for my speakers.
The amplifier has a dual mono design. You'll see4 that there are separate transformers, bridge rectifiers and PSU moothing caps for the left and right channels. Also note the useful chart in the bottom right corner telling how to reconnect the transformers for different voltages. The brown gunk around the main smoothing caps is glue which was always there.
This shows one channel. The layout at the other heatsink is nearly identical. There is evidence of some care with selecting components. See the polystyrene and other plastic capacitors and large output transistors, two of each in parallel per channel. 2SB817 are PNP and 12A, 160V, 100W.2SD1047 are NPN, 12A, 160V, 100W. Not a bad complement of output transistors for an amplifier rated at just 50 W per channel.
A June 1988 review of the RB-850 and matching RC-850 pre-amplifier in Hi-Fi News and Record Review appears below. The RC-850 pre-amplifier was always considered to be the weaker half of the pair and I never bought one. Instead, I built a passive preamp and a John Linsley-Hood design phono pre-amp which are still used with my RB-850s.
What the review doesn't tell you is the weight ! These amplifiers weigh over 8 kg each.
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