MORE POWER
It was evident by 1975 that the human limits to designing power amplifiers was just a bit over 150 watts per channel.
More than this and the loudspeakers would fall apart, the output tubes and transistors could not be fabricated, and your ears would fall off (this part is still unfortunately too true—listening at today’s levels is self-defeating in that you lose your hearing).
So how did one achieve higher sound levels? High efficiency loudspeakers are always a good approach, but back then speaker power handling was not what you expect today.
More (total) power can be achieved with an electronic crossover (bi-, tri and quad-amplification) using separate power amplifiers for the low frequency loudspeakers (woofer) and the high frequency devices (tweeters and/or compression driver/horn assemblies).
Also, with bi- and tri-amplification, you eliminate the passive high-level crossover network at the loudspeaker, which has some power loss. Plus, you had a separate amplifier for each loudspeaker, which is still not a bad idea.
At least all this was the case until Bob Carver came on the scene with his Phase Linear 700 power amplifier. Promising 700 watts (more or less), these units were relatively lightweight, had a relatively tiny power transformer, and minimal heat sinking. The Phase Linears sounded pretty good, with the sense of unlimited power, or at least a lot of peak power.
Instead of using transistors intended for audio, the Phase Linear had a load of video amplifier transistors that could take very high voltage. With the power transformer primary and secondary having close turns ratios, its purpose was more of line isolation then voltage/current transformation and so it could he very small, light, and inexpensive.
The lack of heat sinking could be described by some as criminal, especially as no fan was included.
I guess Phase Linear figured that everyone would figure out a fan was needed in the first few minutes. For many, this realization came on the opening night of a disco installation, with the amplifiers arriving only hours before opening. “Finally,” one would think, “enough power to drive those Bose loudspeakers!”
Luckily, in many cases the fire marshall came by in the nick of time and temporarily closed the joint down for the usual fire code violations. The amplifier cooling fans were usually installed later that same night. Early Phase Linears liked to blow up, shorting out from input to output, and often taking out the loudspeakers for good measure.
Eventually most of these problems were worked out. In any case, the age of big power and (relatively) light amplifiers had arrived.
Neil A. Shaw works with Menlo Scientific Acoustics in Topanga, CA.
