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An Early History Of Modern Power Amplifiers
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Expectations in power amplifiers have come a long way over the years.

We expect higher power, lighter weight, cleaner sound, less heat and increased reliability, all for less cost and with less weight per watt.

Power amplifiers are almost a commodity today. Large power outputs from 1U and 2U high units are ubiquitous, cheap, and one could say that there is a surfeit of choices.

BETHEMOTHS

Last century we were at an archeological dig at the site of an old sound contracting building and we found valuable historical data that we will report here.

In the mid 1960s sound contractors counted on power amplifiers that were still mostly using tube designs. We are always amused when we hear young audio guys fondly talking about the “big tube” amplifiers of those days.

If you wanted a high power rugged amplifier and had the budget, you installed a Mac 275 (McIntosh MC 275), which put out over 75 watts per channel. These were big black and chrome monsters with gigantic output tubes that generated a lot of heat.

Another choice was the Marantz 8E which was very clean sounding but less powerful. Less reliable, but also less expensive, were the Harmon Kardon Citation tube designs, which ran really hot. You could calm them down by reducing the bias toward Class B, but they would not sound as good.

For budget installations that needed “big power” there was the Dynaco 60-watt single-channel or the Dynaco Stereo 70 dual-channel units.

All of these were big, heavy, hot, and had large power transformers as well as big and large audio output transformers, yet none of these really had anywhere near the kind of power we count on today from even the less powerful members of many companies power amplifier offerings.

Hot n’ heavy (literally), from left to right, the Mcintosh 275, Dynaco Stereo ST-70 and the Marantz 8B.

The heat from the behemoths cooked the passive components, like capacitors and resistors, as well as the circuit board, and the output tubes had to be replaced every so often (often at a critical time), and then there were the pathetic distortion specifications at the frequency extremes (mostly due to the output transformers). Power amplifiers of those days are best left to ‘fond’ memories.


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