Back in the day
Back in the last century–we could even suggest millennium–we produced a power amplifier called the 200C, with two inputs: direct and coupled. Designed by Dr. Bob Odell in the early 1980s, this was a killer amp in more ways than one. The 200C, much to our horror, had lit on fire in the living room of TAS reviewer Tony Cordesman…but that's another story for another post (and he loved the sound enough to buy the amp as well as a fire extinguisher). But I digress. Two-input power amplifiers are common practice today: one RCA, one XLR. But in the early 80's only pro gear had balanced inputs, and preamps could not be trusted. This era saw the rise of direct coupled solid state preamplifiers and they weren't always done well. They could have high levels of DC (battery voltage) on their outputs; more often than not. If you build a power amplifier that is direct coupled from input to output, a pure path without any restrictions, that amplifier will perform its task perfectly, irrespective of what you feed into its inputs; AC music signals, or DC offset from a preamp. And if a power amp amplifies DC, small problems turn into big ones that can damage loudspeakers (and set amplifiers on fire). We placed a lot of trust in our customers to first read the owner's manual before connecting their gear. We were young, idealistic (and frankly naive–bordering on stupid), back then. We asked new owners of the 200C to connect their preamp to the amplifier through the safe Coupled input first. That was their default. It would protect from any preamplifier problems upstream. The Direct input should be used only by those who knew for a fact their equipment hadn't any DC on its output. It didn't take long for owners to discover the better sounding input was not the safe one. And anyway, how were they to know if their preamp's were properly designed or not? Yikes. The difference between the inputs was simple. There was a single high quality coupling capacitor inserted between the RCA jack and the power amplifier's input. That capacitor did not help the sound, in fact, quite the opposite, but it could save the amplifier and speakers from trouble. Tomorrow, the mighty capacitor.
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