Paul’s rule number two
In yesterday’s post we learned a little something about capacitors. They only work with AC voltages, like music signals. They do not respond to DC–battery voltage. The lowest frequency of AC that can pass through one of them (bass notes), is determined by whatever they are connected to and their size. The bigger they are, the lower bass notes they permit. These characteristics makes them incredibly useful in a number of application, from filters to coupling capacitors. It is the coupling capacitor we’re focusing on.
The only difference between AC and DC voltage is movement. AC moves from + to – at regular intervals (called frequency), while DC is un-moving, like the Rock of Gibraltar. A capacitor is a very handy tool for designers who want to separate DC from AC. When it comes to amplifiers, we’re only interested in getting the music from one end to the other. The power supply DC required to operate the amplifier needs to stay behind. Capacitors serve this purpose well. But they have limitations and they are not without affect on sound quality.
Paul’s rule number One: don’t be afraid to break the rules.
Paul’s rule number two is a take off on this old chestnut, no good deed goes unpunished.
Rule number two: No signal passes through any medium without change. Wires, capacitors, transistors, opto couplers, transformers–none are perfect, all change what passes through them.
Back in the days of early designs, coupling elements between amplification stages were common. Transformers and capacitors littered circuit boards passing audio signals. It was a safe and easy way to design circuits. As the art of audio design matured and people started acknowledging my second rule (one I certainly didn’t invent), new minimalist designs emerged that focused on removing as many parts as possible. The first candidates for execution were interstage coupling elements, capacitors and transformers, replaced by clever designs and dual voltage power supplies.
And today, most modern well designed hi fi electronics honor the minimalist design approach. But, a growing number of retro ideas are emerging: output transformers on DACs (well, at least one, our DirectStream), interstage transformers (like Mod Wright), and a bevy of other excellent sounding designs too.
We’ve learned over the years that great sounding hi fi designs are necessarily a compromise, blending the best of minimalism, new and old technology.
The constant is performance achieved by carefully listening before products get to market.
From these tenets spring fresh designs that delight the ear.