Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Inc.

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.

Asheville, Walnut Cove, Biltmore Forrest and Western North Carolina’s Audio and Home Theater specialists present Cane Creek AV and Paul McGowan – PS Audio, Inc.

Optimizing limitations

Capacitors are limited in what they can do. And it is this limitation that makes them so valuable in many applications.

A capacitor is a rather simple device, and has nothing to do with flux; Doctor Emmett Brown’s time traveling DeLorean notwithstanding. The easiest way to describe its construction is to picture a sandwich rolled up into a tube. The two pieces of bread are conductors, the meat is an insulator. Here’s a picture of a film capacitor.

This was made by rolling our conductor/insulator/conductor sandwich into a tube and attaching a lead to each end of the roll. There are other types of capacitors, used mainly in power supplies, that are called electrolytic capacitors, but they are based on the same principals of the conductor/insulator sandwich, executed with chemicals rather than film and metal.

A capacitor only works with AC. Put a capacitor in series with a battery and nothing passes through the capacitor. But put an AC signal, like that found in music, and it passes right through as if the capacitor were a wire. How does this help designers? There are many cases where DC and AC are present at the same time, like the output of a tube or transistor gain stage. In most designs the DC is a problem for the next stage, the AC is all that we are after. Passing the signal through a capacitor at the output of a gain stage allows the AC to get through, but the DC is locked out. Just what we want!
But, of course, life’s rarely so simple as all that. Capacitors have limitations. Depending on their size and the load they are working into, only music above a certain frequency will pass through the capacitor. Lower frequencies, like bass notes, are left behind, along with the DC. This can be good if you want to have a rumble filter, bad if you want all the bass present in the music.

This frequency limitation can be put to good use, because if we want to make a filter–to eliminate frequencies above or below a certain point–the mighty capacitor is just what the doctor ordered. Its limitations are its strength.

Tomorrow, how capacitors were used before direct coupled circuits.