Recently I promised we’d jump into Class D amps and how they work – but before going there I wanted to take a short detour – one that a few of you have been on my case to get out in the open for a while: direct coupling.
I really hesitate to write “back in the day” because it makes me feel like such a geezer, but if the shoe fits …… Anyway, there used to be a time when just about every piece of audio kit was capacitor coupled on its input or output or both. A few brave souls started direct coupling their products and featuring the removal of this capacitor as a major selling point and PS Audio was certainly amongst those that stepped out into the cold and unforgiving land of direct coupling. So what does it mean and why do we care?
Everything we listen to in music is AC – meaning it’s electricity that moves back and fourth between positive and negative rather than sitting still – and not DC like that from a battery. If you connect a battery up to your stereo’s input you won’t hear anything because it’s not moving – and we all know that music is movement – if the musicians just sit there quietly we don’t hear any music.
What we want is for a preamp or power amp to amplify only the AC, or the moving electricity, and we do not want it to amplify or have anything to do with DC on its input or output. Seems simple enough.
Why is this important? If your power amplifier somehow puts DC (battery voltage) into your loudspeaker, you’ll fry the speaker’s drivers because their magnets will heat up and die. Not a good thing to have happen and amp manufacturers forever have been very careful to make sure this did not occur.
Capacitors will not pass DC – they will only pass AC – placing one of these on the input to your preamp or power amp blocks any potential DC that might be lurking around – not that many products had DC but just to be careful it was good engineering to add a capacitor and it meant that no company had to worry about DC on their customer’s speakers. The mighty capacitor saves the day and life’s good and safe for amp designers.
Except. Except capacitors have a sonic signature that isn’t beneficial – and even the very best capacitors don’t sound as good as no capacitor.
It didn’t take too long for audio designers to figure out that removing these input and output capacitors provided significant improvements to the sound quality. We referred to this “feature” as Direct Coupled.
But what of the DC? Weren’t we still worried about getting DC and if so, what would we do about it?
I can recall that one of our early products, the 200C power amp we spoke of a few days ago, had two inputs: a Direct Coupled and a Capacitor Coupled input. User beware. That took the issue from our hands into the customer’s hands – but most customers didn’t have a clue what it all meant – and rightfully so.