Paul's Posts — 23 October 2012
By Paul McGowan
We’re starting part 2 of our continuing story of how AC power affects the quality of our hi-fi systems, how vibrations and microphonics do the same and then part 3 will be how we deal with them all together for great benefit.
In part 1 we learned how and why power affects the sound quality – and part of that learning showed us that without perfect DC to run our amplifiers, we get a not-so-perfect output feeding our loudspeakers. This is called distortion and the term distortion simply means a deviation from what we started with – an imperfect version of the original.
Whatever was on our disc, in the form of music, needs to be amplified without distortion to make perfect music in our rooms. In other words we need to transfer a perfect copy of what was on the recorded medium and get it into the room. The closer we get to perfection the better it sounds.
We know that if we have less than perfect power, our goal of a perfect copy of the music cannot be achieved because we’ve added or subtracted something from that music. The same can be said for another form of distortion: microphonics.
Microphonics is a well understood distortion but most of us don’t thoroughly understand it and, even if we did, don’t know how to minimize it effectively.
So let’s start with the basics and understand what it is. Microphonics is the tendency of something to act like a microphone picking up sound. A good example of microphonics might be a tube in a preamplifier. If you were to open up a tube preamp, turn the volume up high and flick your finger against one of the amplifying tubes, you’d hear the sound of your fingernail striking the glass envelope of the tube in your loudspeaker. That’s the tube acting like a microphone. Tubes are notorious culprits as are capacitors. In fact, just about everything in your analog hi-fi electronics is microphonic to some extent.
I remember an ad for software you could load onto a PC that turned any sound card into a microphone (without a microphone) and allowed you to snoop in on some poor unsuspecting person’s conversations. This worked because of the tendency of the capacitors on the sound card to act like microphones and if you amplify it enough you can actually hear what’s going on in the room.
Many modern microphones of the highest quality are actually capacitors optimized for full range sound recording.
Your DAC, preamp, power amp, CD player etc. are all microphones that pickup sound and add it to the music.
Interesting, eh?
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Soundminded
The other day we agreed shape matters. I think it should be pointed out that size matters too. One thing I’ve noticed about audiophiles who do not have a strong technical background (obviously this is not the case here) is that they speak in generalities, theories, notions, sometimes true, sometimes false but always without context. So they will talk for instance about RF noise being induced in a speaker wire without understanding that even for a good antenna properly terminated to capture RF, the order of magnitude is in the microvolt range. A speaker wire terminated at a loudspeaker makes one of the most awful ineffective radio antenna’s and RF detectors imaginable. If you want to know how much RF noise is being captured by your speaker wire, you can get an excellent indication by turning off you amplifier and putting your ear to your speaker. In theory, it’s there but it’s so small no human can hear it. This is by way of example showing how audiophiles grab on to a notion and stick with it without really understanding its significance based on size.
And so in what hints at a discussion of micriphonics in wires and other components, I’m sure Paul will put it all in context of magnitude as it compares to the size of audio signal in the audible passband range, whether or not it’s sufficiently large to be audible, to what degree, how and when it happens to that degree, what can be done about it, and whether or not anything should be done about it based on its size. Engineering as opposed to audiophiledom is about tradeoffs and costs. Is a problem large enough and important enough to be worth addressing? How much improvement of useful value to you get? What other factors have you affected possibly for the worse, that is in solving one problem have you inadvertently created another? And how much bang do you get for the buck especially in light of what it could have been spent on elsewhere?
Among the sillier audiophile notions I’ve encountered that have had far more impact on advertising copy than on real world performance is that you can’t have too much bandwidth. One manufacturer once advertised an amplifier, flat, DC to light. Well almost, it was claimed flat to over 1 megahertz. Bad engineering idea. That made it vulnerable to all kinds of serious problems including real suceptabilty of RF noise induction. Another is that you can’t have too much amplifier power. That concept also flies in the face of solid engineering judgment. Concepts like these are notions that arise from insufficient knowledge. Everyone who assembles an audio system from components is acting as an engineer. But real engineers know size matters and they don’t build for what they can’t use or is of far less value than the costs they incur imposes. So in that context I hope this discussion will include an order of magnitude of size as well.