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Original Message

RE: A salute to Dickie and Macovski

Posted by cpotl on November 4, 2012 at 19:18:22:
Hi Banat,
Let's proceed with an academic discussion, with the sub-text that the isolation transformer is in use...
I think there tends to be an excessive paranoia about the use of direct mains-connected equipment. It is true in the past there was no polarisation of power plugs, and that therefore the polarity of which wire in the power cord would be live and which neutral was essentially random. That is not the case any more. If one were to verify the proper polarisation of a power socket today, then it would remain the same tomorrow, and indefinitely into the future.
Anyway, as Dickie and Macovski say, the need for caution is eliminated by using an isolation transformer.
Of course, one could always construct scenarios where some something goes wrong: "what if an unknowing person took the amplifier and plugged it into a different socket that was wired the wrong way?" Actually, one could build in safeguards to protect against that. But in any case, one could construct scenarios where *any* mains-powered equipment could become dangerous: "what if an idiot child took the equipment on an extension lead and climbed into the bath with it?", for example.
About hum, I've only recently been making detailed measurements for my various OTL amplifiers. My day-to-day amplifier is a totem-pole design by Tim Mellow. My DVM is pushing its limits to measure any hum at all on the speaker outputs. I'm finding about 300 microvolts, which is just on the borderline of what my meter can detect. There is certainly no audible hum, even with Lowther speakers of about 100dB (1 Watt at 1 metre) sensitivity. Next time I have my circlotron running, I'll measure that. I would agree that, other things being equal, one might expect a circlotron to do better than a more asymmetric topology. Of course, in practice other things aren't equal, but I'll be interested to see how it fares in comparison.
Best,
Chris