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Old 30th September 2003, 04:39 PM   #1
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Default Is this statement about autoformers correct?

Taken from an article about transformer volume controls:

This is not a new concept, though it is fairly new to the home-audio market. Pro-audio components in recording studios have used transformer attenuators for years, mainly because they decouple DC, eliminating ground loops, and can handle either balanced or single-ended signals. Aside from a few low-cost products from Antique Sound Labs and others that use voltage attenuators known as autoformers (basically a single coil with a movable center tap -- you get voltage down-conversion but not current up-conversion), several true transformer attenuators are available as kits.
Is that correct that an autoformer doesn't multiple the current by the turns ratio? If it is, where does the difference in power go? There's hardly any DCR to burn this as heat (and in an ideal case there wouldn't be any DCR anyway). It can't be stored in a magnetic field somewhere, because this is lost power and not lost energy (the field would eventually 'fill up' or saturate). So... am I missing something fundamental here, or is the statement incorrect?
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Old 30th September 2003, 05:00 PM   #2
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It's a pretty sloppy piece of writing. You certainly can get up-conversion with an autoformer. And the I-V relationships are exactly as you surmise, not what's written there.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 30th September 2003, 05:06 PM   #3
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Thanks. The prospect of having to re-think everything about how my linestage worked wasn't very appealing. And besides, it's always good to be right
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