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Old 7th June 2012, 02:18 AM   #1
autox is offline autox  United States
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Default Variacs - testing, listening?

Hi,

First, I'm a total noob on all things diy audio, so apologies if I've missed something obvious.

Two questions:

1) Can anyone recommend a good way to test a kit power supply before connecting to an amp? If the diy audio bug doesn't bite me hard enough to build more than one kit, I'd rather not buy a variac.

The source referenced here is dead. Would a home lighting dimmer switch work?

2) If, on the other hand I pick up all 3 solid state kits I have my eyes on (Patek gainclone - AB, classdaudio.com - D, diyparadise Charlize - T), it would be great if they could all share a power transformer - after all, I can only appreciate one at once. A variac seems to fit this bill. Is there any problem using 2 variacs in lieu of a single center tapped transformer?
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Old 7th June 2012, 06:40 PM   #2
sbrads is offline sbrads  United Kingdom
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1) If it's a normal linear PS, I would just check for any shorts across the smoothing cap(s), perhaps even connect a 9v small battery across it (or them) in the correct polarity and check there's not a large continuous current flow. Then, assuming there's a fuse in the mains side (a 1A may blow too easily, try 3A), switch it on and measure the output DC voltage(s). If you meant testing under load, light bulbs are handy, a few 12v car ones might do. A dimmer switch is a nasty device that chops up the mains and a transformer will have to deal with high levels of harmonics and possibly a DC shift it's not designed for, so I wouldn't try it, it's also likely to blow the dimmer with back emf.

2) Two variacs would require you setting up the 2 DC voltage outputs BEFORE connecting to the amps as you don't want much imbalance, say <0.5v difference between +ve and -ve supplies, so a bit less than that if measuring the AC instead.
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Old 7th June 2012, 06:44 PM   #3
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Variacs will not isolate the amplifiers from the AC mains.
That would be very dangerous. You could use a variac to control the intput voltage to a transformer with a center-tapped secondary.
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Old 8th June 2012, 06:56 AM   #4
autox is offline autox  United States
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Thanks for the responses. I think I'll hold off on picking up a variac, use some of the suggested methods for testing and just focus on one amp kit to start with.
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