1-bridge PSU for center-tapped transformer: What's the disadvantage? - diyAudio
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Old 22nd March 2006, 04:29 AM   #1
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Default 1-bridge PSU for center-tapped transformer: What's the disadvantage?

As a rule of thumb, most PSU designs use multiple discrete bridges, and multiple windings. However, it appears (I'm going by NUUK's page here) that one can also use a center-tapped transformer with the AC leads going to the bridge, and the center tap to the power star ground.
What is the disadvantage to this method? I'd like to build a Carlos Filipe style unregulated PSU; would this defeat the point? Should I go for multiple smaller transformers, or perhaps just find a way to cut the winding in half?
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Old 22nd March 2006, 05:33 AM   #2
sangram is offline sangram  India
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Hi

I've tried to cut the Center tap winding of a transformer and it works, I had flyout leads and the windings behave perfectly, if correctly identified.

It also works fine the other way, with a single bridge and the Center tap of the transformer connected to ground. That works too, and sonically I'm not sure I can hear any difference between the two configurations.

There is an explanation somewhere on this forum which says that two windings/bridges allow each half of the supply to get its own return path to ground (I assume without having to include a big fat inductor in the said path). How that makes sense I'm not exactly sure but someone can hopefully drop in and shed some more light on it.
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Old 22nd March 2006, 09:02 AM   #3
Nuuk is offline Nuuk  United Kingdom
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I have had excellent results with a single rectifier bridge and if you have a centre tapped transformer and don't fancy opening it up to separate the windings, try it with one bridge. It's really not that inferior to using two!
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Old 22nd March 2006, 09:16 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I believe the centre tap is not inferior at all.
It gains in reducing the diode volts drop by half (from 2.8V to 1.4V). It's disadvantage is that the rectifier is never allowed a cooling rest period between alternate charging pulses.
It also allows current to take shortest route cap to cap and cap to transformer.
Theses charging currents inherently have a very high peak value and both methods of wiring need to take this into account to minimise injection of buzz into your clean ground.
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Old 23rd March 2006, 10:02 PM   #5
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Quote:
There is an explanation somewhere on this forum which says that two windings/bridges allow each half of the supply to get its own return path to ground (I assume without having to include a big fat inductor in the said path). How that makes sense I'm not exactly sure but someone can hopefully drop in and shed some more light on it.
I'm not sure how you would take advantage of this either. I suppose you could keep two separate ground busses from the rectifier all the way to the ground point at the chip. But only the ps caps (and possibly snubbers) would enjoy the separate grounds. They still need to be connected at (or before) the chip, so the signal currents for each rail are mixed anyway. Incidentally, I don't think I've seen an actual implementation that kept the grounds separate after the rectifiers, but I've hardly looked at all the implementations out there.

On a purely theoretical basis I can't see a big advantage, but we're talking about playback systems for music, so the proof of the pudding is in the eating. If using separate windings with separate rectifiers sounds better then it does, even if I can't see an obvious reason why it should. Chip amps do seem to be especially sensitive to power supplies, so who knows ...

In the end, if you've got a CT transformer then I would go ahead and use it. You can always try dual secondaries on your next project.
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