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Old 5th June 2003, 10:01 PM   #11
pooge is offline pooge  United States
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The purpose of a bridge on each of two secondary windings is to reduce or eliminate any DC component from the transformer, which can occur when a single winding supplies both the + and - rectifiers. This is according to Robert Grodinsky et al, US patent # 4594561, "(W)hen a single winding is used, a small, but detrimental, current that is proportional to the difference between the B+ and the B- currents will appear in the secondary winding. This condition is produced by the non-symmetrical nature of audio signals, i.e., where the positive cycle does not contain the same energy as the negative cycle.... When two secondaries are used, one to generate the B+ and the other to generate the B-, the unbalanced current in the core cannot occur despite the B- load current imbalance."
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Old 6th June 2003, 02:21 AM   #12
sam9 is offline sam9  United States
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It seems like this is saying that music is asymetric with respect to 0V (or system ground) and that therefore the rails draw current asymetrically. So far so good, but each rail draws current from each side of the transformer, namely at a "corner" of a bridge recifier.

I'm thinking in terms of a full wave (i.e., four diodes in a diamond) bridge recifier. Maybe he was thinking of a two diode recifier. That might be different.
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Old 6th June 2003, 03:24 AM   #13
pooge is offline pooge  United States
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The patent shows two center-tapped secondaries, with each secondary having a two-diode rectifier to center tap, if I can recall. I don't have to schematic in front of me right now. I'll have to check it out again, tomorrow.
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Old 6th June 2003, 07:33 AM   #14
2Bak is offline 2Bak  Denmark
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Default same problem

I had the same problem. I use one trafo for four channels and couldn't get it work with two small 6A bridges per channel (8 total). Blew my fuses all the time. I'm sure I wired correctly (like on your schematics), no doubt about it. I couldn't figure out why it blew 'em.
So eventually I used only two larger bridges for all four channels in parallel.
Perhaps I will give it try again sometime, cause sooner or later I'll try with better MUR diodes as bridges and also do some new wiring and a few other things ;-)
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Old 6th June 2003, 07:47 AM   #15
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Maybe a dumb question, but did you take care of the relative phase of the secondary windings?

Cheers

Andrea
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Old 6th June 2003, 08:23 AM   #16
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Quote:
Originally posted by pooge
The purpose of a bridge on each of two secondary windings is to reduce or eliminate any DC component from the transformer, which can occur when a single winding supplies both the + and - rectifiers. This is according to Robert Grodinsky et al, US patent # 4594561, "(W)hen a single winding is used, a small, but detrimental, current that is proportional to the difference between the B+ and the B- currents will appear in the secondary winding. This condition is produced by the non-symmetrical nature of audio signals, i.e., where the positive cycle does not contain the same energy as the negative cycle.... When two secondaries are used, one to generate the B+ and the other to generate the B-, the unbalanced current in the core cannot occur despite the B- load current imbalance."
This is useful information - thanks. Another, perhaps more obvious advantage of using a dual bridges is it saves having the massive charging pulses passing through the star-earth point...

BTW, just to be clear, my post was talking about using seperate rectifiers for each amplifier channel - not 1 vs 2 rectifiers as discussed in the quote.

Personally, I'm slightly uncomfortable about using seperate multiple rectifiers, and I'm perhaps not that surprised that people are having difficulties with it.

Quote:
Originally posted by Andypairo
Maybe a dumb question, but did you take care of the relative phase of the secondary windings?
Andrea,

Could you expand on this a little? I'd be interested to hear your thoughts here...

Cheers,

Mark
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Old 6th June 2003, 09:40 AM   #17
Raka is offline Raka  Europe
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Relative phase?

What do you mean?
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Old 6th June 2003, 11:20 AM   #18
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I was meaning that one winding must provide the positive rail for boht amps and the other the negative.

Did you try leaving the gnounds unconnected and measuring the voltage between them?

Cheers

Andrea
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Old 6th June 2003, 12:14 PM   #19
Raka is offline Raka  Europe
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Yeah, of course I took this into account. Yes, I also measured the output, but only of the first bridge. The second one was built identical, and I didn't measure it.
BTW, I've just fitted the 220K from positive pin to ground in one channel. DC offset went to 0.00.1mV, measured with a pretty yellow multimeter.
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Old 6th June 2003, 01:41 PM   #20
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If you can't use two bridges with a centre-tapped transformer, how could you use four bridges with one transformer (even with independent windings)?

Forgive me the question, Raka, but are you a woman?
Or it's only a photo?
You must be the only one here.
It's good to see that.
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