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Old 27th October 2007, 06:51 PM   #1
john65b is offline john65b  United States
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Default Power Supply Question

Hello all, got a quick PS question for all you smartguys...

Just cruisin' the ebay pages and saw an amp (Forte Model 4) that had a power supply pic that made me question the rail voltages - its pic is attached...

Its a single tranny with dual secondary, dual rectifiers and two pairs of smoothing caps (values are not important for this discussion)

Please read next post
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File Type: jpg 8efa_12.jpg (37.4 KB, 281 views)
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Old 27th October 2007, 06:54 PM   #2
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Now from what I think I see, the both amp channels are independently fed by each pair of the Powersupply caps

Now let's assume dual 20VAC secondaries...as shown on the crappy schematic I have drawn (see attached pic)...


Now, the $64K question is what are the rail voltages - around +/-28VDC or +/-56VDC?

Or do I have the PS Schematic all wrong? Maybe V2 and V3 are connected (CT) somewhere and connected to the ground plane?
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Old 28th October 2007, 01:39 PM   #3
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Anyone?
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Old 28th October 2007, 01:51 PM   #4
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You have it drawn incorrectly. There will be center taps from each of the transformer secondaries going to ground. Also the leg of the tranny marked 0 is now 10VAC and the 20V is 10V also
So what you have is essentially two 10-0-10 secondaries, giving you separate +/- 14 VDC rails
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Old 28th October 2007, 02:08 PM   #5
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Audiopro,

Thanks for the reply, and totally see and agree with you if each secondary had its own centertap, but a tranny with dual secondaries and each winding has its own centertap? Does such an animal exist?

I have never seen a toroid with dual secondaries each with their own centertap...of course I could be mistaken...

Sorry for the lame questions.
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Old 28th October 2007, 02:19 PM   #6
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It's difficult to see, but I suspect there is dual secondaries each with it's own centre tap. The black leads.
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Old 28th October 2007, 03:00 PM   #7
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It's pretty easy to build a toroid with dual center tapped secondaries. These are not off the shelf transformers, they are custom builds. This would also be the only way to build an amp with dual polarity DC rails competely separate from each other for each channel. Otherwise there would have to be common supplies, and thus not dual mono configuration.
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Old 28th October 2007, 03:52 PM   #8
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How do you know it's not a regular center tapped transformer feeding separate bridges and caps?

I can make out two green wires and one red one on the front of the tranny. The two green come out the other side from under the aluminum plate, but the red does not. That leads me to believe the red is the center tap and therefore connected to the aluminum plate on the underside.



Ahh yes, if you look very closely the rectifiers are "daisy chained". It is a center tapped tranny.

What do I win?
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Old 28th October 2007, 04:03 PM   #9
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A blind date with Stevie Wonder?
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Old 28th October 2007, 05:33 PM   #10
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OK, I see the rectifier on the right has a green wire to the AC connection and then goes to the other rectifier on the left, while the other rectifier has the same, so they are daisy chained...

Good deal! I have been scratching my head for hours on this...I am a power supply retard
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