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Old 8th December 2007, 08:15 PM   #1
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Default Using virtual center tap for a power amp ?

Hello

I did found a nice big 50 volt ac power transformer, but there is no center tap and it would be quite trouble to take it apart to do a center tap.

I know that we can do virtual center tap by using two big capacitor, like the drawing I've done.

But is it ok to use that virtual center tap for a power amp, is there any possible problems ?

Thank

Bye

Gaetan
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Old 8th December 2007, 09:16 PM   #2
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Sorry, but that won't work. You need a low impedance DC path, so you'd have to do a divider and a whole 'nuther power amp to derive the ground.
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Old 8th December 2007, 09:50 PM   #3
srh is offline srh  New Zealand
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Hi Gaetan,

Quad have been doing it for years. Google for QUAD 306 to see how they do it.

Cheers,
Steve.
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Old 8th December 2007, 11:26 PM   #4
Nrik is offline Nrik  Denmark
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Quad thing looks strange to me. - Does that really work?
But you can definitely do this:

Solution 1)

------------>|-------------- +70V
|
|
-------------------------------------- 0
|
|
--------------|<-------------- -70V

...gives you double voltage which might limit the use, and also gives you only half the rectified signals, as positive and negatives are split. This gives more ripple when you use heavy currents.

Solution 2)
Use it for a singlesupply poweramp, one with a capacitor in the output. It is not as bad as you would think.
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Old 8th December 2007, 11:39 PM   #5
srh is offline srh  New Zealand
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@ Nrik,

There have been alot of Quad amps built and sold so I suppose the technique works.

Cheers,
Steve.
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Old 8th December 2007, 11:57 PM   #6
Nrik is offline Nrik  Denmark
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By studying it closer I can now see, that for the speaker output the two capacitors act as the output capcaitor in a single supply amplifier.

Hmmm you are right it works, but the capacitors are in the signal path.
I would keep the T11/T12 arrangement, if I would ever copy this.
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Old 9th December 2007, 12:19 AM   #7
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hi,
take a closer look at the voltage divider provided for the rail caps

http://www.audio-circuit.dk/Schemati...0Schematic.pdf
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Old 9th December 2007, 12:47 AM   #8
srh is offline srh  New Zealand
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Nrik,

I would think that the power supply caps are in the signal path regardless of the power supply technique used.

Cheers,
Steve.
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Old 9th December 2007, 12:50 AM   #9
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The Quad circuit is cute and obviously works, but it's also been designed as a system to work with that divider scheme. I question whether you could take any arbitrary power amp, hook it up that way, and have it function as well as it should. Caveat emptor.
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Old 9th December 2007, 12:56 AM   #10
eyoung is offline eyoung  United States
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sometimes the fun(learning) is just in building it...sims are wonderful guidance, but nothing beats solder and copper.
build your ideas and see what happens. the foundation to a good
amp is a good power supply and thats a simple fact.


Elwood
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