500va transfomer 3 secondaries no centre tap - Page 3 - diyAudio
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Old 28th May 2010, 05:07 PM   #21
djk is offline djk
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Sell transformer, buy a 50-0-50 (for 200W/8R).
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Old 28th May 2010, 06:42 PM   #22
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Quote:
Originally Posted by sregor View Post
Somebody correct me if I'm wrong, but for any given current, wouldn't doubling the capacitance in a half wave circuit give the same ripple (but different frequency) as a full wave circuit? (based on V=q/C where q would be the integral of the current over time) Another vote for big capacitors and beefed up diodes. Another 2 cents down the drain.
To a first approximation, yes. Just a little bit more is needed because the on-time of the diodes doesn't increase much while the period doubles so the off-time is slightly more than doubled. The capacitor ripple current is about a factor 1.5 times higher than the typically used non-doubler circuit so watch out for that too.

As long as the current draw is symmetrical there is no problem using this kind of circuit except the obvious drawback that it needs more smoothing capacity for the same ripple. If the current draw is nonsymmetrical however, a DC component will be drawn from the transformer which could push it into saturation due to the resistance of the primary winding. Full-wave doublers are routinely used in SMPS front ends in 120V mode even at multi-kilowatt levels if PFC isn't needed.

The current draw of an audio amplifier should be symmetrical over a sufficiently short time interval for a full-wave doubler circuit to be usable. That is, if you have enough space and money for the capacitors

Last edited by megajocke; 28th May 2010 at 06:45 PM.
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Old 28th May 2010, 07:49 PM   #23
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34+34 volts of AC obviously is not enough for 200W ... if this transformer has a bit of a gap between the coils an dthe case ...winding another 34 turns to make it a center tap might be possible ...or at least worth looking at it ...it might as well be

may you never make it to 200 W in a single mode but may be something brigged might do the trick or simply come closer to what you want ....

if you manage to proper 125W the diference from 200 is very very low
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Old 28th May 2010, 08:07 PM   #24
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Quote:
Originally Posted by infinia View Post
well using a SMPS as an example in favor of this scheme is not fair because
Older SMPS (non PFC) using full wave voltage doubler make no loading connection to the center of the cap string, so the ripple is 2x line rate, unlike audio amps that use the speaker return (load) directly to the center point. That is the main problem with using this.
Problem compared to what? Do you suggest that 50Hz ripple is worse than 100Hz ripple when valley, ripple RMS and pk-pk voltages are the same? The big drawback is that much more smoothing is needed - but if you can live with that then...

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Symmetry is not guaranteed esp when the power amp is clipping!
Good catch. There could be asymmetric current draw if clipping asymmetric waveforms heavily. A typical DC protection circuit would be triggered though in such a case. The same "problem" exists if you use a single bridge symmetric PSU but use two separate transformers instead of a center-tapped/dual secondary one.
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Old 28th May 2010, 08:29 PM   #25
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What about using a full bridge power amp in conjunction with the full wave voltage doubler. No appreciable power is drawn from the capacitor center.
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Old 28th May 2010, 09:14 PM   #26
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You could probably get that to do 200W into somewhere around 4-6 ohms bridged, but is it worth the trouble of making two amplifiers and needing double the power supply energy storage compared to a non-doubler circuit?

I'd get another transformer, like djk suggests.

Last edited by megajocke; 28th May 2010 at 09:18 PM.
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Old 28th May 2010, 09:25 PM   #27
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Well it depends, if (since) , we (OP) have the amps and transformer what the hay? All this discussion is just a mind exercise since the OP seems to be missing.
Me I'm just on a full bridge crusade today.
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Old 28th May 2010, 11:10 PM   #28
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Quote:
Originally Posted by AndrewT View Post
no!
tube/valve amplifier assemblers use doublers regularly and they don't suffer from inadequate power delivery from their high voltage supplies.
It is a current delivery problem.
yes, but i doubt the poster was interested in tube amps.....by the looks of it he was asking about ss amps....

its a power delivery problem....
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Old 28th May 2010, 11:32 PM   #29
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properly designed transformers for tube amps using the full wave voltage doubler topology does not supper much from inadequate power delivery, this is a good place to look: powertranschokes
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Old 29th May 2010, 01:23 AM   #30
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"properly designed transformers for tube amps using the full wave voltage doubler topology does not supper much from inadequate power delivery, this is a good place to look: powertranschokes"

So now he has to build a doubler and scare up a big power choke?

No thanks.

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115/220V Input Dual 41VRM 6.1A $20
(should be good for 200W/4R)

15-0042 93VCT 250Va $25.00
(should be good for 180W/8R)

Antek - AN-4450 $45 (new 50-0-50, should do 200W~250W/8R)
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