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Old 19th November 2003, 04:13 PM   #1
Serow is offline Serow  Canada
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Join Date: Feb 2002
Location: Regina, SK, Canada
Default Using a transformer without CT?

Hi, I am in the process of acquiring parts for a Mini-Aleph.

I am able to obtain a cheap transformer with two separate 24 VAC secondaries. This is twice as much as I can use - the filter caps I have are rated 20 VDC.

If I parallel the secondaries to get 24VAC, I could create a virtual ground using resistors bypassed by caps. Should this be done before or after rectifying to DC?

Approximately how much current passes through this ground? - if it's too high this method may be impractical.

Alternately, could I parallel the secondaries and solder in my own center tap?

Any other thoughts on how to make this work would be great.

Thanks,
Eric
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Old 19th November 2003, 05:07 PM   #2
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Location: Nottingham, England
In order to derive a usable 0V using resistors the values would have to be so low that most of the power would be wasted heating them up and unless you're prepared to unwind and rewind the transformer adding a centre-tap is also out of the question.

Surely it won't cost that much to buy the correct transformer, I can't think of any viable alternative.
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Old 19th November 2003, 08:58 PM   #3
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The front end doesn't take much in the way of current, so that's not a problem. The problem is that the speaker goes to ground, which amounts to a (relatively, I mean we're talking about a pretty small amp, here) fair amount of current.
You could regulate the rails, but you'd be dropping a lot of heat that way, too. Another possiblity would be some large inductors (wind your own--it's cheaper) and use them as the lead element in the filter in your power supply (an L filter, in other words). A large enough inductor will actually reduce the rail to below the value of the incoming AC (.8? .9? of incoming AC...I don't remember offhand), while offering more current than just caps or a PI filter. Or you could use a combination of inductors and regulation.
Of course, you could just build a higher power amp, but given the rails you'd end up with, you'd have to keep an eye on heat dissipation in your output MOSFETs. You'd also be better off with a relatively high Z speaker in that case, as it would allow for somewhat lower bias, hence lower heat dissipation.
Finally, depending on what else you have available and the current capability of your transformers, you could build one of the smaller Alephs. Yeah, I know none of that works well with your 20V caps, but it's what comes to mind at the moment.

Grey
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Old 19th November 2003, 09:50 PM   #4
Serow is offline Serow  Canada
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Join Date: Feb 2002
Location: Regina, SK, Canada
Thanks for the help guys. I had come to the same conclusion as you; there's no easy way to make it work.

Someone may be interested in the said 500VA 24V Plitron transformer, current auction price 10 USD from a gentleman in Toronto:

http://cgi.ebay.ca/ws/eBayISAPI.dll?...gory=3278&rd=1

Cheers,
Eric
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