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Old 23rd May 2007, 09:11 PM   #1
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Default NooB PSU question (gc)

Greets...

A couple of months ago, I happened upon a CMOY site and had to build one. Too cool, except of course, now I need another project

So, I've been looking at GC info... another MUST build. I just rec'd my LM3875s and I'm itching to get started, but I have a transformer/PSU question. I have a 28VAC 175va transformer that has a single pair of secondaries (no center tap) and I'm at a loss. Every PSU that I have run across for a GC is based on a tranny that has either dual secondaries OR a center tap. I ASSUME, that I can rectify the AC and create split rail system ala CMOY, but I thought it prudent to ask the experts as to the best course of action.

Thanks for your time

Darren
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Old 23rd May 2007, 09:26 PM   #2
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If the transformer has a single, untapped secondary winding and thus only two wires you can't use that transformer by itself to power an LM3875 chip amp. If you had another transformer, preferably identical to the first, you could do it.

If the transformer is a toroid you can try adding your own winding to get a second output for a bipolar supply. The core will still limit you to 175VA.

Otherwise, get another transformer with the right secondary configuration. Tricks for creating a ground with a couple resistors works (sort-of) for op-amps because they don't need much current. A power amp needs to send amps through ground.

I_F
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Old 24th May 2007, 12:35 AM   #3
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Thanks to your quick reply, I was able to run back and grab a couple custom/surplus units built by Toroid for a local firm. Dual primaries, single 20VAC 100VA sec @ 20 bucks ea.
Let the fun begin...


Darren
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Old 25th May 2007, 09:55 AM   #4
AndrewT is online now AndrewT  Scotland
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Hi,
connect the four primaries in parallel (for 110Vac to 120Vac).
Connect each of the secondaries wires to individual termianls in an insulated terminal block.
Build up a mains light bulb tester consisting of a plug top, bulb holder and socket outlet. Keep it for ALL future projects.
Plug in the bulb tester, plug in your two transformers into the bulb tester. Insert a 40W to 60W mains bulb into the bulb holder.
Switch on.
If the bulb lights then you have wired the primaries incorrectly. Rewire correctly to ensure the phase of each primary winding matches it's three partners.
If the bulb stays out, then measure the voltage going into the transformers. Measure the voltage across eqch pair of secondaries.
Come back with some answers.
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regards Andrew T.
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Old 25th May 2007, 10:20 PM   #5
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Thank you Andrew.
It actually went quite well... even though these are custom units and therefore not listed on the Toroid site, I was able to see the factory conventions/ color codes for dual primaries. PSU complete.

Darren
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