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Old 26th June 2012, 08:08 AM   #1
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Default Toroid transformer and big voltage drop

Hi,

Im building a bass amplifier with a Hypex Ucd180 (class D module) as power amp section. It's almost done, but I have a problem with my PSU:

The power amp accepts max
50V, and will shut down if either rail exceeds 52V. So I bought a 38V 180VA toroid trafo. I chose the smallest (physically) that I could find, in order to fit it into my enclosure.

I knew that a transformer needs a load to drop to the rated voltage, but I've built a similar PSU before for a chip amp, with a 26V physically larger toroid, and it dropped to the the rated voltage with a load of 4k4.

I didn't read the specifications carefully enough and it showed the slimmer size of the trafo also means it has a bigger voltage drop, and the
38V holds true only when it's driven close to its VA rating. When measured unloaded it gives 56V (DC rectified with a standard diode bridge + caps), but It seems to need lots of load to drop the voltage to decent levels.

I tried different bleeder resistors, and I now have a 500Ω 40W resistor from + to - (112V), which should dissipate around 25W. This drops the voltage to
54V, which is still too much for the the power amp, and it generates way too much heat to be practical. It also seems like a big waste of power.

Now, I've tried to run the power amp with this PSU, and it doesn't go into protection mode for some reason, but I don't like being this close to its limit.

I've thought about regulation, but the standard regulators available don't seem to be able to handle the current draw of a 180W power amp. I may be wrong though?


Any help would be greatly appreciated!

/Fredde



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Old 26th June 2012, 09:04 AM   #2
DF96 is offline DF96  England
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You really need a different transformer, 35V would be about right. Failing that, and if you are happy and safe working on mains voltages, you could try using a buck transformer to reduce the mains input voltage to your power transformer.
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Old 26th June 2012, 09:59 AM   #3
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Quote:
Originally Posted by DF96 View Post
You really need a different transformer, 35V would be about right. Failing that, and if you are happy and safe working on mains voltages, you could try using a buck transformer to reduce the mains input voltage to your power transformer.
Aargh, now I feel really stupid. This was something I actually knew from the last time i built a PSU, I just forgot about it. Rod Elliott says "The DC output is approximately equal to the secondary voltage multiplied by 1.414", so the DC output will never go down to acceptable levels. I'll just save this trafo for some later project and buy a smaller one.

Thanks for your help!
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Old 26th June 2012, 02:10 PM   #4
AndrewT is offline AndrewT  Scotland
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Even 35+35Vac is a bit high for an amplifier that shuts down @ +-52Vdc.

You would probably be better going to <=33Vac.
Or play safe and use 30+30Vac.
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Old 26th June 2012, 02:20 PM   #5
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unwind a few wraps of the secondarys. check with a meter repeat if needed.
then rewrap with super 33 ele tape then rubber isolation tape.
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Old 27th June 2012, 07:56 AM   #6
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Quote:
Originally Posted by sakellogg View Post
unwind a few wraps of the secondarys. check with a meter repeat if needed.
then rewrap with super 33 ele tape then rubber isolation tape.
Thanks for the suggestion, but I don't feel that comfortable altering a mains transformer. Also, this one has a bottom cup fastened with some kind of plastic filler or epoxy or something, so it wouldn't be possible anyway. Found a decent priced 2x30V, 160VA, it should output around 42VDC which will be enough for my use.

/Fredde
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