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Old 12th July 2004, 07:55 PM   #1
ImWhoIm is offline ImWhoIm  United States
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Question Aplifier Power Supplies

Hello everyone!

Admittedly, this is not a true audio question but I can't think of anywhere else to ask.

I am in need of a power supply for a project that I am working on that requires 30-40V out with a current of 6 or more amps. Obviously the most expensive part of this will be the transformer...

I also have a couple of "old" (they are solid state) stereo amplifiers lieing around - One is 85W/channel and the other 185W/chan that were working when removed from service (replaced by newer models). I know these things have massive transformers and many caps in them... I was wondering what you thought the odds might be of my being able to use one of the transformers out of these amps to build my supply with?

Knowing little about amplifiers in general, I really don't want to tear one of these apart if the possibility of failure is higher than success...


Any ideas?
Thanks
Don W.
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Old 13th July 2004, 03:57 AM   #2
Jay is offline Jay  Indonesia
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85V to 40V is too much But I can't see any possibility of failure by tearing the TR apart, except for

I was so lucky I could manage the same problem by the fact that the TR had many windings (primary and secondary) or outlets. I choose 240V (the highest/longest) for the primary, and tailored the (phase of) secondary outlets. Not sure if this would introduce new problems.
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Old 13th July 2004, 03:26 PM   #3
K-amps is offline K-amps  United States
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40v out at 6 amps is 240VA and dual rails makes it 480VA.

The 185 watt amplifiier might just have that sized transformer but it will probably give you something like 50-0-50 volts @ 4-5 amps.

The 85 watt amp's PSU may give you around 35-0-35 v @ 5 amps or 40-0-40 @ 3 amps for a smaller trafo.

This is all guess work, the actual ratings may differ based on design.
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Old 13th July 2004, 03:49 PM   #4
jcx is offline jcx  United States
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Default great, now a post about vivisection

If you can’t calculate the expected volts/current that these amps’ transformers must supply from the power/load specs then maybe you’re not ready to start building up mains operated power supplies from scratch

Power supplies can be found for ~US$ .5 –1/W - even less from surplus outlets – search for “open frame”, “unregulated” for lowest cost
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Old 13th July 2004, 08:38 PM   #5
ImWhoIm is offline ImWhoIm  United States
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Thanks for the Info K-Amps -

Sounds like it's something I might be able to use at those ratings

And thanks for your input jcx -

I am thoroughly capable of calculating anything I need for building a power supply of this type. I was hoping that someone might know (in general) how many voltages/amps these transformers might put out so that I could avoid disassembling a perfectly good power amp just to find out the output voltage is no where near what I might need.

Hmm.. vivisection... good word for it too!
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Old 13th July 2004, 09:43 PM   #6
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There is a few things we don't know i.e. whether the 185 watts is into 4 or 8 ohms but I'd have thought that if these amps are "split rail' or "output capacitor less" then the secondaries will be centre tapped and if you run the transformer secondaries in parrallel rather than series you will get close to what you want with regards to volts and amps. You can always put a meter across the rail voltages and see what you've got.
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Old 13th July 2004, 09:58 PM   #7
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Another thought. You obviously know how to do the maths and calculate things but just take care with the current rating. I'm not an expert on this but you can't always take the published power out put of an amp and then work your back and work out the current rating of the tranformer. If the figures you quote are from a brochure or spec sheet you would be wise to see whether the numbers refer to "RMS" output or what some manufacturers call "music power" which is the short term peak output. The latter is always higher but can only be sustained momentarily. If the supplies in the amps you speak of are regulated (doubtful) the figures will be the same. It depends on what conditions you are using the new siupply for. Some manufacturers will fit a slightly undersized transformer because they know that music won't draw the full current for long and so they rely on a relatively long thermal lag in the tranny to cope with momentary full power output. But this means it might start smoking if asked to do this same work load for 15 minutes. And most transformers are too young to start smoking. Also I take it they are stereo amps....so you have the power from two channels.
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Old 14th July 2004, 12:46 AM   #8
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Default power supply

Jonathan has it right. if the secondary windings have a centertap, then you can use a half wave rectifier. Just make sure the filter caps are big enough to handle any current spikes you may need in your project to reduce ripple.

compare the RMS current needed with the RMS current of the transformer.

Keep in mind that even the best amplifiers can't get around the maximum power transfer thereom This simply states that Z out must be matched with Z load to get maximum power into the load. In other words, if the amp outputs 50W RMS to a load, then the heat generated by the outputs will be 50W.
50W+50W=100W
This assumes that the amp is 100% efficient!
most good class AB amps are maybe 70-80% efficient.

Take the rated RMS output power of all amp channels, multiply it by 2 to get a VA estimate; divide by secondary voltage, and determain the current that way.
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