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Old 29th October 2013, 03:02 PM   #1
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Default Help with PS design

I just picked up a Sure Electronics TAS5630 board to cobble together an inexpensive instrument amp. I've got a giant power transformer from a failed Sansui AU-D7 that puts out about +/- 57v with a low load/idle. The schematic suggests the transformer is 41v-0-41v.

This amp will likely take a beating, so I'm looking to have a PS that can handle about 50v 12A. Unfortunately I can't do a simple CRC design where the board idles at about 100mA which is probably way less than the AU-D7, and I can't exceed 52v.

So I likely need a voltage regulator of some sort. I've searched. I've googled, but I've not had success finding anything that can handle 50v 12A. Any suggestions? It doesn't need to be quiet or super accurate.

I suppose I can just scrub off some power with some big resistors, but that's not very nice.
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Old 29th October 2013, 03:37 PM   #2
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How about something simple that isn't quite a "regulator"? What if you just build an enormously beefy, oversized emitter follower or source follower, put a fixed reference voltage on its base/gate, and accepted the voltage on its emitter/source as "good enough"? There's no feedback control system; you don't sample the output, compare to a reference, and use the difference ("error voltage") to adjust the regulator.

Here's the outline of the idea; details such as fusing, Emitter-Base reverse bias protection, start-up inrush current limiting, load disconnects, and protection when the other rail fuse blows, are omitted. But perhaps they are crucial.
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Old 29th October 2013, 03:44 PM   #3
Elvee is offline Elvee  Belgium
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A simple and effective solution is to use a small transformer connected as an autotransformer in front of your transformer: for example, a 120V to 12V transformer will shave off a little more than 10% of the voltage and only needs to have 10% of the VA rating of the main transformer
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Old 29th October 2013, 04:03 PM   #4
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@elvee: Not sure I understand. So put the primaries of the autotransformer in line with one of the primaries of the main transformer?

I could reconfigure the iron for 220v on the primary side. That would about halve the output on the secondaries, but I may have trouble getting anywhere close to 50v.
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Old 29th October 2013, 04:15 PM   #5
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If you do want a feedback control system including a potentiometer that lets you fine-tune the output voltage, perhaps a few parallel Nchannel MOSFETs and an NFB loop would be a possibility. I've shown 2 parallel devices but since they are so inexpensive ($1.56 qty 1 @ DK), you might prefer to build yourself an enormous SOA by connecting 4 or 6 of them in parallel.

Again, many crucial real-world details are omitted from this conceptual sketch.
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Old 30th October 2013, 09:02 AM   #6
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by whitelabrat View Post
@elvee: Not sure I understand. So put the primaries of the autotransformer in line with one of the primaries of the main transformer?
No, here is what I mean. You keep the input of your main transformer configured for 120V, but you subtract the voltage from the auxiliary transformer.

The drop will be more than 12V, because the regulation of the aux transformer will work backwards
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Old 30th October 2013, 11:06 AM   #7
Johno is offline Johno  Australia
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How beautifully clever and simple - first time I have seen that solution.
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Old 30th October 2013, 02:41 PM   #8
domyboy is offline domyboy  Scotland
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Have a look at the PSU im building New PSU for Power Amp.

i ve made a few changed since my last upload but will upload the final version tonight.

Cheers
Dom
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Old 31st October 2013, 10:07 PM   #9
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Thanks everyone for your posts. I have a mountain of transformers so that's a real option.

I haven't gotten any big ps capacitors yet, so I'm going to use what I've got for now in a CRCRC config for initial testing. I think big 2 ohm resistors for the R if I can find them. And then a bunch of parallel 47uF 63v LOL for the first C, and then massive 600uF 500v film capacitors for the other two C's. Just for testing.
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Old 3rd November 2013, 01:23 AM   #10
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So I found a 120v to 15v transformer that is about 45 VA. After wiring it up my first observation after getting the 15v phase backwards was a bit of a voltage boost. After reversing it I got a nice solid 51.9V DC with a 56k ohm bleeder resistor in place.

nice!
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