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Old 31st December 2003, 01:00 AM   #1
bbksv is offline bbksv  United States
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Default Pwr supply/transformer question

I pulled a transformer out of an old sony rcvr I had lying around and was planning on using it for a amp I'm gonna build...but had a quick question. I am not sure of the VA rating of the transformer...but it has 3 output voltages...all center tapped. The 2 that I am interested in for possible amps are the +/-18v rms output or the +/-38v rms out. The internal amp was only 70x2 and 10x2....but with the 38v rails...I can run over 140w rms per channel. Would it be safe not knowing the actual VA rating of the transformer?
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Old 31st December 2003, 01:05 AM   #2
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U must know the actual VA of the transformer
Until or Unless u will wind up in smoking it!
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Old 31st December 2003, 05:16 AM   #3
azira is offline azira  United States
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Actually you can pretty easily figure out a good estimate. If you (hopefully) still have the rcvr around look for the onboard fuses and, more importantly, the bridge rectifier. They will probably be set at slightly above what the rating of the transformer is. If the receiver was still function you could probably reuse some of these parts but new is always good too.

Basically what I'm trying to say is you can use the transformer but don't exceed the current draw that it originally was used for.
Secondly, you actually get higher rails. Your DC rails will be about AC voltage * 1.414 (sqrt(2)) -1.5V (rectifier drops).
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Old 31st December 2003, 04:48 PM   #4
bbksv is offline bbksv  United States
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Well...here is what I have. On the 18-0-18 out...I have 2 3.15 amp fuses. On the 38-0-38 out..there are no fuses. There is an RBA-402 bridge rectifier (which I cant find any info on....RBV but not RBA) The RBV ratings are 200V max reverse voltage and 4 amps...but thats the only info I can find on it. It looks to be a pretty puney rectifier. Actually...I dont see how it could have worked very well..without using high impedance loads...
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Old 31st December 2003, 05:01 PM   #5
Magura is offline Magura  Denmark
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if im not mistaken, the difference between RBV and RBA is the configuration of the terminals. 4A sounds reasonable with a 3.13A fuse.

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Old 31st December 2003, 05:06 PM   #6
bbksv is offline bbksv  United States
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the 4 amp rectifier isnt on the same output as the fuses though. The 18-0-18 output goes to 4 small stand alone transistors and the 38-0-38 output goes to the rba rectifier...but no fuses..
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Old 31st December 2003, 05:15 PM   #7
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on the back of the VCR, did it state its power consumption? that would be the figure I would use as a proxy for the trransformer power rating.
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Old 31st December 2003, 05:20 PM   #8
bbksv is offline bbksv  United States
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it was a sony STR_D590 audio receiver and it was rated for 2x70 and 2x10. It says 140 watts on the back though. The problem is....using the 38-0-38 output I would have approx 50v rails...which would give me alot more the 70 watts out in an 8ohm load and would draw too much current...correct?
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Old 31st December 2003, 10:41 PM   #9
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Hi, I'm thinking either before the transformer or after there has to be a fused link or at least a breaker between the supply and load on the 38-0-38 outs or POOF if there was a failure.

But aside from that you need to think of the load resistance your amp will be tailored for. These design projected loads require different supplies. An 8 ohm load up to 70 watts would be fine for a 106v +/- supply @ a couple amps but a 4 ohm load would require less voltage but a higher current load of as much as 5 amps ( high efficiency) to achieve 70 watts into a 4 ohm load.

Don't forget transformer ratings are in rms and not peak as well as most ac side fuses.

Also it is possible to guestimate a transformers load target by measuring the wire size on the former and secondary versus the former/secondary winding ratio with respect to the no load current on the primary but I can't explain this right now.

Just find the primary to secondary ratio, note it, and gradually apply load to the secondary until the sec/pri ratio is changed so that the primary voltage no longer decreases with the secondary and your transformers saturation limit is more or less known, and if like most UL listed transformers just 25% above this saturation limmit is the transformers rated VA.

Don't take this as scripture, it's only an estimate, as to much saturation can actually cause the primary coil to start acting on the core and as the load increases the primary coil load fights with the core while the secondary output decreases due to the primaries magnetic field now acting on the core instead of the secondary.

Just contact Sony and ask.
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Old 31st December 2003, 10:47 PM   #10
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Oh, I was assuming this is an IE transformer and not a toroid, toroids behave differently.
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