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PSU for ebay amp build
PSU for ebay amp build
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Old 3rd December 2019, 06:36 PM   #1
giro1991 is offline giro1991  Scotland
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Default PSU for ebay amp build

I tried to build a cheap amp using this PCB using TDA7923

It can do '100W x 2', however this is 10% THD. A more realistic handling wattage is probably half, 50W x 2, for ideal THD. I don't expect 8ohm bookshelf speakers to demand 50+ anyway.

So, if one doesn't need 100W, can the required PSU be correspondingly minimized (*VA or volts)? Thats my main curiosity. Regulation is built into the chip and datasheeet says it accept 12-50V AC.
I tried to use a fairly beefy 24-0-24 transfomer sourced from a Hitachi AX-M140. The hitachi is rated 2 x 35watt. I wired it up and played signal: no output. I since isolated transformer and measured one secondary with MM with mains applied and it does not reveal any voltage anymore (upstream fuses are intact).

If I had realised in advance how expensive a 28-0-28was going to be I would have just bought a second hand amp..... 50 for a transformer, REALLY?. and thats 25-0-25, let alone 28-0-28.
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Last edited by giro1991; 3rd December 2019 at 06:52 PM.
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Old 3rd December 2019, 06:59 PM   #2
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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Sorry, but I have unclear the question. Using a smaller trafo or PSU surely it will be "overloaded" at full amp power output, but mainly in stronger music passages. For normal listening, you will not see any difference. A lower traffo or PSU voltage will correspondingly reduce output voltage following the squared rule: power is voltage squared divided by the resistance of the load. Maintaining the latter constant, power varies with voltage squared.

I don't understand why using the Hitachi trafo the set didn't performed.
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Old 3rd December 2019, 07:10 PM   #3
FauxFrench is online now FauxFrench  France
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Yes, you can take off some maximum power by using a lower supply voltage. The TDA7293 accepts up to 50V DC. Assume the output to go 3V from the rails in both directions.
Test the transformer windings with an Ohm-meter. I have never seen a transformer winding burn.
Like you, I am reluctant to pay for new transformers and in particular the freight. I pay some 15-20 Euros (possible new currency for you) for a defect amp. Nice quality transformers made in Japan in those days.
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Old 3rd December 2019, 08:31 PM   #4
giro1991 is offline giro1991  Scotland
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Thanks and thanks.
Quote:
Yes, you can take off some maximum power by using a lower supply voltage.
Great, thanks for clarification. Also, buying broken amps for transformers is definately more economical, strangley enough. While I agree a transformer that was in 35 mini-hifi cant do 50, for quiet listening levels it should be ok IMO.

As for winding burn - It is strange that there is no volt reading on the secondary. How many ohms should it read with ohm-meter?
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Last edited by giro1991; 3rd December 2019 at 08:40 PM.
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Old 3rd December 2019, 09:15 PM   #5
FauxFrench is online now FauxFrench  France
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My guess, in the order of 2 Ohm.

Defect amp. -> ask price for repair -> reply: 100 Eur minimum -> either dump it or let someone else pick it up for 15 Eur. I have managed to get many good transformers this way.

You write "no volt reading". Use Ohm for a start, not Volt. If all windings are not broken, you will have a voltage (if you measure at the right terminals).
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