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Monsoon PM-9 Transformer
Monsoon PM-9 Transformer
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Old 3rd December 2020, 01:01 PM   #21
thoglette is offline thoglette  Australia
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Monsoon PM-9 Transformer
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Originally Posted by yorab View Post
Is there some way that I could determine the VA rating of the original PT so that I can confidently fit a properly spec'd replacement?
Weigh it. Similar architecture (toroid if i recall correctly) transformers of a similar weight will, generally, have similar VA ratings

The 100VA is the max rated draw from the mains, not the rating of the transfomer. A quick examination of the circuit will show you that there's no way in hell the PM-9 amp draws 100VA (quite what it is I leave as an exercise for the reader but rail voltages and DC woofer resistance will get you close)

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Originally Posted by yorab View Post
Also, the original PT has a thermal fuse while most of the PTs that I saw at the shops did not have one. How critical would this be?
It's critical for most of the markets that the product was sold into. It also allows a smaller VA rating (as if it gets hot POOOF rather than fire)

Have either have a thermal fuse or a separate mains fuse (200mA slow blow) for safety's sake.

In the absence of a fuse, a genuine short across the mains will generate several thousand amps at 230VAC, which in a domestic setting will be shortly followed by your smoke alarm going off and an unpleasant conversation with your insurers.
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Old 5th December 2020, 11:01 AM   #22
yorab is offline yorab  Singapore
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Ok, so I think the rail voltage is 12.5VDC. At least that's silkscreened to the board at the output of the bridge rectifier (see pic). I also know from measurement that the original PT secondary is at 12VAC.

I don't have access to the subwoofer at the moment to measure, but a quick search of a similar Eastech driver shows 3.56Ω (DCR) for a similar 4-ohm (Z) speaker.

So it looks like a ballpark of 12.5V/3.5Ω= 3.57A across that load.

That means a PT secondary coil would need to carry 3.57A*12VAC= 42VA. Then there are some losses to be concerned with.

Am I on the right track here?
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Old 6th December 2020, 03:38 AM   #23
thoglette is offline thoglette  Australia
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Monsoon PM-9 Transformer
Quote:
Originally Posted by yorab View Post

That means a PT secondary coil would need to carry 3.57A*12VAC= 42VA. Then there are some losses to be concerned with.

Am I on the right track here?
Yup. Throw in a few watts for the planars & preamp plus a bit for losses and you're in the right ball park. 60VA would be about right, based on what you've said
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