power xfmr specs vs audio xfmr specs - Page 2 - diyAudio
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Old 3rd June 2012, 03:21 PM   #11
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> If you don't want to know whether the transformer
> is suitable for purpose, that's up to you.

It's just that I know there are many nay-sayers
on the subject. For my purposes it works.
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Old 3rd June 2012, 03:24 PM   #12
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Quote:
Originally Posted by bear View Post
low power output transformers are cheap. plentiful, buy one??

_-_-bear
Not as cheap (nor plentifull) as low power toroid power xfmrs.
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Old 3rd June 2012, 03:29 PM   #13
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> determine the turns ratio of the power transformer.
> then look up the formula that relates turns ratio to
> impedance ratio.

Then when an output xfmr is speced @ 8 Ohm output Z
and 5 kOhm primary Z, those numbers are based on having
a 8 Ohm load rather than something intrinsic to the xfmr ?
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Old 3rd June 2012, 04:04 PM   #14
EssB is offline EssB  United Kingdom
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Quote:
Originally Posted by hitsware View Post
So ............
((234/unloaded out)^2)*8 ~ primary Z ?
I'm not trying to replace the 1 example with the other,
just to find a method of calculation ........... Thanks
Yep, you got it!
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Old 4th June 2012, 07:02 PM   #15
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Quote:
Originally Posted by hitsware View Post
> determine the turns ratio of the power transformer.
> then look up the formula that relates turns ratio to
> impedance ratio.

Then when an output xfmr is speced @ 8 Ohm output Z
and 5 kOhm primary Z, those numbers are based on having
a 8 Ohm load rather than something intrinsic to the xfmr ?
Yes and no. That's why some are hesitant about just giving answers without an explanation of why.

Intrinsic to an output transformer is a filter with a pass band barely adequate to fit the audio band in. An audio transformer has not only the right impedance ratio, but also puts its barely adequate pass band in the right place for its specified loading. A good one will optimise Q and properly balance the two halves for PP.

So, those numbers are based not just on having particular load, but also on the frequency of operation. Theoretically for a mains transformer, worst case scenario, the numbers are only true for the specified mains frequency. In practice for a toroid they will be true over a broad band but probably not broad enough, and it will have a full-amplitude low frequency of just a few Hz below specified. Q could be anywhere, and if the halves are matched they will not be so across the bandwidth.

For an audio transformer, if you change the loading, you shift the pass band sideways.

All these things and more make the answer to your questions yes or no, depending. 8:1 impedance ratio is very different from 8k:1k. Only the turns ratio is the same.

The reason you get hassled with stuff you don't want to know is perhaps because "impedance ratio" is a misnomer, in that it's not just a ratio. It's a relationship between one specified impedance and another, for a particular pass band.
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