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Old 2nd June 2012, 05:38 PM   #1
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Default power xfmr specs vs audio xfmr specs

I do not want to explore why or why not power supply toroids
will work as audio output transformers.
I would like to know how to translate power supply toroid
specifications into audio transformer specifications.
For instance how would one convert these specs:

http://www.toroid.com/standard_trans...701.062_M4.pdf

Into this context:

VTP15671-1100 - Firefly Push Pull Output Transformer 22.5k to 8 ohms 1.5 watts

And vice-versa

Last edited by hitsware; 2nd June 2012 at 05:40 PM.
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Old 3rd June 2012, 02:23 AM   #2
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Old 3rd June 2012, 03:03 AM   #3
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determine the turns ratio of the power transformer.
then look up the formula that relates turns ratio to impedance ratio.

that will get you in the ball park.

it will NOT tell you if you have sufficient primary inductance or if you have enough turns in the primary to handle the current, nor will it tell you if you will have any voltage breakdown issues... and tells you nothing about DC imbalance nor leakage inductance and capacitance issues...
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Old 3rd June 2012, 04:54 AM   #4
jcx is offline jcx  United States
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pri magnetizing current should be on most pwr xmfr datasheets - can calc Lm

valid only for specified line V, freq VT product - pwr xfmr core often working into saturation - 2x increase in VT can give 10x increase in Im

conversely if you can operate at lower V (VT really) the reduction in Im is similarly disproportionate

pwr xfmr cores are not gapped, don't tolerate DC bias current - use push-pull only

Last edited by jcx; 3rd June 2012 at 05:00 AM.
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Old 3rd June 2012, 09:48 AM   #5
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> determine the turns ratio of the power transformer.
> then look up the formula that relates turns ratio to impedance ratio.

O.K ...... If I have the ratio, then how do I determine the impedance ?
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Old 3rd June 2012, 09:52 AM   #6
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> pri magnetizing current

Would that be 'rated primary current' ?
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Old 3rd June 2012, 11:53 AM   #7
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Quote:
Originally Posted by hitsware View Post
> pri magnetizing current

Would that be 'rated primary current' ?
No. It's related to the "< 2mA" on the power transformer datasheet. You don't need to know this to calculate the parameters you have asked for. If you don't want to know whether the transformer is suitable for purpose, that's up to you.

There is insufficient information in the power transformer datasheet. The turns ratio is, near enough, the unloaded voltage ratio, but you only have the loaded ratio. To calculate one from the other you need the regulation, which isn't given. It could be approximated from winding resistance, but that's not given either. Some power transformers have more infomative datasheets. If you knew the turns ratio, it's really not hard to find out how to calculate the impedance ratio.

The rated primary current, at a known voltage, provides for a rough estimate of maximum power before meltdown. I trust you know how to calculate power, given a current and a voltage.
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Old 3rd June 2012, 02:31 PM   #8
EssB is offline EssB  United Kingdom
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Forget this nasty little toroid a substitue for the VVT Firefly transformer.
What you want to know is the impedance ratio is the square of the turns ratio.

There is the text 'Uo=8.1V RMS' in the spec, this may well be the unloaded secondary voltage. This makes the regulation about 25%. The temp rise of 63 degrees C seems high as well.

Assuming you connect the secondaries in parallel and 234V in gives 8V out then the turns ratio is 29.25 and impedance ratio is 855.5 An 8 Ohms speaker load will give Ra-a = 6.84K which is way too low.
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Old 3rd June 2012, 03:09 PM   #9
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low power output transformers are cheap. plentiful, buy one??

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Old 3rd June 2012, 03:18 PM   #10
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Quote:
Originally Posted by EssB View Post
Forget this nasty little toroid a substitue for the VVT Firefly transformer.
What you want to know is the impedance ratio is the square of the turns ratio.

There is the text 'Uo=8.1V RMS' in the spec, this may well be the unloaded secondary voltage. This makes the regulation about 25%. The temp rise of 63 degrees C seems high as well.

Assuming you connect the secondaries in parallel and 234V in gives 8V out then the turns ratio is 29.25 and impedance ratio is 855.5 An 8 Ohms speaker load will give Ra-a = 6.84K which is way too low.
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
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