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2nd June 2012, 06:38 PM  #1 
diyAudio Member
Join Date: Mar 2002

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: VTP156711100  Firefly Push Pull Output Transformer 22.5k to 8 ohms 1.5 watts And viceversa
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3rd June 2012, 03:23 AM  #2 
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bump
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3rd June 2012, 04:03 AM  #3 
expert in tautology
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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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3rd June 2012, 05:54 AM  #4 
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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 pushpull only Last edited by jcx; 3rd June 2012 at 06:00 AM. 
3rd June 2012, 10:48 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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3rd June 2012, 10:52 AM  #6 
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> pri magnetizing current
Would that be 'rated primary current' ?
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3rd June 2012, 12:53 PM  #7  
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Quote:
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. 

3rd June 2012, 03:31 PM  #8 
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Join Date: Jan 2009

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 Raa = 6.84K which is way too low. 
3rd June 2012, 04:09 PM  #9 
expert in tautology
diyAudio Member

low power output transformers are cheap. plentiful, buy one??
__bear
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3rd June 2012, 04:18 PM  #10  
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Join Date: Mar 2002

Quote:
((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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