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30th November 2010, 06:35 AM  #1 
diyAudio Member
Join Date: Nov 2010

Output Audio Transformer Impedance
Is there a standard frequency at which the impedance of an audio output transformers primary winding is stated? I have a Hammond output transformer whose primary winding measures 3.3k @ 100cps, 20k @ 1000cps and 40k @ 10000cps.
Regards, Richard 
30th November 2010, 07:45 AM  #2 
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Join Date: Mar 2010

The primary impedance is NOT the open circuit impedance. It is the loaded impedance, with the secondary having the rated load.
Having said that, the 'standard' frequency of measurement is 1 kHz. 
30th November 2010, 03:12 PM  #3  
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Join Date: Jan 2002
Location: nowhere

Quote:
So since Z of inductance = 2*pi*L*f, u get an inductance of L = Z /(2*pi*f) which gives u 5.3H, 3.2H, and 0.6H at those frequencies. The capacitive part of the OT is obviously taking charge as frequency goes up. Tho I'd think it shouldn't vary as much as this below 1kHz? 

30th November 2010, 04:21 PM  #4  
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Quote:
Above approx 5Khz the iron vanishes Yves. 

1st December 2010, 12:55 AM  #5 
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Join Date: Nov 2010

Thanks for the reply; since a voltage drop measured across a component is, in fact, a measure of inductance, placing a known inductor in the test circuit along side the test component provides a comparsion of the two individual voltage drops providing a good measure of inductance at any particular frequency. Obviously the test circuit must, as closely as possible, reflect the working circuit. The original question still stands: is there a standard test frequency for stated transformer inductance ?
Regards, Richard 
1st December 2010, 01:22 AM  #6  
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Location: Orlando, Florida

Quote:
I use 1kHz to measure unknown transformers to find the impedance ratio. 

1st December 2010, 01:42 AM  #7 
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Join Date: Sep 2002
Location: Sacramento, CA

Transformers don't have any particular "impedance" per se.
What they do is reflect impedances as a square of their turns ratio. Of course the reflected impedance appears in parallel with the primary inductance so you don't want to do your measurements at too low a frequency. 1kHz is about as good a "standard" as any as if it's a decent transformer the reflected impedance shouldn't be too drastically effected by primary inductance. se 
1st December 2010, 06:48 AM  #8 
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Thanks again, I think I got it.
Regards, Richard 
1st December 2010, 07:26 AM  #9 
diyAudio Member

Just a couple of details to make things harder to comprehend... of course!
The impedance formula given is correct, for 3 db at the frequency of interest. To achieve 0.1 db at this frequency multiply the derived inductance by 2.76. If you measure with 120Hz @ 1 volt, you will be measuring the core at rest, which is not zero flux but remnant flux, what it will provide with minimum excitation. To understand what happens with more power applied you must have permeability curves and an inductance formula. There are two permeability curves, one for AC signal and one for any core with DC on it and a gap. The impedance of a primary is equal to the inductance of the primary. To match impedance to a tube you need to add the transconductance and turns forced load line in parallel, then add the DCR of the primary in series. This is the number to use to decide if your transformer will match with a tube at the lowest frequency you care about. For E/I commercial grade core the core is all done with power transform, except as a ferrous bounding box for the coil antenna event, from 250 Hz to 400 Hz, core grades M50 to M3 respectively. The capacitive coupling and the direct EMF flux transform from primary to secondary provides everything else.Other metals work at higher frequencies, but there are trade offs to be made when using them so don't assume that the very best core will provide you with what you are looking for, once you decide to move beyond Hammond and EDCOR levels of performance. Not that either of these two have anything to apologize for. Bud
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1st December 2010, 07:33 AM  #10 
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Join Date: Sep 2002
Location: Sacramento, CA

Uh... what was that middle part again?
se 
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