# A question about audio output impedance

Discussion in 'Audio' started by Y2KEDDIE, Oct 30, 2012.

1. ### Y2KEDDIE

259
15
Sep 23, 2012
I’ve been studying audio output transformers and experimenting.

I’m using a Chicago A-3831 Universal Output transformer.

The transformer spec’s indicate 10,000 ohms plate to plate, center tapped, primary and multi-tapped secondary. (I’m using the 8 ohm tap).

Using my HP 200, audio signal generator, I applied a 1000 Hz sine wave to the primary ( blue and red leads). I adjusted the sine wave generator output until I measured exactly 1.500 vac, with my Fluke digital meter, across a precision 8 ohm carbon resistor attached to the ST and 8 ohm tap (secondary). I then measured the voltage across the blue and red leads). I measured 30.100 vac.

Dividing 30.1 by 1.5 I find the turns ratio of 20.067 : 1. I squared 20.067, to get 402.68 When I multiply 402.68 (the turns ratio squared) by 8 ohms, I get 3221.44 =Z.
With a spec of 10,000 ohms plate to plate, and a measured value 3221.44, I feel I am doing something wrong. Can you please point me to my error?

Thanks,

Eddie

2. ### Laplace

1,252
184
Apr 4, 2010
One may infer the turns ratio as the square root of the impedance ratio, but this transformer type is made for push-pull circuits where the center tap is connected to the plate bias voltage power supply. Is it possible that the 10K primary is the plate impedance or is it the plate-to-plate impedance? I don't know how these specifications are given.

One way to determine this empirically is to measure the turns ratio from primary and secondary voltages, but devise a test setup that removes the transformer from the real world effects of internal resistance, leakage reactance, exciting current, core distortion and losses. Not exactly sure what is the best way to do this but I would try a secondary load of higher resistance (32 ohms?) on the 8 ohm tap, and matching the primary impedance to the signal generator with a 10K series resistor. (Yes, I know the reflected 32 won't match 10K.) Use the lowest driving voltage possible to still get an accurate reading. Take the voltage readings from the transformer terminals with a high impedance meter/probe so the presence or absence of the meter has no effect, or use two meters simultaneously.

3. ### Y2KEDDIE

259
15
Sep 23, 2012
Impedance matching

The transformer spec is printed on the housing: 10,000 ohms Plate to Plate.

The formula Z out is the turns ratio squared times Z in,, what I want to do is measure it.

I tested the transformer at both 400 and 1000 HZ. The ratio is the same.

My next test will be 8 ohms resistance on the 8 ohm tap,( the secondary, I will then put a variable resistance load on the primary and measure the voltage across it at different values, while applying a test signal. I expect to see the voltage dip across the primary resistor, and peak across the secondary resistor when the primary resistor is the correct Z for maximum power transfer.

( I can then disconnect the resistor and measure it.)

Wish me luck!

4. ### Y2KEDDIE

259
15
Sep 23, 2012
Part of my error was I was using only 1/2 the winding: the center tap to one side. Also my voltage ratio is actually 19 v / .5 or 38. 38 X 38 = 1444. 1444 X 8 = 11552 which is closer to the spec of 10,000. I'm thinking measurement error accounts for the discrepancy.

I put a 10 ohm load on the 8 ohm winding and wired a variable resister in series with the full center (not used) tapped side, I applied a 1KHz signal to the winding and variable resistor (in series). I adjusted the variable resistor until I had (almost) equal voltage across the winding, and then the variable resistor.. When measured, It was close to 12,200 ohms.

Again , I'm thinking measurement error.

Do you think the spec 10,000 ohm Plate to Plate is a "nominal value" ?

The reason for my experiments is: I have a junk box full of unmarked audio transformers and I would like to know their values. It's relativity easy to find the turns ratio, but measuring actual matching plate resistance is somewhat of a challenge. If it's nominal /marginal, maybe it's not that important.

Thanks,

Eddie

5. ### Laplace

1,252
184
Apr 4, 2010
When you put the 10 ohm resistor on the 8 ohm winding, was that the marked value on the resistor or the measured value of the resistor? The actual value of that resistor is what gets reflected onto the primary winding. I would tend to believe that the turns ratio can be reproduced accurately from transformer to transformer so it could be more appropriate to use the specified turns ratio (sqrt(10K/8))=35.36 or at least investigate why that does not exactly equal the measured voltage ratio. Just be sure to use the actual reflected value for the primary impedance rather than 10,000 ohms.

6. ### Laplace

1,252
184
Apr 4, 2010
Don't miss this reference given in the references provided by "The Electrician".
http://www.jensen-transformers.com/an/Audio Transformers Chapter.pdf
Pay particular attention to the section on "Realities of Practical Transformers" and the parasitic elements that are part of the transformer circuit model. While the transformer designer has supposedly done his best to minimize the effects of these parasitic elements, please realize that while your test setup is trying to measure the performance of the 'ideal transformer' these parasitic components are a part of the test setup and do have an effect on your measurements. Of course, if you knew the actual values of these parasitic components, then it would be possible to account for their effect in your test calculations. Might there be some clever way to devise a ratiometric test that would cause parasitic cancellation? Just something to think about.

7. ### The Electrician

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11
Jul 6, 2012
It absolutely is a "nominal" value. I have made impedance measurements on a bunch of "identical" transformers, using a high end impedance analyzer, and it's quite typical to see variations in measured impedances of around 10%. There are many variables in the building of transformers, such as the tension provided by the winding machine, losses in the laminations, variations in the diameter of the wire used in the windings, etc.

This is of the essence. I put it much stronger in the thread on two ports I linked to.

The existence of the various parasitics is just what causes the transformer to have an optimum pair of impedances between which it provides the best performance. If it weren't for those parasitics, a transformer (an ideal transformer, in other words) would provide the same performance between any impedances having the same ratio (the square of the turns ratio).

Rather than try to make a measurement that cancels the parasitics, just measure the image impedances. Those are the optimum impedances and are the impedances that the manufacturer (ideally) would specify (and design in) as the rated impedances of the transformer. The image impedances take into account the parasitics without any need to derive them separately.

"If you want to know the "rated" impedances of an audio transformer, find the image impedances. The best way to do so, is to use an LCR meter and measure the impedance at the frequency of interest with the other winding successively shorted and open and take the geometric mean of those values.

A method that can be done with minimal equipment is this:
Use an audio generator with a low output impedance; 50 ohms will be good. Connect a potientiometer whose maximum value is several times the expected open circuit impedance (Zoc) in series with the primary winding. Set the pot to zero ohms and set the generator to 1 volt or so. Connect a DVM on AV millivolt range to the secondary. Make note of the secondary voltage with the pot set to zero ohms. Now adjust the pot to higher resistance until the reading on the DVM decreases to 1/2 the value when the pot was set to zero. Disconnect the pot and read its value with an ohmmeter; that value is approximately equal to Zoc.

Disconnect the potentiometer and connect a low value resistor (perhaps .1 ohm for our output transformer; for an arbitrary transformer, use a value of no more than one tenth the expected value of Zsc) across the secondary. Connect a different potentiometer as before, but with a lower maximum value, perhaps several times the expected short circuit impedance (Zsc). Set the pot to zero ohms and set the generator to 1 volt, or so. Note the reading across the low value resistor connected to the secondary, in millivolts on the DVM; adjust the pot until the reading is reduced by 1/2. Disconnect the pot and measure its value with the ohmmeter. That value is approximately Zsc.

Calculate the value of SQRT(Zsc*Zoc), which is approximately the input image impedance, and which is a good approximation to the "rated impedance" of the primary winding.

The transformer windings can be reversed to find the image impedance of the other winding. It might be easier to also measure the voltage transfer ratio and use the square of that ratio to get the nominal impedance ratio. That impedance ratio can be applied to the primary image impedance to get an approximation to the image impedance of the secondary winding."

8. ### Y2KEDDIE

259
15
Sep 23, 2012
To Electrician:

I looked up the two references you pointed me to. Excellent! And your answer of "yes" to nominal values was what I was looking for.

The graphical image of open and short cicuit impeadances was most interesting.

My experiments proved I don't have an "ideal " transformer. LOL

Thank you!