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I need to select the proper transformer to use in a device to convert from a hi-z input to a lo-z input. As far as I understand transformers have no impedance, rather a percent impedance which is actually a voltage, expressed as a % of rated voltage, that is required to circulate rated current through the transformer.

When I shop for audio transformers they list primary and secondary impedances. Am I to assume theyre providing me two parts of a ratio that is the % impedance, and that as a parameter it doesnt have a direct effect on my use case of wanting to go from hi-z to low-z? Also it lists the impedance as "CT" which is center tapped as I understand. i also dont understand why the values are provided as center tapped primary and secondary impedances.

Here is an example transformer that calls out primary/secondary impedance. https://www.mouser.com/ProductDetail/Triad-Magnetics/TY-250P?qs=Yx4IQMNd5u7UhIpi75ZQsw%3D%3D

Thank you

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  • \$\begingroup\$ The example transformer hasn't shown up yet. \$\endgroup\$
    – Transistor
    Sep 19, 2021 at 22:03
  • \$\begingroup\$ If you're not sure why the audio transformers are centre-tapped my answer here may be of interest. \$\endgroup\$
    – Transistor
    Sep 19, 2021 at 22:04

3 Answers 3

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For an ideal transformer, you're perfectly correct.

However, you've not got a catalogue of ideal transformers, you've got a catalogue of 'audio transformers', that is, transformers designed for use in an audio amplifier or distribution system.

The listed impedances are the ones the transformers have been designed for.

Designing a wideband audio transformer is quite difficult, more so than designing a single frequency transformer. To push the low frequency end down, you need high primary inductance compared to your operating impedance, which needs lots of turns and a large core. To push the high frequency end up, you need a high SRF, which needs low self capacitance, which needs the number of turns minimised and a large core. To maintain a low loss at the operating impedance you need thick wire. To have a low cost, you need thin wire and a small core.

Transformer designers first decide on a target impedance range, power and bandwidth range, then optimise a design, and place it in the catalogue.

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  • \$\begingroup\$ This is the answer I was looking for thank you very much. As I understand it % Z or % impedance is a characteristic that is not called out for audio transformers, but rather more commonly for power transformers. "Primary Impedance" seems extremely misleading as a listed parameter in component selection, making it sound like its an attribute of the transformer, not the expected user input impedance range. I also could find absolutely no documentation on this stuff. Thanks \$\endgroup\$
    – GodJohnson
    Sep 20, 2021 at 17:11
  • \$\begingroup\$ @GodJohnson Yiu can use a transformer at any impedance you like. However, if you use it at significantly different to the design impedance, you won't achieve the bandwidth, or the power handling, that the transformer is advertised as having. Do accept the answer if you feel it addresses your question. \$\endgroup\$
    – Neil_UK
    Sep 20, 2021 at 19:09
  • \$\begingroup\$ Just to explicitly state for future readers: primary impedance is the impedance you anticipate the device connected to the transformer primary side to have, I.e. the device connected to the input will have an impedance equal to primary impedance. For secondary impedance you anticipate the device connected to transformer output to have the specified impedance. To me this was confusing. \$\endgroup\$
    – GodJohnson
    Sep 24, 2021 at 18:52
  • \$\begingroup\$ @GodJohnson You're not entirely correct there. When the secondary is loaded with the rated secondary impedance, the primary will present the rated primary impedance to the device that's driving it. If the criterion was maximum power transfer, then that would also be the output impedance of the driving device. However for audio amplifiers, max power transfer is usually sacrificed in favour of some other criterion like minimum distortion, or amplifier dissipation. \$\endgroup\$
    – Neil_UK
    Sep 24, 2021 at 20:02
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Let's consider an ideal transformer with just two coils: Primary and secondary.

The key characteristics of a transformer is the ratio between the number of turns of wire creating primary and secondary coils.

The easy to understand part is that the voltage between the coils follows the respective numbers of turns:

If a primary coil has p turns and the secondary coil has s number of turns then the ratio is \$r=s/p = Vs / Vp\$ where the Vp and Vs are Primary and Secondary voltages.

The currents, however, are transformed by 1/n (to keep the power at the input the same as the power at the output). (Remember: We are discussing *ideal transformer" with no losses)

Now, let's define the primary side impedance:

\$Zp=Vp/Ip\$

you may see that the secondary impedance is:

\$Zs = (Vp*r)/(Ip/r) = Zp* r^2\$

The impedance is "transformed" by \$r^2\$ (squared) where r is the ratio between the number of primary and secondary turns.

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  • \$\begingroup\$ I think i understand this concept. What I dont understand is why does digikey list audio transformers with parameters "primary impedance" and "secondary impedance" when these parameters are dependent upon what is placed at the input/output of the transformer \$\endgroup\$
    – GodJohnson
    Sep 20, 2021 at 1:43
  • \$\begingroup\$ There is a difference between "ideal" and a "real" transformer. The real one has inductance, resistance, parasitic capacitance, losses in the magnetics, max magnetic flux where it may still be considered linear etc. The technical parameters are probably converted into a typical or characteristic impedance where a transformer is expected to perform as intended. The example you linked has three coils with the same number of turns and specifies 1 kOhm, 1 kOhm and 250 Ohm. My best explanation would be that you can configure it 1:1 (1kOhm : 1kOhm) or 2:1 (voltage) ==> 4:1 impedance. \$\endgroup\$ Sep 20, 2021 at 4:32
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Consider a cascaded arrangement of an op amp followed by a 12:1 step-down transformer followed by an 8 ohm speaker.

The impedance that the op amp "sees" will be 144*8 ohms. This is because, moving right to left from speaker to op amp across the transformer, the voltage has increased by a factor of the turns ratio but the current has decreased by the same factor (by the turns ratio). Since R = V/I it means that inserting the transformer in between the op amp and the speaker has increased the impedance that the op amp "sees" by a factor of the turns ratio squared.

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  • \$\begingroup\$ I think i understand this concept. What I dont understand is why does digikey list audio transformers with parameters "primary impedance" and "secondary impedance" when these parameters are dependent upon what is placed at the input/output of the transformer \$\endgroup\$
    – GodJohnson
    Sep 20, 2021 at 1:42
  • \$\begingroup\$ @Godjohnson Sorry, I can't help you with that. \$\endgroup\$
    – James
    Sep 20, 2021 at 12:13

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