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I have a line-level audio output stage with an output impedance between 100-600 Ohms and an input stage with an input impedance of 10-20kOhms. I am looking to use an isolation transformer to connect these stages, isolate them and eliminate common mode noise between them. I am looking at audio transformers, and I have the option between 600CT:600CT or 10,000CT:10,000CT impedances.

Maybe I'm misunderstanding this, but it has been mentioned in previous posts that these impedance ratings refer to the load impedance for which the transformer is designed, as it will have the flattest output response in the region in which it is designed to operate. So am I correct in assuming that I need a 10,000CT:10,000CT transformer?

I feel like this specification is confusing, as 1:1 signal isolation transformers are not designed to transfer power, and the specification suggests that both input and output stages should have the same impedance. It also could be easy to interpret it as meaning that 10,000 Ohms is the impedance of each coil itself. Can someone please clarify?

EDIT: For anyone else looking for similar information, check out this very informative article on designing circuits using audio transformers by Rod Elliott.

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I believe the most important feature of transformer impedance matching is to achieve the maximum transfer of power between the output and input impedance. Nowadays that is rarely required due the adopting of lowz outputs and Hiz inputs. There may be an advantage in terminating a 600:600 tfmr with 600 ohm resistor before connecting to your 10K input. A 600:10k transformer will also give a voltage gain of x4 which may not be required - or may even overload the 10k input. edit..Sorry..600:10k was not one of your options in the OP..Go with the 600:600 every time.

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  • \$\begingroup\$ Thanks Robert, I'm adding a link as an edit to my original post to an excellent article I found on designing circuits with audio transformers, it seems I'm not the only one who finds the ratings confusing and not extensively helpful. \$\endgroup\$ – User7251 Jun 21 '17 at 13:58
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I would use a 600:600 transformer just as is.

Depending on the quality of the transformer, you may want to add a damping resistor to the output side. You can determine whether you need this resistor by driving the input stage with a square-wave signal and observe the output stage signal. Look for ringing on the signal.

You adjust the value of the damping resistor across the secondary of the transformer to minimize any ringing.

In general, you want to drive the primary of the transformer with as low impedance as possible. This maximizes low-frequency response.

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