i understand turns ratio and sq root of impedance etc. but what is the difference btw say a 600:600 and a 10K:10K as they are both 1:1. is it the amount of turns on the core or the wire size?

Inherently, there is nothing like a 600:600 or a 10k:10k xfmr.

In order to qualify as a X ohm winding, the impedance must be significantly higher (typically 5 to 10x) than the rated Z,

**at the lowest specified frequency**.

Let's say you want a 600ohm impedance over an operating range of 20Hz-20kHz.

The impedance is Z = L. 2pi.f

As f is a variable, you need to consider worst case, which is obviously the lowest frequency; so the calculation goes:

L = 600/ 125.6 = 4.77 Henries then you want your 5-10x safety net, which gives roughly

L = 23-47H

The same calculation for a 10k:10k xfmr would give Lp = 40-80H

But in fact, if you restrict the LF -3 dB point to 40 Hz, the same xfmr will be usable as a 20k:20k.

Alternatively, restricting the LF bandwidth allows using less turns for a specified impedance.

Wire size and core size will be dictated by the operating level and the optimization of DCR of windings.