Audio transformer: What to do with an unused winding?

Question

For the purpose of impedance matching and floating a single-ended, ground-referenced signal, I am planning to use an audio transformer like this one.

Here's the schematic from the data sheet:

My source has 150 Ω and I am using an output impedance of 600 Ω. Therefore, I am connecting pins 6 and 7 and feed the secondary (5-8) into my 600 Ω load. For the source, there appear to be several options:

1. Feed the signal to 1-2 only, connect a 150 Ω resistor across 3-4. This will increase the power that has to be provided by the load, but the impedances on all windings will match and the desired impedances cannot be achieved: 150 Ω source impedance matches 1-2, the additional 150 Ω resistor only appears to matches 3-4 but really messes up everything else, and the output resistor matches 5-(6,7)-8. [edited after Neil_UK's answer]
2. Feed the signal to 1-2 only, leave 3-4 open. No additional power required for unused winding 3-4, impedances on 1-2 and 5-(6,7)-8 will match, but any parasitic capacitance across unused winding 3-4 might build a resonant circuit across this winding and degrade the fairly linear frequency response of the system. Might be a bad idea (?).
3. Connect the two primaries in parallel. No additional resistor required, thus no additional power dissipation. Impedances on all windings will match: 150 Ω source impedance matches (1,3)-(2,4), and the output resistor matches 5-(6,7)-8. However, any possible mismatch between the two primary windings might degrade the performance. Mains (50/60 Hz) transformers that can be configured for either 115 V or 230 V are usually connected exactly this way to use the additional copper of the paralleled primary windings, but how about audio frequency transformers, where stray capacitance is a concern because (a) it is larger due to the high number of turns and (b) it matters more because we're using frequences up to 15 or 20 kHz instead of just 50/60 Hz?

Which option is preferred? Why?

Answer 1

Option 3 is best

It does not matter whether you connect the primaries together or not, you are still stuck with the primary voltage appearing on pins 3 and 4 as a result of transformer action, and still stuck with that interacting with whatever parasitic capacitance exists.

At higher frequencies, the leakage inductance will tend to decouple winding 34 from 12, so the parasitic capacitance on 34 would be better controlled by connecting it directly to 12. This favours option 3 over option 2 somewhat.

Option 1 does not do what you want. The 150ohm resistor will appear in parallel with the transformed 600ohm load, and present a 75ohm load to your primary.

Answer 2

Generally, for audio, matching isn't really that useful so I'd feed the signal into 1&2 and take an output from 5&6. This means that the 600 ohm load is seen as a 600 ohm load on the 150 ohm source i.e. it lightly loads it. If there is a compelling reason to achieve matching then spell it out.

Which option is preferred? Why?

I'd strongly consider a 4th option. I'd be keen to know that the output is representative of the input so I'd be tempted to use winding 7&8 as feedback to the driver thus, high frequency losses due to leakage inductances can be somewhat mitigated. But, it may not be that important to you?

Answer 3

As I understand it, each winding has an impedance of 300 ohms, therefore there really is no choice if you want to match the required input and output impedances. You have to connect in parallel the input windings (150 ohms), and in series the output windings (600 ohms), and that's option 3.

Options 1 & 2 are using the false assumption that a single input winding has an impedance of 150 ohms.