balanced to unbalanced audio conversion

[veedub565]

I have an old military HF radio with 600ohm balanced audio in/out that I need to connect to the PC.

Actually the 600ohm transformer board is an option that is fitted. (Diagram is attached). I need to connect "aux in" and "line out" to the pc soundcard 

As far as I can work out I have two options

1) use another set of transformers to convert balanced to unbalanced. So unbalanced soundcard output, convert to balanced, into the radio... where it is converted back to unbalanced again by the optional transformer board. 

2) ditch the optional 600ohm balanced transformer board, and then its just easy unbalanced to unbalanced. It would probably still need a 1:1 isolating transformer though.

Option 2 seems the simplest. Does this seem a reasonable plan?

I'm not sure about impedances, not sure what the normal line in/out impedence is on a soundcard. I guess with a isolating transformer in play then it doesn't matter too much?

The specs for the radio say -16 to +10dBm is an acceptable level  range.

[flynwill]

I would probably start by just connecting it directly to the sound card, in both cases just use one wire of the balanced pair plus the grounded center-tap.  You might need to add an attenuator between the line-out and your sound card to prevent the output from driving your sound card's input into clipping.  In the other direction you may need to crank up the gain on the HF radio's input to get enough level.

[veedub565]

Thanks, I will give that a try. So I can just use one leg and centre of the balanced input?

[themadhippy]

cheapest/simplest/most comon way is to connect  one of the balanced legs to the center tap for ground and the other balanced leg as signal,

[David Hess]

The transformer isolated balanced outputs and inputs will happily interface with balanced or unbalanced lines.

[minifloat]

The transformer isolated balanced outputs and inputs will happily interface with balanced or unbalanced lines.

Only if the center taps are not connected to the radio ground. One possible solution would then be disconnecting the center taps.

Oh, btw. The transformers may need proper source and sink impedances for correct frequency response.
For the 600Ω radio input, it might be necessary to put 600Ω in series with the low-impedance PC source, when only feeding one leg, would be 600Ω/sqrt(2).
For the radio output, 600Ω across the whole Output or 600Ω/sqrt(2), because Line-ins are normally the in the 10kΩ range.
Why 600Ω/sqrt(2) for half-leg operation? Transformers transform impedances with n² .

br, mf

[David Hess]

The transformer isolated balanced outputs and inputs will happily interface with balanced or unbalanced lines.

Only if the center taps are not connected to the radio ground. One possible solution would then be disconnecting the center taps.

That is right; the transformer outputs need to be uncommitted.  But that should be easy to fix.

At higher frequencies this also breaks down because common mode coupling through the transformer unbalances the outputs but this should not be a problem with audio.

[H713]

The beauty of transformer balancing is that it's pretty flexible in this regard- this is one of the reasons why some pro audio equipment still uses them.

-One side goes to the hot connection (center of RCA, tip in a TS connector)
-Other side goes to the negative connection (shield of RCA, sleeve in TS connector)
-Center goes to a separate shield on the cable, if it exists, otherwise leave it open. If you're like me and use extra starquad cable for your interconnects, tie the braided shield to the center tap.

Careful about tying the center to one of the two sides of the transformer- this creates a shorted turn. While the transformer might not be damaged (low power level), it will load the driver circuit heavily which will lead to distortion.