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Voltage and power matched audio in 2014

Filed under: audio,audio-video systems design — 2014-04-26 by Richard L. Hess — Last Edit 2014-04-26 by Richard L. Hess

I received an email requesting clarification on my 1980 AES Preprint about the use of voltage audio distribution vs. power matched audio distribution for analog audio signals.

The confusion seemed to be about equipment being rated for driving a 600 ohm load. Yes, most professional audio equipment will drive a 600 ohm load, but might (repeat might) lose a small amount of headroom doing so. The better reason to be able to drive a 600 ohm load is to drive long cables which might create slew-rate limiting if they load the output to the extent that they slow it down. It’s all about current. In fact, merely being able to drive a 600 ohm load may not provide enough current to drive very long cables.I will let the designers working on these details determine the proper amount of drive current in order to adequately drive the desired cable length, but please allow me to suggest that 300 m is not too long a minimum design criterion.

I have not seen a professional broadcast plant that had 600 ohm terminating inputs on analog audio signals in a very long time. I think the Canadian Broadcasting Corporation migrated away from this in the 1980s as they built new facilities. To the best of my knowledge they were the last hold-outs in North America on a system-wide basis.

Now, I’m not talking about long cable runs using analog technology, for example a studio-to-transmitter link. I’m talking about normal in-plant wiring where 100-300 m is the typical longest length.

Since we are seeing more fibre transmission and embedding audio into digital video (and then possibly putting that on a fibre), the days of huge analog plants seem to be numbered. Honestly, I have been away from large facility design for about a decade, so I do not have my nose in this space, but from what I’ve read and heard, it seems to be the way facilities are going…if not completely file-based.

So, yes, design an output stage to drive a 600 ohm load, but do NOT terminate the following input in a normal broadcast or recording plant.

If there is a very long run (studio-t0-transmitter link, for example), I would suggest going fibre for many good reasons, unless this is a wireless point-to-point system, then that should be a digital microwave system.

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