Harold Hallikainen wrote:
> The fact that the speakers are amplified and have a 10k input
> impedance makes me think you can bridge a bunch of them across a
> single line and not worry about loading it much. In broadcast, we
> used to have 600 ohm sources and loads (brought over from telephone
> lines). Inputs were either terminating (600 ohms) or "bridging" (10k
> or higher). You could hang a bunch of bridging loads across the
> terminated line with no problem. One other issue is ground loops
> since both inputs and outputs are unbalanced. But I don't think even
> that is an issue if the amplified speakers are powered by ungrounded
> wall warts, so the inputs to the amplifiers are floating.
>
> So, overall, I don't think you need to add any hardware. How about
> testing it and seeing?

I agree.  I don't see why this is supposed to be a problem.  As Harold says,
the real issue is picking up common mode noise, like power line hum, that
gets interpreted as differential signal at the powered speakers.  Shielded
cable will probably help some, although it may actually make things worse if
the speakers aren't isolated.  If you do have hum, it is probably cheaper
and likely more effective to put small 1:1 audio transformers right at the
inputs to the powered speakers.  Then you should be able to run cheap
twisted pair out to each set of speakers.  I haven't used audio transformers
in many years, so I'm just assuming these things are still available
somewhere.

OK, I go curious so I checked.  Jameco has a "audio transformer" at $28 for
10 (630459), and a "modem coupling transformer" at $17 for 10 (125357).
Both are 1:1 at 600 ohms both sides.  The modem transformer probably was
designed for a narrower frequency range with more slop allowed, but most
likely will work fine for your needs.  A PC sound card output or anything
designed to drive headphones should be able to easily drive these.  The
extra cost of the transformer will be more than offset by being able to use
cheaper cable if you do have a hum problem.

However as Harold said, try it first.  You may not need to do anything
special.


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