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As part of my my consideration of possible solutions to my multi channel variable speaker number problem arising from my attempts to restore radio functionality to an old hotel intercom system (see previous post on the subject if interested) I have been considering how to distribute say three audio streams delivered to multiple locations (by way of three/four cores per location), where a switch at those locations can be used to select an audio line.

My inexperience is leaving me slightly unsure of what approach to take in terms of the big picture / over all design. Basically what I want to achieve is this...

enter image description here

BUT... I want it for many (10+) rooms (the room being the right hand side of the diagram, where the switch and speaker is).

The problem is this leads to a design that looks more like this:

enter image description here

That's three 'rooms', but with a maximum design consideration of 4 audio channels and 13 'rooms' this means over 50 amplifiers. Small amplifiers are cheap, but still, I can't help feel it's a bit of a mad design!

Trying to come up with something more sane, I considered this:

enter image description here

BUT... The system should obviously survive usage, and here the impedance of the load applied to the amplifiers will change dramatically as variable numbers of speakers are turned on and the audio lines tapped... Obviously it's going to halve just from a second speaker being brought online with the same channel, and whilst it's not a likely use case, what if someone went round turning on 10 or more speakers all on the same channel!? The impedance of the load is going to be REALLY low now, and I have no idea if it is allowable to have such a variable impedance load for an amplifier. I do realise it's not normal or ideal, but is it acceptable if the amplifier is big enough to handle it?

I'd really appreciate any comments and/or help with the above questions or any suggestions for an alternative approach. I'm obviously more than happy to put time and effort into researching and working to expand the ultimate solution, but it's not currently clear to me which direction to go in at the moment!

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  • \$\begingroup\$ Seems like you are thinking about this backwards. Is it completely impossible to instead put the amplifiers at the speaker end? The result would be that you would be running and switching line level, rather than speaker level signals. \$\endgroup\$
    – uint128_t
    Dec 30, 2015 at 16:46
  • \$\begingroup\$ It's a sensible comment. The problem is I have to work with the/an existing system, which appears to transmit to the 'hotel' rooms a centrally amplified signal. When I first looked at the system I immediately assumed it would be amplified at the room, because that seemed sensible! But now it seems more likely to be a pre amplified high voltage transmission (a so called constant voltage speaker system). I tried to cut out as much extraneous question detail as possible to make it more suitable to Stack Exchange. Maybe over did it. \$\endgroup\$
    – Michael
    Dec 30, 2015 at 17:00
  • \$\begingroup\$ Ok, that makes sense. If it's a constant voltage system, then the variable load isn't such a big deal, as per user44635's answer. It doesn't sound like your speakers are very large, so you have less to worry about. \$\endgroup\$
    – uint128_t
    Dec 30, 2015 at 17:08
  • \$\begingroup\$ Thanks, I appreciate the feedback and confirmation, it's unfortunate I didn't know about constant voltage systems when I inspected the whole thing last week (so much to learn) but as soon as I am back I will analyse it in greater detail and measure what sort of voltage range is being transmitted. \$\endgroup\$
    – Michael
    Dec 30, 2015 at 17:13

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Yes, it is allowed to have that variable a load on your amplifiers, given certain precautions. Most amplifiers can tolerate open circuit on their outputs, and, subject to limiting the output so they don't get too hot, somewhat lower than their 'rated' load.

A common method for supplying multiple speakers from a central location is the '100v line' method. Each speaker has a transformer before it to raise its impedance. This can get expensive in transformers, that have to be rated for the audio range.

You should be able to buy high impedance speakers, 32 ohm is not uncommon. Ten of those in parallel would present around 3ohms to your amplifier, which most audio amps should be able to drive at less than max output with little distress. How much fi and power do you want, I suspect these higher impedance speakers will be lo-fi and low power.

You could precede each speaker by its own local amplifier, which would raise the per room loading to a level where it ceases to be an issue. If powering the amplifiers via the CAT5, you would want a beefy electrolytic across the supply at the amplifier.

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  • \$\begingroup\$ Interesting... I've come across the 100v line thing, someone kindly mentioned it in connection with my original post (linked in my question), and it's rather possible the hotel speaker system in this case already uses that. I had to ask about impedance effects of such systems in another question but from that I understood the 2 transformers didn't change the effective impedance of the speaker, ahh, unless the transformers are deliberately unbalanced I guess... \$\endgroup\$
    – Michael
    Dec 30, 2015 at 14:20
  • \$\begingroup\$ I will measure the resistance of the speakers when I am back in Torquay with a multimeter and other kit, but I agree, the speakers must be lo-fi and low power, the intercom part of the system works ok, it's just the radio functionality that doesn't (and probably was never installed I suspect) and the intercom sounds pretty ropey it must be said, so it's all really old probably quite robust but low tech low performance gear. \$\endgroup\$
    – Michael
    Dec 30, 2015 at 14:34
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Warning: potential danger

I have came across this in passing and didn't read the whole thing so my concern here may be baseless. However, having Cat5E wire at the output of an audio amplifier is potentially dangerous. Cat5E has very limited power transfer ability and the cable is rated to 0.5A max current or thereabouts. Anything above this is a potential fire hazard.

In addition, there may be different (and more conservative) ratings for residential cable and running speaker level signals through installed Cat5E may also be a violation of local building codes.

Therefore I strongly that you have any solution you come up with checked out by licensed professional who is familiar with local building codes.

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  • \$\begingroup\$ Hello. Thanks for your post, it's always good to raise stuff like this. The existing system as it stands is probably low current because it's probably using a constant voltage speaker system, as referenced in my comments to the other answer, but that's something I will have to check next time I am there. It was entirely recabled by a qualified electrician, so one would certainly hope the choice was sensible! Any additional equipment doesn't need to TX anything more powerful, but I will certainly be sure to get any additions looked at and certified as you say. \$\endgroup\$
    – Michael
    Dec 30, 2015 at 16:33
  • \$\begingroup\$ Cool, thanks. This may be perfectly fine, however in cases like this it's always good to have a second set of eyes, just to be safe! \$\endgroup\$
    – Hilmar
    Dec 31, 2015 at 18:03

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