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I understand from my research that the RPI has a fairly high impedance audio out, and that whilst it may drive small headphones, it's really not man enough to drive real speakers properly, so clearly I must amplify the output for that purpose.

My question therefore, is it safe and sane to connect a lot (say 10 to 15) small amplifiers in parallel to the audio out, each of which drives a speaker (via 10-30m of Cat5)?

I know this might sound a bit mental, but I am trying to resurrect some radio functionality of a circa 1960s (perhaps, I guess) hotel intercom system, a relic that my parents decided to retain in their new house during its extensive renovation as a nod to its history and heritage, and I'm finding many gaps in my knowledge whilst considering various options.

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    Ahh, sorry about that! It's an original/early/V1 version B device. In case that doesn't narrow the field enough I ca – Michael Dec 30 '15 at 1:23
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    Right, yes, my RPI has a yellow RCA and blue audio out. – Michael Dec 30 '15 at 1:24
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    I've done something not totally dissimilar in the past (a 6 unit headphone unit for a TV in an Old-peoples home) - you may want to have a "line-driver" unit to amplify the audio next to the Pi and use that to send out a boosted audio-signal over the lines and then those separate amps (with it's own volume control?) at each speaker. Are you planning a mono or stereo system? – SlySven Dec 30 '15 at 1:28
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    Right, this is what I was unsure of, I don't have much idea of how much power an amplifier input requires, and when you get to 10 to 15 of them, clearly an odd requirement, I thought I'd better do a bit of consulting! It's all mono, because the units on the walls of the former hotel rooms just have one speaker in each. – Michael Dec 30 '15 at 1:32
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    Any idea of impedance and power rating - if you're into building your own circuits I'd take a look at the TDA7233M - the mono version of the one I used (TDA2822M) for those headphones for the speaker amps nearly 2W with a 12V supply into 8ohms. You then want a single-ended op-amp with a fairly decent... hang this is getting into EESE territory! – SlySven Dec 30 '15 at 2:13
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Apparently, you have access to a lot of amps. If not, you can get one more, I'm sure of that. Make sure that the amplifier can drive 10 to 15 more audio devices in parallel. If I were in your position, here's what I'm going to do:

  1. Connect the pi to one amplifier.

  2. Connect the hotel amplifiers to the amplifier connected to your pi.

Here's a visual guide:

Your plan:

Raspberry Pi   <--->   Lots of devices

Proposed plan:

Raspberry Pi   <--->   "Primary" amplifier   <--->   Lots of devices

Contrary to what SlySven said, nothing bad will happen if you connect a pair of good ol' headphones or a normal 3.5mm audio connector to the audio/video jack. Actually sounds better than the previous models in my opinion (even without tweaking any equalizers).

Take note that there well be some moderately loud and annoying noise when plugging in and unplugging the audio connector. That's what happens when it touches the video out pin.

  • It won't in this case as the OP's unit has stereo connector - but I have reservations on whether it is really the best thing to short out the video on the later models with the black 4 pole socket... – SlySven Dec 30 '15 at 20:36
  • @SlySven Shorting with ground should not damage the Raspberry Pi since a standard TRS (yes, only one R this time) will short the video and ground, so safety measures have been implemented to prevent damage. projects-raspberry.com/raspberry-pi-2-quick-n-easy-rca – PNDA Dec 31 '15 at 8:54
  • @SlySven BUT!!! In the rare event that your Raspberry Pi does get damaged by shorting video with something else means literally something else (GPIO, +5V, +3.3V, etc.). projects-raspberry.com/raspberry-pi-2-quick-n-easy-rca – PNDA Dec 31 '15 at 8:56
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    Your second comment actually precedes the bit which contains the article writer's disclaimer: "...something else, it is IN NO WAY MY FAULT since I am not forcing you to do this (and I see no reason why anything would get damaged)." ! However, you have backed this up with empirical evidence in the third - all I can say is that wiring it up with the right connector still seems to be the safest option in the absence of anything from the Foundation telling us how it is... 8-) – SlySven Dec 31 '15 at 18:46
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    @SlySven Found a great article: cablechick.com.au/blog/understanding-trrs-and-audio-jacks Mixing Plugs and Sockets section – PNDA Jan 1 '16 at 1:13

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