Multiple headphone amp outputs.

[mrpackethead]

Looking for a low cost way of being able to distribute audio ( via headphones ) to groups of people at events that need it translated into a 2nd language.   Am thinking of radio, but am considering if a wired solution is possible.   The seating is fixed ( theatre sytle )..  in rows of up to about 16 

Can assume a balanced professional line level input. and AC mains in avaialbe.

[Rerouter]

Cheapest would just be use $5 FM radios set to 1 stations, rip out the battery and power it from a supply, and broadcast using a limited power transmitter inside the theater?

Otherwise, a minimally loaded local amplifier, just using the balanced line as the feed, then using a small headphone balanced to single amp, in each seat,

[JS]

Small-ish power amplifiers, like 100W would do many headphones at once, or big and do a lot of headphones at once. Use an attenuator I each headphone, it can even be adjustable.

Not kidding, this is widely used in recording studios, a signal is fed from a few power amps to all places and then a box would select the source (from a stereo connection, select stereo, mono only L or R) and attenuate.

You have to do some math to calculate the attenuator resistor for the given power and impedance and then estimate how many HP would run from one. It's not a very efficient way, but the more quantity of smaller amps you use is more efficent. Or, if you need efficient, you could use a big ass transformer in the output of a big ass power amplifier and get an output voltage closer to what you need, then a small attenuation or level control in each seat should be enough.

JS

[NiHaoMike]

Get one of those amplifier modules that doesn't have a bridged output, then run wires to the seats with each one tapping off the bus through some resistors. Try 470 ohms for a start, keeping in mind that the higher you go the more headphones you can run from the amplifier. (Although at 470 ohms, a single 4 ohm rated amplifier module will easily handle over 100 headphones.)

[SiliconWizard]

Looking for a low cost way of being able to distribute audio ( via headphones ) to groups of people at events that need it translated into a 2nd language.   Am thinking of radio, but am considering if a wired solution is possible.   The seating is fixed ( theatre sytle )..  in rows of up to about 16 

Define low cost?
Behringer makes a quite decent 8 headphones amp:
https://www.sweetwater.com/store/detail/HA8000V2--behringer-ha8000-v2-8-ch-headphone-mixing-distribution-amplifier
for $149. That would be about 300 bucks per row.

They also make a much cheaper 4-channel headphones amp for like $25.
You would need 4 per row, thus about $100.

The plus is that you wouldn't have a single point of failure. If you use a single power amp and it fails, you end up with no sound.
With seperate modules of 4 or 8, it's more distributed. You can always store 1 or 2 extra in case one fails, and you're all set in a matter of a couple minutes since this would all be standard connections.

[bson]

Most small consumer headphones only need a fraction of 1mW to get really loud.  So how about running the audio over a cat5 100ohm twisted pair (or two pairs for stereo), use plain old RJ-45 jacks, and magnetically couple it:



Voltage in on J1 should be around 0.5V rms, with 10V peak to peak (crest factor of 10) as worst case for music.  Speech can be compressed to bring the peaks down (as is commonly done).  The transmitting side can be driven by an op amp with a resistor and a TVS diode to protect it.

[NiHaoMike]

Most small consumer headphones only need a fraction of 1mW to get really loud.  So how about running the audio over a cat5 100ohm twisted pair (or two pairs for stereo), use plain old RJ-45 jacks, and magnetically couple it:



Voltage in on J1 should be around 0.5V rms, with 10V peak to peak (crest factor of 10) as worst case for music.  Speech can be compressed to bring the peaks down (as is commonly done).  The transmitting side can be driven by an op amp with a resistor and a TVS diode to protect it.

The main flaw with transformers is that they're not cheap, which adds up if one set is needed for each seat. Now one set shared among several seats can be reasonable, in which case common 70V line transformers can be used.

If you use a single power amp and it fails, you end up with no sound.

Given how cheap a 50W or so amplifier module is, it's not at all unreasonable to keep a few spares. And especially in such light duty, they rarely fail.

[bson]

Sharing between seats makes sense, this also will reduce the amount of wiring needed.

In that case add a resistor to each headphone, between the secondary and the volume pot, to limit current (and max volume) so a single volume pot or bad jack or headphone cable can't effectively short it out.  Might a good idea regardless.