Listening to multiple Headphone outputs simultaneously through one speaker.

Hi everyone,

I’m working on a small Arduino project where I want to monitor the outputs of 5 radio-sets simultaneously on one loudspeaker.

When I just connect all the lineouts in parallel, I’m afraid that the output of the amplifiers will damage the others?

What’s the best (easiest) way to do that?

Thanks in advance
Andreas

Connect a 10K resistor in series with each line output before you join them together.

When I just connect all the lineouts in parallel, I'm afraid that the output of the amplifiers will damage the others?

GOOD THINKING!!!!

The general rule is NEVER connect outputs together. It's generally OK to connect inputs together, such as driving 2 or 3 amplifiers from one CD player.

If you need to mix audio signals, use a mixer. Resistors will work as a passive-fixed mixer, but with all five amplifiers connected, the signal from each amplifier will be reduced to 1/5th of the original voltage (a 14dB loss). There is actually potentially some more signal-loss, depending on the input impedance of the amplifier connected to the output of the mixer-circuit.

That's assuming you run the mixer-circuit output into another amplifier... You won't get ANY sound through 10K resistors directly into an 8-Ohm speaker.

And, you shouldn't connect headphone-output to a 4 or 8-Ohm speaker either... Headphones/earphones have higher-impedance than speakers, and they require a lot less power.

"I'm working on a small Arduino project where I want to monitor the outputs of 5 radio-sets simultaneously on one loudspeaker."

You did not tell us the radio specs, so let me guess :
Each radio set uses a 16 ohm speaker that you will tap wires onto.
You have one 16 ohm speaker to combine the sounds.

It is safe to put a 16 ohm resistor in series with each of 5 output wires. Join all 5 resistors. Add a 16 ohm resistor at that new node. Drive one speaker from that common resistor and a ground. The resistors need to be able to dissipate 1 watt or whatever your secret spec is.

You can buy 500 ohm headphones or 50 ohm headphones to use instead of one 16 ohm speaker.

When I just connect all the lineouts in parallel, I'm afraid that the output of the amplifiers will damage the others?

Line out is typically 1V p-p, with a typical AC impedance of 600 Ohm although it could be lower, say 100 Ohm. Headphone jack output is in the area of 32 Ohm.

The best way to deal with (multiple) line out is with 1K pots. All outputs go to the top of the pot, all bottoms of pot are connected together, use separate 10uF capacitors back to the lineout common of each device. Set all pots to 50%.

Exact same for multiple headphone outputs BUT use 50 Ohm pots.

Connect the wipers together, connect gnd/common to common of other inputs. Feed this single input into your line input of amp. If using (mono) headphones, just connect the center to the wiper connection and shield to gnd.

For mono headphone, try to find a model with no less than 32 Ohm impedance... That is, NOT an 8 Ohm speaker... If you want a speaker, look for a cheap transistor amplifier AND add a 470 Ohm resistor in series with the connected wipers and line input.

Adjust pots +/- to balance.

Thanks a lot for all your replies.

The Radios are commercial Kirisun 2-way-radios which have an output connector for an earphone - no line-out. Sorry for mixing things up.
I'm not sure whether I'll use headphones or a speaker as output - but with all the information you gave me, I think I can figure out a proper way to get that working.