I have several voice-radios with amplified mono outputs. The radios are designed to connect directly to a 4Ω speaker. I would like to connect two (or more) of them to the same speaker.

The radio transmits human voice (think walkie-talkies), so the signal is relatively narrow at 300-3kHz. Each radio has an independent volume knob.

Naively mixing two (or perhaps more) audio signals is relevant, but the solutions suggested are active. In my case, my signals are already amplified, so in theory I don't need further amplification. Furthermore, an active solution could have an undesired impact on a certification effort.

Using resistors is likely to substantially alter the frequency response, as explained in https://electronics.stackexchange.com/a/29871/248767, so that looks like another dead end.

Question: can these amplified signals be mixed together passively, maybe via some intelligent usage of decoupling capacitors and/or transformers, or is the simplest solution just to run each radio to its own independent speaker and call it a day?

  • \$\begingroup\$ Mixing without mixing controls? \$\endgroup\$
    – Andy aka
    Commented Aug 15, 2023 at 16:21
  • \$\begingroup\$ @Andyaka yes, so it basically becomes a summer. \$\endgroup\$ Commented Aug 15, 2023 at 16:25
  • \$\begingroup\$ Simple way is to connect the two radio's GND together... then connect one end of a speaker to one radio audio-hot and the other end of the speaker to the other radio audio-hot. Only works for 2 radios. \$\endgroup\$
    – glen_geek
    Commented Aug 15, 2023 at 16:25
  • \$\begingroup\$ @glen_geek that is simple! Very nice. Question: would independent volume control still work, and would it still work if one radio were simply off? \$\endgroup\$ Commented Aug 15, 2023 at 16:38
  • \$\begingroup\$ @KennSebesta You need to be careful with this, many two-way radios use balanced amplifiers and will NOT like having their speaker terminals grounded or connected to another radio. Can you tell us what model radios you plan to use? \$\endgroup\$
    – GodJihyo
    Commented Aug 15, 2023 at 16:59

3 Answers 3


Question: can these amplified signals be mixed together passively, maybe via some intelligent usage of decoupling capacitors and/or transformers

Connect each radio output to a transformer (for isolation) and connect both transformer secondaries in series to achieve a summed output. No mixing controls or anything special.

  • \$\begingroup\$ I lpve the simplicity. Do you have a reference I can read up on that? I wasn't able to find much on people doing this with high-power, although gearspace.com/board/showpost.php?p=666410&postcount=6 sounds very pessimistic about the technique. \$\endgroup\$ Commented Aug 15, 2023 at 19:44
  • 1
    \$\begingroup\$ No reference; just 50 years of experience and electrical common-sense on transformers @KennSebesta \$\endgroup\$
    – Andy aka
    Commented Aug 15, 2023 at 20:00
  • 1
    \$\begingroup\$ Just try it and see but, realistically for perfection, you need to use the same transformer model and, that needs to be frequency spectrum rated for audio (not power) and ideally both will step down 2:1. \$\endgroup\$
    – Andy aka
    Commented Aug 15, 2023 at 20:22
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    \$\begingroup\$ @JasenСлаваУкраїні please justify your concerns by writing an answer (comments aren't enough). Try and show how cross-over distortion would occur (given that the OP may be using class A amplifiers). How close to perfection is "near-perfect". I see your comment as an unwarranted and unjustified "spoiler" at the moment so clarity is needed (not guesswork or speculation). Sure, what I propose is rough and ready but, that's the whole point. Your answer would be seen as acceptable if it tells the OP the passive idea is badly flawed because of this, that and the other etc.. \$\endgroup\$
    – Andy aka
    Commented Aug 16, 2023 at 7:43
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    \$\begingroup\$ I'm sorry Kenn but I think I have gone past the point from advising you to solving problems that can only be done with a bench test; sometimes you just have to try things out carefully at first then, see what happens. These are proprietary radios so I suggest contacting the supplier now. \$\endgroup\$
    – Andy aka
    Commented Aug 16, 2023 at 13:09

The two receivers have internal amplifiers that drive the loudspeakers.

It's hard to be sure how amplifiers will behave when they have another amplifier connected to their output terminals,

One possible way to isolate them from one another is to use a hybrid transformer


simulate this circuit – Schematic created using CircuitLab

If galvanic isolation between the two sources is not needed (only power isolation is needed) a simpler circuit can be used. Here L1 is a 1:1 (centre-tapped) autotransformer. This can be constructed from a 1:1 transformer by connecting together one leg from each winding. Or just using a centre-tapped winding on a transformer with other windings.


simulate this circuit

  • 1
    \$\begingroup\$ (Was just about to look up whether circulators where applicable for audible frequencies.) \$\endgroup\$
    – greybeard
    Commented Aug 16, 2023 at 13:53
  • \$\begingroup\$ :) circulators for audio, it would hard to do that and have it be portable! - apparently accousic circulators exist but you'd need to two loudspeakers. \$\endgroup\$ Commented Aug 16, 2023 at 13:54
  • \$\begingroup\$ Nice! Why tie together the outputs with the 4R (16R) resistor in the top (bottom) schematic? \$\endgroup\$ Commented Aug 16, 2023 at 14:13
  • \$\begingroup\$ It absorbs the power that would otherwise flow to the other input, that wikipedia page explained it well last time I checked, \$\endgroup\$ Commented Aug 16, 2023 at 21:11

One speaker per radio is the simplest solution. A multi-gang rheostat can be used for output volume control. It won’t affect the frequency response enough to matter. Each radio’s own volume knob is then like a level knob in a mixer: the relative volume of each radio in the “mix” is adjustable. The neutral volume knob position for each radio would be at 50%-75% depending on the desired maximum volume.

  • \$\begingroup\$ I don't fully understand. Did you mean one resistor per radio? If it's one speaker per radio, then wouldn't the rest of the answer not be applicable since the radios are isolated at this point? \$\endgroup\$ Commented Aug 16, 2023 at 14:30

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