I have a Radio that I am modifying to be able to play a old radio programs off of a SD card.

Basically I have a mp3 player and radio in one case the mp3 play is powered off of its own power supply and the radio is powered normally but with a relay in line so that the mp3 player can cut power to the radio. Then put it own signal to the + - lines of the speaker.

Can I have both devices soldered to the + - lines of the speaker or will this cause problems?

They should never be playing at the same time. If the mp3 player is playing there should be no power going to the radio and if the radio is playing that means that the mp3 play is not playing otherwise it would have cut the power to the radio.

Is it safe to do or would I have to make some kind of mixer? Is there any way there could be some kind of feed back that could damage ether device?

The mp3 play is powered of 5v usb if that helps and the Radio is 110v. The only connection would be the - and + of the speaker with both connecting to the - and +.


1 Answer 1


Can I have both devices soldered to the + - lines of the speaker

Not in the general case, no.

or will this cause problems?

Quite possibly, yes.

  • Amplifier outputs aren't really designed to be backdriven by something else. Ideally they are somewhat robust against electrical faults (for example shorts) but not always. Before IC amps with build in protection circuits, blown fuses or transistors could happen. Note that two amplifiers trying to drive opposite voltages are similar to a shorted speaker wire.

  • The specifications of many active semiconductor devices specifically prohibit having voltages on pins when the device itself is unpowered.

  • Even if there is no damage, the unused source may load the speaker lines, taking power and quite possibly causing distortion.

A "mixer" would only be a solution for low level outputs which you then fed into a power amplifier - you don't really mix power amplifier outputs.

A switch (possibly even more contacts on your relays) could be an option. Running an amplifier without a load isn't a good idea either, but many will tolerate this briefly even if they don't have protection against other kinds of faults.

  • \$\begingroup\$ You seem to know the thing. I was wondering: what if I want to use two sources and I want to simply join them with + and -, however, using two diodes on each pin, would that be okay? \$\endgroup\$
    – AgentFire
    Dec 24, 2017 at 20:12
  • 1
    \$\begingroup\$ No, not at all. Audio signals are AC, you can't combine them with diodes. \$\endgroup\$ Dec 25, 2017 at 4:56

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