My fire department uses a Kenwood TK690-H as a base radio. The system was used to (poorly) power 3 in ceiling speakers throughout the station.

Recently, I rewired the paging system in the station with a dedicated amplifier and 4 speakers. With Bluetooth, or 3.5mm mp3 connection to the amplifier the speakers sound good, no buzzing or hums, and is quiet when nothing is playing.

I attempted to splice the BTL output of the Kenwood to a 3.5mm cable that goes to 1/4" microphone input to the amplifier and it hums loudly and constantly.

Mic in is preferred as in paging mode it will override any other sources if our station is paged.

How can I fix this problem?

  • 1
    \$\begingroup\$ Hard to say, because we don't know how you wired it and how the devices are powered, and with what kind of power supplies. \$\endgroup\$
    – Justme
    Commented Jul 2, 2022 at 20:39
  • \$\begingroup\$ Please add a diagram showing how the system is wired up. Please use separate lines to represent separate conductors in a cable. The buzzing almost certainly comes from some mains AC finding its way into the audio path, but there are multiple ways this can happen. Ground wires are important, which is why I ask to be shown all the conductors, and not merely and overall block diagram of your system. \$\endgroup\$ Commented Jul 2, 2022 at 20:45
  • \$\begingroup\$ I did a search and BTL stands for "bridge tied load" and it's a type of speaker output? Are you connecting a speaker output to a microphone input? \$\endgroup\$
    – Bryan
    Commented Jul 2, 2022 at 20:46
  • \$\begingroup\$ That unit has auxiliary outputs (and inputs). I'd investigate those. You need to give us a some sort of schematic showing how you have connected things. If you are taking output of Kenwood TK690-H as input to your amplifier, then that would account for the buzzing. \$\endgroup\$ Commented Jul 2, 2022 at 21:15
  • \$\begingroup\$ Chances are one side of your MIC input is tied to ground, and both sides of the BTL output need to float (you are shorting one of them to ground). Use an audio isolation transformer and the problem should go away. \$\endgroup\$ Commented Jul 2, 2022 at 22:01

2 Answers 2


Two things connected together:

  1. Isolation transformer, Something like this: https://www.amazon.com/Smof-Isolator-Speaker-Eliminate-Completely/dp/B0171PQLB8/
  2. level attenuation, something like this: https://www.amazon.com/Inline-Control-3-5MM-Headphones-Black/dp/B008DJTB32/

Connect the output of your Kenwood VHF radio to item #1. Connect output of #1 to #2. Connect #2 to your other sound system. Adjust the volume on #2 carefully to a level that is loud enough but does not overload your external system.


With a bridge output to a Mic input, you either use a transformer to make it single-ended or just choose one output. Locate the 0Vdc= gnd from the Kenwood which will be floating to connect to mic gnd.

This is my guess with a lot of unknowns.


simulate this circuit – Schematic created using CircuitLab

The Pot is optional.


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