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I've been trying to DIY a home audio setup. We have a pair of speakers in the ceiling of an upper floor (previously installed), and a new TV on the same floor. We wanted the sound of the TV to come out of the speakers.

The speakers are connected to an amp in the basement, and there are existing CAT5 cables running behind the new TV down to the basement. Rewiring is challenging, so we took advantage of the existing setup and got a cheap pair of audio-to-CAT5 baluns (https://www.amazon.ca/LINESO-3-5mm-Stereo-White-Extender/dp/B01HHO8382), hooked one side to the TV (3.5mm jack) and the other side to the amp in the basement. The TV sends audio to the audio-out port, through the balun, through the CAT5 cable, back through another balun, to the amp, which amplifies the sound and sends it to the speakers.

Now, for the fun part: when the TV is on, everything works great. But, if the TV is turned off and the amp is set to a loud enough volume, there's an audible mains hum (~60Hz) over the speakers. My best guess is that the CAT5 cables are picking up small currents from the mains over their length (not surprising as they run through multiple floors). When the TV is on, the TV's line out presumably sinks these stray currents (if not playing anything) or otherwise produces enough current to drown out the induced currents. When the TV is off, it disconnects its audio output; the amp's 47kΩ input is high enough and has high enough sensitivity to amplify the induced currents into a buzzing noise.

What can I do to reduce or eliminate the unwanted sound, ideally with a passive solution? I have thought about adding a resistor somewhere but I am not sure whether it would be in parallel or in series with the connection, nor what value it should have.

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I wonder if that's really a balun or just a straight-through connection?

Anyway, the output impedance of "line out" is apparently in the 100-600 ohm range. So if you want to try a passive solution I would start with a 1k resistor across each line. This will attenuate the output somewhat along with attenuating the noise. If it's too much, add 1k resistors in series with each other; if it's not enough, add one in parallel.

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  • \$\begingroup\$ Thanks - when you say to put a 1k resistor across each line, do you mean putting a resistor in parallel with the load (= the amp)? My analysis of this using my high-school understanding of electronics is that it would reduce the load resistance seen by the mains current from 47kΩ to a bit over 1kΩ, which would (presumably) drop the voltage by a factor of 47^2, or about 33dB. Is this right? The other side of this would then be - what's the effect on the source for driving this lower-resistance load? \$\endgroup\$
    – nneonneo
    Commented Oct 19, 2020 at 8:56
  • \$\begingroup\$ In parallel, yes. The source should generally be fine as long as the source impedance (100-600, maybe) is lower than the impedance it's driving. There's also a voltage-divider effect there, which is why the intended signal will be attenuated as well. \$\endgroup\$
    – pjc50
    Commented Oct 19, 2020 at 9:10
  • \$\begingroup\$ I put a 2.2k resistor in parallel across each line (using a Y splitter and splicing a spare RCA cable) - worked a treat! Noise is gone & the sound still comes through clearly. Thanks for the suggestion! \$\endgroup\$
    – nneonneo
    Commented Oct 27, 2020 at 2:24
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What can I do to reduce or eliminate the unwanted sound, ideally with a passive solution?

I think your analysis sounds reasonable so...

Your best bet might be to use an audio coupling transformer at the input to your amplifier. There's a good chance that your amplifier doesn't have a fully differential input stage but, if it did, the noise that you are hearing would be greatly balanced-out to near zero levels. So, try and get hold of an audio 1:1 transformer - it stands a decent (and passive) chance of solving the problem.

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  • \$\begingroup\$ I'll confess - when the problem first manifested, my searches led me to transformers as well. But, I don't really understand the principle behind how they would help (i.e. - why would a transformer not pass induced mains currents, while passing the currents produced by the TV?). Any light you could shed would be very helpful in deciding what to do! \$\endgroup\$
    – nneonneo
    Commented Oct 19, 2020 at 9:00
  • \$\begingroup\$ Because a transformer has a balanced input, the noise voltages picked up on each wire will be nearly identical. It isn't a current thing; it's a voltage pick-up thing. \$\endgroup\$
    – Andy aka
    Commented Oct 19, 2020 at 9:27

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