Cable Connection Problem

Hi AV gurus (I hope!)

I'm puzzled. I'm in a modern(ish) flat, with wiring conduits built into the walls. I've run 2 cables from one side of my room to the other. There's a speaker cable, terminated with banana sockets for the right rear speaker. The other cable is terminated with phono sockets. Plan is to use an active sub at the rear (and wire the left rear speaker separately).

I've used a mono amp and single speaker to test both as speaker cables, and they're fine. Connect the amp input to the phono cable (with different sources connected to the other end at different times) and the amp hums as it does when no source is connected to its inputs.

So both cables are intact, and work fine with speakers, but the phono cable doesn't connect a line level signal at all. All I can think of is that it's attenuation. It's a long cable run, from room to a hub in the hall, and out again to the other side of the room. So 15m or so of 1.5mm cable, but attenuation doesn't seem to be the most likely cause, as I'd expect at least some of the signal to get through.

Any suggests at all why the phono cable doesn't work? A passive sub would work, but they seem to be like hens' teeth at the mo', or I could use a wifi phono sender to connect the sub, but my preferred option would be the cable connection.

TIA
Chris

Is the phono cable 15m in length? Do you know what cable is used?

Are the plugs/sockets terminated (ie wired up) correctly both ends?

Hi - thanks for replies
Barry - original plan was for speaker wiring, so both are 1.5 mm speaker cables. Length of 15m is approx.
Scotty - yes, terminated to phono sockets and banana plug sockets respectively at each end. Wired the sockets myself. Double and treble checked
Thanks,
Chris

1.5mm² copper wire has a resistance of 11.5mOhm/m, so the loop resistance for a 15m pair will be 0.35 Ohm.

Assuming the amplifier has an output impedance of less than 0.1 Ohm, and your speakers have a nominal impedance of 10 Ohm, the damping factor will be ~ 21, which ought to be sufficient. The loss in the cable will be ~ 0.3dB, which is negligible.

If the speaker cable is of coaxial construction, as used for signal interconnects, then because the diameter of the central core is quite small, the loop resistance of a 15m length could be quite high. Exactly what coaxial cable are you using?

Thanks for confirming my uninformed assumption that losses would be negligible Barry. Unfortunately, I bought the cable a couple of years or so ago (speaker cable from Richer Sounds), and can't remember the brand. By now, I've tried a stereo amp as well as the mono. No hum - just white noise.

Looks like I'm dead-ended on reasons the cable won't connect. Looks like plan B or C - passive sub or wifi sender.

Cheers,
Chris