Teensy 4 Audio headphone port noisy when unpowered

[StewartG]

Hi all,

This is my first post here, so I'll begin with a big thanks to Paul and the community contributors for a great and fun development platform.

Here is a small problem I noticed with the Teensy 4 Audio board. I use the headphone port to connect to an external amplifier. The sound is good when operating normally, but when unpowered it emits a loud hum. Presumably in this state the headphone outputs are floating in such a way that the Teensy and any attached cables are acting like a big antenna for ambient 50Hz mains noise.

I managed to fix this problem by adding two 1k resistors from both HPL and HPR to HPVGND. I figure that this provides a return path for any induced electrical noise, without significantly decreasing the load resistance. Seems harmless to me, but is there a better approach?

Cheers,
Stewart

 

[MarkT]

That's a good fix - you can use 100 ohm if you want even less noise, as the headphone outputs can drive low impedances. Your diagnosis is correct, audio amps will typically generate significant noise without a low impedance at their inputs as they are usually something like 50k to 100k at the input, and noise power at the amp output is proportional to the input noise resistance.
Keeping the interconnecting cable low impedance will also make it much more robust to external EMI.

 

[StewartG]

Thanks Mark, 1k is working well for me, but its good to have some options.