Because I want to improve my skills in producing music, I've started taking piano lessons. To start out with, I'm using a borrowed 61 key, touch sensitive keyboard. I'd read around about this. Some people suggest you should always learn on a piano, but my teacher suggested that's not necessary when first starting out, at least until reaching ABRSM Grade 1 level. This has the benefit of not having to invest a substantial amount of money into a digital piano, should learning prove too difficult.
The first thing you realise when practising is that it's unlikely that anyone else within hearing distance is going to appreciate the false starts and wrong notes of a beginner playing piano. This is a big plus of keyboards or digital pianos: a headphone socket means you can practise without irritating anyone nearby.
So, duly plugging headphones in, I realised there was a substantial hum (possibly ground hum) from the keyboard's output, even at zero volume, and white noise which appears as the volume is cranked up. The same happens in the speaker output, but is much less apparent and distracting as it tends to get masked by ambient background noise.
I'm usually quite fussy about unwanted noise, but I found this particularly distracting when trying to concentrate. I would also say that you do tend to get used to it over time and can maybe forget it's there a little, but ideally I was looking to reduce or eliminate it.
Quietening things down
Solutions I considered but didn't try
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Try another power supply. It's possible that this might not fix it, if the power supply isn't the source, so I didn't actually buy another to test this theory. (In this case, the keyboard can be powered by a substantial number of batteries, which would be an ideal test to diagnose whether the power supply is at fault here.)
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Route the keyboard to a PC via MIDI to USB, play the keyboard via a digital audio workstation, and use a computer audio output instead of the keyboard's own output. This would work if you have PC audio that is noise-free. But, it means you then have to rely on having a PC around and powered on. Instead, I'd prefer to use an instrument directly; this way it actually keeps me away from a computer for once. If you're on a Windows PC, also bear in mind that you'll likely need decent ASIO drivers for your audio hardware, otherwise you might have to deal with considerable input latency.
Solutions I used
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Use worse headphones. The budget Monoprice headphones I have did seem to reduce the noise a little compared with the Sennheiser HD25 I'd first tested out.
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Add a ground loop isolator into the headphone socket on the keyboard and plug headphones into that. This did the trick for me. Since it's filtering the signal, it does also attenuate the volume, but that's a small price to pay for quiet. The going rate seems to be under £5 for cheap and cheerful generic kit, you can spend more on ones from the likes of Behringer, while some people even make their own DIY solutions.