The thing to be aware of is that you’re going to have a strong 400kHz signal coming in together with the audio. Some audio equipment will simply filter that out, which solves the problem. Other audio equipment will happily pass it through. I could imagine two kinds of audio equipment that wouldn’t like it:
- an analog amplifier with no filters, it would try to amplify the 400kHz signal. The speakers wouldn’t be able to reproduce it, and you wouldn’t hear it even if they did, but the amplifier might end up running hot or dying from the extra signal.
- A digital sampler/mixer/amplifier/whatever that doesn’t filter properly before sampling. On equipment like this, the 400kHz signal could create significant noise on the incoming signal, depending on how the sampling works.
In both of these cases, an in-line low-pass filter with a cutoff around 20kHz would solve the problem, not sure where you get one of those though.
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Looks like a fly has landed in the ointment of my Bluetooth tests that I linked to above… 
It turns out that the system I used isn’t Bluetooth, it’s a basic UHF transmitter and receiver. Not sure how I missed that - presumably because it was listed amongst all the other Bluetooth audio stuff.
That in itself wouldn’t be a problem - as we saw, it works great - the rub is, it’s illegal, at least here in the UK! 
Turns out the frequency it transmits on is 730Mhz which used to be a license-free frequency until about 15 years ago if memory serves, but now has to be licensed through Ofcom. Today the only UHF license-free band is 863.0 - 865.0 MHz.
What’s amazing is that this particular model of guitar transmitter is available to buy everywhere - not just ebay but also any number of reputable sellers. Most people probably don’t know it’s illegal, and the only reason I know is because knowing a little about frequency licensing is part of my day job working with TV radio microphones.
In truth, using it at home is unlikely to cause major problems, but I would be very cautious using it at any kind of big event or anywhere that’s likely to have a more crowded radio spectrum.
Years of hearing the phrase ‘pirate radio’ suddenly make sense.
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Oh No! I am so sorry if I led you to believe I used BT. I never specifically mentioned it’s just RF I suppose. Oops.
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Don’t worry Brian - it wasn’t you. I used to have a similar type of thing that was just a receiver that was Bluetoth but I lost it, so I assumed this was the same idea. There are various Bluetooth versions of this on the market I think, I just assumed this was one of them as it was listed amongst lots of other Bluetooth gadgets. But as we all know we should never assume! Lesson (re)learned! LOL! 
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