Ian Iveson wrote:

I wonder if gain is partly controlled by carrier amplitude,
to compensate for changes in IR signal strength as you move
about? That could explain why noise, from receiving circuit
and background, gets so much worse in the absence of a
signal.

The receiver in mine seems to be a normal FM demodulator connected to an
IR receiver instead of a tuner, so you get all the benefits of marginal
FM reception with no squelch control as well as the insensitivity of a
cheap IR transmitter/ receiver pair. The signal comes over the link as
an IR signal modulated with a standard FM stereo signal. Harder to
explain than design. The whole lot cost less than a reasonable set of
earbuds. The IR emitted by a head is pretty well shielded from the
receiver in this case, not to mention being in the wrong band and
unmodulated, but flouresecent lamps put out quite a lot of modulated IR,
& incandescent lamps or the sun in the room can put out enough to almost
swamp most IR transmitters the makers can build for a sensible price.

Assuming the listener's head is itself not a noisy IR
source, then the the high noise when the receptors are
blocked must come from the combination of a noisy receiving
circuit and high gain.

Perhaps it might be worth trying to return them for a
refund?

You could *try* for a refund or exchange, but the chances are that the
headphones aren't defective, just badly designed & built. If they get a
signal you can hear, they're probably going to be regarded as fit for
purpose under the Sale of Goods Act, or whatever the local equivalent
is. Just possibly, they're a defective set, in which case another set
may work better.

--
Tciao for Now!

John.