I have two Tascam DR-05 recorders and they both exhibit the same
phenomenon: when they are recording complete silence, there are faint
irregular ticking noises on the left channel.

This occurs when using an external circuit which disconnects the
internal mics, but the circuit only needs to be plugged in, not
connected to anything, for it to happen. If there is any sound on the
channel, the noise immediately stops, but re-starts about one second
after complete silence returns.

The noise only occurs if the recording gain is set low; as soon as it is
advanced, the noise floor of the pre-amp is enough to stop the effect
(it doesn't need to be high enough to mask it). it isn't audible on the
headphone monitoring port, even if that is connected to the input of a
high gain amplifier; it only shows up when the recording is played back.

Can anyone suggest why this is occurring (bearing in mind that it only
affects the left channel) and can anyone suggest a cure?

--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk