Hi,
Low-cost commercially available audio pre-amplifiers are usually very vulnerable to interference from adjacent cellular phones switching, in the UK, at around 217Hz.
When designing an audio amplifier with high immunity to such interference, I can see the obvious value of adding suitable RF filtering (ferrite chokes and shunt capacitors with low-Z in the UHF band) to the input circuitry but wonder if a pre-amplifier using differential inputs (FET or Bipolar ?) rather than single-ended would be better. Transformer coupling of source to the amplifier input is another possibility, of course. To meet a current requirement that I have to meet, amplifier noise should not be too much of a problem since the input signal levels are in the region of 20-50mV pk-pk, with a bandwidth from around 300Hz to 5KHz, and the output level required is around 2 volts pk.pk. into a nominal
600 ohm load.
Does anyone with practical experience of dealing with this problem have any comments that they'd care to make about how to minimise such interference?
TIA - Dave
David C.Chapman - Chartered Engineer. FIEE. ( snipped-for-privacy@minda.co.uk)
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