Jim Lesurf wrote:



The Armstrong 626 I had refurbished recently was an early version. These
were prone to clicks, etc. The first impression might be that they were
getting in via the mains.



** Nearly everyone imagines that pops and clicks heard through stereo systems as appliances are switched on or off MUST arrive conducted via mains wiring.
But this is only rarely the case, iron transformers and other PSU components do a thorough job of blocking RF energy arriving that way.

The source of the energy is the arc that briefly forms when a mains switch opens - or closes and then bounces. Much like a spark transmitter, RF energy from the arc is then radiated by the active and neutral wiring carrying current to and from the switch.

There can be a fair bit of power involved too, particularly when the load is inductive - like a fridge compressor or washing machine motor. The arc's energy's spectrum spreads from audio frequencies right up to the top of the VHF band interfering with radio and TV reception too.

So the actual path is through the air and into audio signal cables (ie RCA, microphone, DIN and speaker leads) where the RF energy can then enter directly into sensitive audio circuitry. In the case of speaker leads, it enters via the NFB loops in output stages.

With well screened cables, the bursts of RF energy are picked up and travel on the shields, dissipating harmlessly IF these are grounded to each chassis right where they enter and leave equipment. Most makers know this while some appear not to and so allow RF to come right in the door.

Many domestic appliances have RF suppression components across their switches and this IS the best solution - but some do not plus the components can fail due to age.

IME adding EXTERNAL (ie plug in) suppression devices to offending appliances rarely helps and adding one to your stereo system never does.


.... Phil