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Buzzing from speakers

We recently set up a new computer system ‘on-site’ and were perplexed to hear the speakers buzzing. Tried rerouting the wiring, different power outlet etc. No change. The buzzing did seem to vary as we moved around the room.

It turned out that it was being caused by certain mobile phones – especially when in this case the location was on the edge of Mobile Phone signals. Mobile phones continue to try call back to base if they loose signal. They switch back to slower and more powerful signals when they have trouble connecting. Some phones switch right back to GSM – and here lies the problem.

The buzzing is AM detected signal.

The reason of audio amplifiers being hit by GSM signal is that contemporary audio semiconductor parts are actually very functional up to high GHz range. For GSM-800-900MHz range any 80mm copper trace works like 1/4 wave antenna, or stripline resonator. The signal is AM detected on any non-linearity (transistors or diode structures in chips) on multiple points of amplifier simultaneously, also including power regulator chips and so on.

It is translated into audio range as tiny but very sharp and periodic dips or pops of averaged conductivity of non-linear parts (AM detection), which are DC powered.

Think of low speed oscilloscope trace showing straight line with beads of UHF flashes. Simple sharp spikes of consumed DC current will become audible with amplifier.” – Rocket Surgeon


Luke found it!