Why do cell phones sometimes interfere with amplifiers

The usual problem with interference from GSM cell phones is caused by rectification of the strong RF signal at the semiconductor junctions at the input of the amplifier.

GSM uses Time Domain Multiplexing to share the RF frequency with other users - it only turns the RF signal on for a short time at a 217Hz rate with other users and the base station using the remaining time.

This short relatively high-power RF can be rectified by any semiconductor junctions to cause a slight change in the bias level of the amplifier for the duration of the RF pulse (~0.5ms). This is a buzz at 217Hz that can then get amplified by the amplifier to appear at the speakers.

It is not necessary for the amplifier to have any response at the RF frequency which can be up to about 2GHz.

The cure is to provide filtering to avoid the RF signal getting to the first stage of the amplifier. The filtering may be as simple as a small high-frequency capacitor across the inputs, an RF choke in series or a more complex filter.

Bipolar junction transistors are more sensitive to this effect than FET front ends so it will be affected by which devices are used.

It's not obvious why SMS and data transfers would not cause noise as well - they use the same mechanism for transferring data.

Even though audio amplifiers won't typically process any signals above 40kHz or so in usable fashion, the early stages of such amplifiers may not completely filter them out. If high-frequency signals are strong enough, they may overdrive an amplifier stage, causing distortion. Applying harmonic distortion to a signal that contains multiple frequencies may yield new spectral content at any frequency which can be formed by adding or subtracting any integer multiples of any frequencies present in the original.

Some wireless protocols use a variety of frequencies which are distributed in somewhat random fashion. If such transmissions are picked up by an audio amplifier, the sum and difference tones will be distributed over a wide range of frequencies, and the result will simply be a slight increase in the level of background noise.

The problem with GSM is that its spectral content is distributed in a way that causes many of the sum and difference frequencies to coincide, thus concentrating their energy at a few discrete audio frequencies. If the designers of GSM had foreseen the extent to which it would wreak havoc on audio, they could have easily adjusted the design slightly to avoid such frequency concentrations. Unfortunately, by the time the problem was discovered it would have been impractical to adjust the design.

The chirps you hear from inactive speaker coils or amplifiers is the AM demodulation of Broadcast sync tones from cell phones exciting the coils.

This carrier burst sync to nearest towers is modulated quite different from the constant amplitude frequency hopping schemes used.

Even some car speakers nearby causes the coils to rub on the magnets from lateral forces of microwave energy on parasitic resonances tilting the coil while moving, which adds some sharper tones to pattern of "dit-da-da-dit" repeated several times.