The conversion from one voltage to another is done by a transformer. You can build a transformer that can take in 220 volts, and tap it at a different point to accept and convert 110 volts also. That is what the switch does. Changes the tapping point in the transformer.
The transformer is designed and made for a particular frequency in the AC. In general, a using a 50Hz system in a 60Hz environment will not matter much. But the other way is dangerous. For one, the transformers inside the units are small and highly efficient. The number of primary turns in the transformer is just enough for the specified frequency. There is hardly any room for tolerances. If the correct voltage and frequency is not supplied, the transformer will saturate and heat up.
A transformer designed for 60Hz will draw more idle current.
When the core starts getting saturated, it will be subject to stray magnetic flux and electrical noise. This will be transferred to the amplifier as noise. In addition, in worst cases, you can also hear the transformer inside the unit 'growling'. All these are dangerous for the power supply as well as the unit in general. In extreme cases, the power supply could become hot enough to explode.
Power supplies inside amplifier use a capacitor to compensate for the AC rectification cycle. If the capacitor, and the related circuitry are designed for 60Hz cycle, a 50Hz input will not be able to charge the capacitor at the required frequency. In general, though this may not matter much, in high powered transformers, when there is the need for high in-rush currents, the capacitor will be emptied quickly, and may not have enough power to feed the next amplification stage.
Unfortunately you can only step-down 220 to 110 volts. Nothing can be done for the AC frequency. It is always better to use a power transformer designed for a particular voltage and frequency combination.
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