The choice of line-up is dictated by the strength of the signal to be reproduced. For example, a 10W circuit incorporating volume and tone controls can be used satisfactorily without a preamplifier if input signals greater than 500 mV are available. Such signals can be obtained from high-output crystal pick-up heads or FM radio tuner units. A 3W circuit with volume and tone controls can be used directly with sources which give signal voltages greater than 100 mV.
However, signal voltages from low output crystal pick-ups, magnetic pick-ups and tape-recording heads (which can have values of upto 500 mV but are usually between 2 and 100 mV) are normally too low for direct use with the power amplifiers, and a preamplifier is necessary ...
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