rumble detector - traces ultra-low frequency signals

A very large low-frequency range is not necessarily a good thing. Often when a record is played at a reasonable volume the woofers occasionally "strike" far further than the normal (acceptable) distance. This is usually due to undesirable dis- tortion in the range of about 1 to 10 Hz. This phenomenon is limited only to rec- ord players; it is not noticed with compact disc players, radio tuners or tape players. The reason is that these last three have a high-pass filter of about 20 Hz fitted somewhere in the electronics or in the signal source. As far as this circuit is con- cerned, therefore, we are only interested in the record player. For most people a record player is still the best source of high-quality sound reproduction. (The CD — compact disc - system may be technically better but is far less popular.) A lot of attention is paid to the MM (moving magnet) and MC (moving coil) inputs in "reasonable-quality" amplifiers as the best results are possible by making use of these...