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POUL PETERSEN

Electronic design and prototype development.


Identifying acoustical feedback to a gramophone.
Acoustical feedback occurs when a gramophone picks up its own signal from the loudspeakers. The audible results spans from a muddy bass reproduction through a slight low frequency rumble and all the way up to where the signal from the record is severely distorted.

This procedure is simple way to identify low levels of acoustical feedback in the system:
Record a piece of music containing low frequency signals on a high quality tape recorder. During the recording, the loudspeaker volume is switched between off and normal listening level.
Play the tape at a normal listening level. If there are audible differences between the parts, where the loudspeakers were on and off, this is a clear indication of acoustical feedback in the system.

Another way to identify acoustical feedback is to compare the music from the gramophone when it is played in mono and in stereo. A more precise bass reproduction when playing in mono indicates a problem with acoustical feedback as most records are recorded in mono at low frequencies.

Some remedies against acoustical feedback are:

  • Place the gramophone on a stable surface.
  • Move the gramophone and loudspeakers away from each other.
  • Adjust the gramophone's tracking pressure and antiscating properly.
  • Insert a rumble filter in the signal path (this will also remove some of the low frequency content of the signal).
  • Use a preamplifier with built in Feedback Canceling Filter.

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