If the instrument uses just a piezo pickup then you will have to leave the GAIN control set on
maximum as this is the best setting you can achieve.
N.B. as the LEVEL indication is driven from after the equalisation circuits, it may be necessary
to re-adjust the GAIN control once the equalisation has been altered.
Phase switch
The ‘PHASE’ switch has two distinct uses, firstly, by reversing the phase of the signal,
feedback from the guitar can very often be considerably reduced, this is dependent upon
the position of the player relative to the amplifier and the switch may need to be either
“in” or “out” depending on player position and the frequency of feedback. This is an ideal
way of reducing feedback as it introduces no change at all to the sound of the
instrument. However this will only work if the amplifier to player distance is fixed as is
often the case with a performer sitting down next to their amplifier.
The second use of the PHASE switch is with the dual pick up system where reversing the
phase of one channel can enhance the effectiveness of the dual pick up depending on the
position of the microphone within the body of the instrument. Again this can also
minimise the chances of feedback.
Description of how the ‘Phase’ switch reduces feedback
If the distance from the instrument to the amplifier is fixed i.e. the player is sitting down
next to or in front of their amplifier then the speakers reproducing the sound of the
instrument set up a series of standing waves, the length of these standing waves is
dependent upon their frequency.
Now as feedback from acoustic guitars is always at fairly low frequencies then the
standing waves associated with it are quite long. If you happen to be sitting at the node
(i.e. the most sensitive spot) of one of these standing waves then feedback is quite likely
to occur as the wave will tend to excite the body of the instrument into resonance.
If however you are sitting at the antinode (least sensitive spot) the feedback will be
unlikely to occur. By simply reversing the phase of the signal through the amplifier you
are in effect reversing the positions of the nodes and antinodes within the standing wave
and consequently changing your relative position in relation to the amplifier i.e. switching
yourself from sitting in a node to sitting in an antinode and thus reducing the likelihood
of feedback occurring.
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