Publicité
What is a Notch Filter?
Basically, what a Notch Filter does is to remove a very narrow band of frequencies from the
response of the amplifier. If this band of frequencies is narrow enough it will not be missed from
the wide band frequency response of the unit. This narrow band or 'Notch' is then made tunable
so that it can be adjusted to the resonant frequency of the body of the instrument, thus reducing
the amplification at that particular frequency and eliminating the feedback.
Why is the necessary?
The pick up on an acoustic guitar is usually mounted on or near the bridge of the instrument, at
the centre of the front of the soundboard. This picks up the vibration of the strings but also picks
up the vibration of the soundboard itself. The soundboard, being a relatively large area, will also
pick up the sound from the speaker. This will be amplified via the pickup and amplifier and cause
feedback to occur at the resonant frequency of the soundboard.
Each guitar will have its own resonant frequency and it is this frequency that you must adjust the
NOTCH FILTER control to find. The most effective way of doing this is to slightly deaden the
strings of the instrument, turn up the amplifier until feedback occurs, switch in the NOTCH
FILTER and adjust it until the feedback disappears.
This has tuned the NOTCH FILTER to the basic resonant frequency of your instrument, the
frequency at which feedback is most likely to occur, and will allow the instrument to be amplified
much louder than if no NOTCH FILTER were available. If the amplifier is turned up further then
feedback will still occur at another frequency. The only way to stop this is to either move further
away from the amplifier or reposition yourself relative to the amplifier.
The range of the NOTCH FILTER has been reduced from the previous generation of acoustic
amplifiers to cover only that area of the frequency spectrum where we know from experience that
feedback is likely to occur.
This makes it easier to adjust the NOTCH FILTER. The depth and width of the NOTCH remains
the same.
PHASE SWITCH ( TA100R only )
The TA100R has a speaker PHASE switch located on the rear panel. Switched to its IN PHASE
position this facility has no effect on the sound of the amplifier. However, when switched to OUT
of PHASE it causes phase cancellation to occur at various frequencies that give enhanced clarity
and extra projection to the sound. Phase cancellation also tends to reduce further the chances of
feedback occurring.
PULL SHAPE
The MASTER LEVEL control is fitted with a PULL SWITCH that activates the SHAPE facility.
The SHAPE is actually a pre determined, fixed EQ that can be switched in to add an instant
'sparkle' to the sound that is appropriate to both rhythm playing and fingerpicking styles.
Please note:- With the Shape switch pushed IN, the graphic set flat, the notch filter switched OUT
and the HI-TRIM and LO-TRIM controls in their centre positions the amplifier has a perfectly flat
frequency response. i.e. this is the actual sound of your instrument when amplified and is a good
place to start from in amplifying an acoustic instrument. If you know the natural sound of your
instrument when amplified you can make more objective decisions about what to do to either
enhance or correct it.
7
Publicité
