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Section 9
Noise Elimination
Speakers/Headphones
OK, so where do we start? What we do here is work our
way backwards through the system. I didn't have a problem
here, but I'll take you through the steps I would use so you
can see the approach you should take.
Listening to your speakers you can hear something which
you believe is clipping. Start by turning down the amp. Is
the clipping still present or has it diminished/vanished? If
problem goes away then you are clipping the amp
internally, or driving the speakers too hard. If not, then try
another set of (preferably the same) speakers at the same
level as before. If these are OK it is the speakers which are
the problem. If it doesn't go away turn down the device
feeding the amp. Keep the volume up, and re-check. If the
problem has gone then you were driving the amp too hard,
or possibly the output of the device you turned down, but
this is less likely.
My RA100 is a very sturdy beast, but it will clip if pushed.
It warns about this, and ANY other non-linear operation
(such as extreme out-of-spec loading—e.g. too many
speakers wired in parallel or a short across the speaker
terminals) by flashing the red clip LED of the channel in
question. If your amp or speakers have something similar,
then check this as a first step. If these are flashing then turn
down the input slightly to avoid clipping. Some units have
the clip indicators cut in 2dB or so before actual clip occurs.
On these units it is desirable to have the indicator blip
occasionally, however frequent blips should be avoided.
Other units only indicate when clipping actually occurs.
Some units only indicate clipping when as much as 10dB
has been clipped. Obviously you will hear this even though
the unit hasn't indicated a clip. Make sure you trust your
ears, and these procedures as they will work regardless of
the clip indication provided. According to my RA100
manual, if the unit has been running for a long time running
under a heavy load the output protection circuitry may have
kicked in. Alesis recommend turning the power off for a
minute or two and then checking to see if the problem has
gone away. If it does they recommend checking for thermal
problems, such as obstructed air-flow. Indeed if you use too
low speaker impedances or the unit starts to overheat due to
poor ventilation it will limit the signal by clipping. This
results in distortion but it still produces some sound. Again,
check your amp's manual to see if something similar can
happen.
Right then. Assuming you still have the clipping and its not
been solved above we continue to work backwards down
the chain.
Next in line in my set-up is my Palmer PGA-05. The input
to my PGA-05 is linked direct to the output via a THRU
connector. This thru link has no volume control and works
even when the power is down. I would assume therefore
© Copyright 2000-2002 Barry S. Pearce et al.
that this is a hard wired thru connection. I therefore don't
believe that this unit is clipping the amp.
So, continuing to move backwards, we come to the dear old
faithful GT-3/5.
The GT-3/5 has an external volume control. It is possible
that this is clipping the PGA-05 input, and thru. Turn down
the volume control significantly; say by 25%. Again check
to see if the problem has disappeared. If there is no change
then maybe the GT-3/5 is clipping internally. We looked at
checking and resolving any GT-3/5 internal clipping in
section 8 so give yourself a quick re-read if you need to.
The meters on the GT-3/5 might well show that we are in
fact clipping the GT-3/5 input.
Let me take you back to where I said that if you have an
input signal that clips the GT-3/5, then you need to turn
down the output level from the device that is feeding your
GT-3/5. If the unit prior to the GT-3/5 is your guitar, you
could turn down the volume control. This isn't very
satisfactory. Instead I recommend that you check your
pickup outputs first (see above) and then buy a simple
volume control device or a decent compressor/limiter. I
recommend the Behringer Composer Pro. Indeed this
solution can use used whenever you cannot control the
output levels of the preceding device. Obviously the device
you use must have a better input range and headroom than
what it is driving or you will merely just repeat the problem.
The output of my Cry-Baby Wah is fixed. I believe it is
governed by the input though—i.e. it has unity gain. If this
clips the GT-3/5 input I can place a compressor/limiter after
it which has a higher input limit than the GT-3/5, or even a
simple volume control device.
OK, so we are working back through the chain, next up is
my Samson Wireless Receiver. This has an output volume
control knob. If you recall in section 8 I talked about level
matching. I set-up my Samson output level to be the same as
my guitar so there is no difference between the two. It is
worthwhile checking at this point that this really is the case.
If it is, the guitar should clip the input as well, and turning
down the guitar volume control knob should help identify
the problem. Anyway, back to the Samson, again we check
for clipping of the WAH by turning down the output level.
Wireless systems also have two more places they can clip.
The air interface signal may be too strong (!) or you might
be clipping the input of the wireless transmitter. Either, can
be checked by bypassing the wireless system with a cable.
Also, the wireless transmitter may have its own level control
(a belt pack gain trimmer - as does the Samson), which you
can turn down.
One problem I do have here is that the Samson receiver has
a set of LEDS indicating signal strength. These are marked
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