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Never coil power cords - inductance really increases (heat,
current draw). Also if you must run a signal cable across a
power cable, do it at 90 degrees to minimize crosstalk.
Let's move on now and take a look at our equipment. I
suggest starting with multiple socket strip, plugged into the
wall. Move all your other equipment out of the way. Plug in
your amplifier ONLY. And when I say 'plug in X' I mean
only the things I mention. Just turning the power off on an
item of equipment is not enough: they must be unplugged
otherwise the earth leads will remain connected and this
could alter the results.
Amp & Speakers
OK, now we shall test your amp and speakers. If you run in
stereo, test each channel individually. If you use multiple
amps and/or cabs test each in isolation.
Make sure your speakers can handle full power (If they can't
you really are dicing with death. At an absolute minimum,
you should have speakers rated 10%-15% higher than the
amp, preferably about 150% and the 'no chance of problem
ever' recommendation of 200%. But I think that's a bit
extreme. My cabs are 140W and my amps were 100W and
75W. This is pretty much an acceptable situation.
Don't plug anything into the amp (other than the speakers!).
Volume 0. Turn it on. Listen. Any noise already? This is the
basic level of amp noise you will get. Start to bring the
volume up – use a clean channel first. If you have a gain
control as well bring up the master first then the gain
control. Try this with all your channels one after the other.
High gain channels will produce noise but hopefully
nothing too alarming.
OK there should be very little noise at all (except high gain
channels). Anything else is a problem at this stage. OK.
Now turn the volume up FULL. Go on. Don't be scared.
BTW I should mention now that you have the volume up
full that I take no responsibility for damage to any part of
your system here :-)—check your manufacturer's warnings
beforehand—if you have the kit correctly matched it
shouldn't be a problem. Now bring up ALL the tone
controls as well.
My RA100 power amp is very silent. Apart from very very
quiet mains type hum which is always present (and you
need to put your ear next to the speaker to hear it) changing
the volume control from 0 to 10 makes little difference,
except on the right hand side where the transformer is in the
amp. But this is not worth worrying about. This is how it
should be.
OK turn it down! Do anything else you use individually
now. If you use more than 1 amp/cab arrangement now is
the time to plug then all into the power supply together.
© Copyright 2000-2002 Barry S. Pearce et al.
Section 9
Connect them up and do the same test. The results should
be exactly the same.
If at any point you have heard more than a hiss/v. low hum
you need to investigate your cables, and the power supply to
the amp itself. At worst case here you may have a noisy
amp. Start unplugging things until the problem disappears—
this last item is the one where the problem manifests itself.
It may not be the piece of equipment at fault—funny
interactions can happen with electronic equipment. At least
you know where the problem is.
The problem may be noise in the power supply. Turning off
other things in the house can check this—washing machines
etc won't help. A noisy amp at this stage is a pain in the
bum. You need to decide if you can live with the noise
generated. If it is only at very high volumes, then it may not
be problem—but remember what you hear at high volumes
is generally present at low volumes; only it may be so quiet
that you can't hear it. A noisy transformer in the amp (i.e. a
buzzing sound rather than a humming) should be
investigated—it can affect signal cables that travel
anywhere near it and if close to your amp will quite
probably induce noise.
I've also checked my Marshall Valvestate VS100 head.
Yikes!!!! It's a noisy beast. In fact I am NOT prepared to
live with this noise—one of the reasons I am selling it. This
appears to be a Valvestate problem as I know a chap who
plays in a band with a guy who has a Valvestate and he says
this is very noisy as well. This thing hums without the
speakers plugged in and the volume zero. Plug them in and
the hum goes straight through to them at a loud enough
volume for me to be concerned. Bringing the volume up
from zero makes no difference on the clean channel. The
channel itself is very quiet it's just the rest of the amp! It
would be useless to mike this up for recording purposes.
BTW you cannot solve this sort of hum with a broken shield
cable. It is NOT a ground loop. If you don't have signal and
shield connected the speakers won't work!
If you have a complex set-up and hook up signal cables
*between* your amps then a hum may be caused by a
ground loop, and we have already talked about how to
remove them. Other than that, with regard to solving the
problem I can only suggest speaking to someone qualified
to comment – the manufacturer, repairman etc.
If you have a simple rig like me—one amp, two cabs this
has been a fairly quick and easy affair. Next we will look at
Mr Guitar.
Mr Guitar
Although we did some preliminary checks earlier now let's
see how it works with our amp in section 3.
Noise Elimination
49
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