GUITAR TECH
Heads Up When You’re Hooking Up
Hooking up a head to a single or group of You probably don’t want your amp catching
cabinets is often a confusing topic. Most of us fire, no matter how cool it looks onstage.
just want to play music and don’t have time
to learn about watts and ohms. But this is an
important topic to understand so you don’t
damage you or your amp.
If you are running your head to one cabinet,
things are pretty straightforward. If your head
next. Parallel means sending one output
of the bass amp head to one cabinet and
a second output from the head to another
cabinet. Parallel is two or more side-by-side connections.
First, it’s important to use speaker
cables when hooking up a head to
your cabinet. Instrument cables used
between your guitar and amp might
look similar, but they are drastically different. Speaker cables are thicker and
not shielded. Even gauge numbers
can be confusing – just remember, the
thicker the cable, the lower the number.
Use at least an 18 or 16 gauge speaker
cable so you don’t damage your rig.
Do the math before hooking up multiple cabinets, otherwise you might have a flaming amp on your hands
When you add a second cabinet, realize
there are now two places for the amp’s
power to go. Adding a second cabinet
causes the total impedance (amount of
resistance) of the load to change.
You will see the term “load” a lot with
amps. A load is something that uses
power to do something. A load could
be a motor, light bulb, speaker, a group
of speakers in a cabinet, or several cabinets hooked together. An amp head is
used to power the load.
Impedance is a measure of resistance.
When electricity flows to a load,
some of the electricity is resisted or
impeded. Impedance is measured in
ohms – denoted with the Greek letter omega ( ). Most cabinets have an
impedance of 4, 8 or 16 ohms. More
ohms mean more resistance and with
more resistance there is less power
flowing to the speaker.
It’s easiest having each cabinet with the
same impedance (e.g. each cab is 4 ohms
or 8 ohms). In order to determine the
total impedance of the cabinets (all with
the same impedance), take the impedance of one cabinet and divide that
amount by the total number of cabinets.
For instance, two 8-ohm cabinets wired in
parallel will have a total impedance of 4
ohms; two 4-ohm cabinets would have an
impedance of 2 ohms. Remember, if your
amp can’t handle a 2-ohm load it could
go up in flames – that’s why this is so
important. If your amp says it can put out
a certain number of watts at 4 ohms, you
can only hookup a total load of 4 ohms, 8
ohms or greater – not
2 ohms!
Amp heads specify how much power
is sent to a load (a cabinet or group
of cabinets) of a certain impedance
(the amount of resistance measured in
ohms). For example, you may see an
amp rated “400W 4 ohms.” Likewise,
a particular amp head might send 200 watts
into an 8-ohm load, but the same amp will
send 350 watts into 4 ohms. More power
will flow from an amp as the ohm rating
(resistance) decreases. If you went down to 2
ohms, that same amp might send 600 watts
to the load.
If you are hooking up two cabinets of different impedances, there’s a little more
math involved. For instance, if you have a
4-ohm cab and an 8-ohm cab, the equation is as follows: 4 x 8 = 32, then 4 + 8
= 12. Divide 32 by 12 and you get 2.667
ohms. If your amp is rated only for 4
ohms, you can’t use this configuration of
cabinets with 2.667 ohms.
says 300W 4 ohms, you need to plug it
into a 4-ohm cabinet that can handle 300
watts. If you plug it into an 8-ohm cabinet,
you might only get 200 watts of power and
you’ll probably get less volume. If you plug
it into a 2-ohm cab, there’s not enough resistance and the amp will overwork itself, which
results in FIRE!
If you are still confused, get someone
with experience to verify that your amp
hookup is okay before you turn it on.
Make sure you use the correct cables and
have extras to avoid the temptation of using
an instrument cable. Most importantly, know
the specs of your head and cabs. If you’re
heads up, you’ll be able to get down! Go
forth and hook up!
All of this is important because amps are
designed to work within a certain resistance.
If there is not enough resistance, the amp will
put out more juice than it can handle, leading
to overheating, and eventually burning up!
When you hook up more than one speaker
to an amp, there are two ways they can be
arranged: series or parallel. Series means
chaining the cabinets together, one to the
Rick Wheeler
Rick Wheeler currently works as Larry Carlton’s guitar
tech and front of house engineer. He is also an accom-
plished jazz guitarist, vocalist, and educator.
You can contact Rick at rickwheeler@hughes.net
