Electrical resistance
within speakers, amplifiers, as well as all basic
electrical connections, is expressed in ohms, named
after George Simon Ohm, a German physicist. While we
needn’t go into the background of him and his
achievements, or even the heavy details about
resistance, ohms, or impedance, this is an area that
is often confusing to people.
When running one cabinet, for example a standard
Marshall 4 x 12 cabinet at 16 ohms, it's easy to
presume and know that the amplifier head should also
be set to 16 ohms for best performance. However, in
other situations, other questions can and do pop up.
What if you don’t have a 16 ohm output for the
cabinet and the head you’re using only has
options for 8 or 4 ohms? Can it still be run safely?
What about the case of using a 4 ohm cabinet but the
amp only has settings for 8 or 16 ohms? And what
about running more than one cabinet – what
setting should the amplifier’s ohms adjustment
be set to then? What if you have two cabinets with
different ratings, say one 8 ohm and one 16 ohm? Can
they be run together safely and if so, again, what do
you set the ohms selector on the head to?
Fortunately, the principles about ohms aren’t
too complex in terms of what musicians need to know.
Ohms
and Amplifiers:
Guitar amplifiers that have multiple outputs use
parallel wiring internally at their output jacks. Why
you may ask? Well, for one reason, if they didn't,
then the outputs on the amp would only work if ALL
outputs would be used of course! Remember again that
a series connection needs to have all connections
present to work and this would apply to an amplifier’s
output jacks as well. So as a result of knowing that
an amplifier’s speaker outputs are wired in
parallel, running two 16 ohm cabinets in a standard
Marshall amplifier head would result in an 8 ohm load
and therefore the amplifier should be set at 8 ohms
accordingly. Some older Marshalls had four speaker
outputs. Using the same principles, four 16 ohm
cabinets run in parallel together would result in a 2
ohm load and you would then set the Marshall to
operate at 2 ohms.
"But what if you don’t have a 16 ohm output
(for the speaker cabinet rated at 16 ohms) and the
head only has options for 8 or 4 ohms? Can it still
be run safely? What about the case of using a 4 ohm
cabinet but the amp only has settings for 8 ohms or
above?"
The answer to the first question regarding whether a
16 ohm cabinet can be run safely with an amp that has
settings for 8 or 4 ohms is yes. However, when
running the head at a lower ohm rating than the
cabinet, the result will be a significant degree of
power loss. In the second case of using a 4 ohm
cabinet with an amp that must be run at 8 ohms, this
will stress an amp and cause it to overheat.
Technically, you’ll get more power output (not
efficient or stable power output mind you!) to some
degree, but again, at the expense of burning out a
transformer and/or other components. Not a good idea!
For the best performance it's not recommended to use
two different cabinets in a setup, one being an 8
ohm, the other a 16 ohm for example, However, if you
insist on having to do this setup, just make sure
that the amplifier impedance is lower than the
combined 8 and 16 ohm parallel load. The formula for
calculating the impedance when not using equivelant
cabinets is different, i.e. the "halving"
or "doubling" that is done to get ratings
when using series and parallel won’t work.
