54 PREMIER GUITAR NOVEMBER 2009 www.premierguitar.com
Sometimes some bass-player amp knowledge
can pay off in multiple ways. At band rehearsal a few weeks ago, our little PA head started
acting up. It worked fine for a few minutes,
but then it started to get fuzzy sounding and
finally faded to nothing. We swapped out mic
cords, but the gremlin kept resurfacing. After
a while, I found out that the amp was set in
bridge mode, a way of combining its two
power amps into a single amp that produces
more power than either one alone.
However, that amp section of the PA was
rated at a minimum impedance of 8 ohms
in bridge mode. Unfortunately, we had two
8-ohm cabs plugged in, which created a
4-ohm load. Aha! There was the cause of
our problem. We switched the head out of
bridge mode, plugged one PA speaker into
each power amp, and the rest of the night
was trouble free.
Because most bass amps are solid state
rather than filled with lovely glowing tubes,
a bass player’s knowledge tends to be different from what a guitarist needs to know—this
impedance mismatch problem was just one
of the things that gets caught quickly by the
alert bass player, but might not dawn on a
guitarist. I’ve run across a bunch of these little amp tidbits over the years that have more
in common with the bass world, but also have
utility for guitar amps and PAs, and that’s the
point of these next few columns.
Choose your bass cabs wisely!
In the bass world, there are 8-ohm cabs and
4-ohm cabs, and a bassist is often mixing
and matching cabs for a specific gig setting,
typically going for combinations of 10" and
15" speakers that can fill the room with the
sound you’re after. There are pretty efficient
cabs that put out a lot of sound for the power
they’re fed, and there are far less efficient cabs
that put out a particularly beefy low end.
That’s the bassist’s first challenge: getting the
right combination of cabs to keep an amp
happy and produce the sound you’re after.
Combining speakers can result in a different
impedance load depending on whether they’re
connected in series or parallel. Most amps run
down to a 4-ohm load, and a few can handle
two ohms. Check your amp and cabs before
plugging things together or you might run into
a shut-down situation from overheating.
Fortunately, almost all bass amps connect
speakers in parallel, and this doesn’t change
whether you’re plugging two cabs into an
amp or daisy-chaining one cab onto the back
of the next. The equation for speakers connected in parallel is a bit of new math, so
that 8 + 8 = 4 and 4 + 4 = 2. Just to keep
things messy, 8 + 4 = 2. 67.
What this means is that to keep your amp
happy, you need to keep the speaker load at
or above its minimum rating. An amp rated
to a minimum 4-ohm load can do fine with a
higher total impedance, such as 8 or even 16.
At those higher loads, a solid-state amp just
puts out less power. For example, an amp
rated at 400 watts at 4 ohms might produce
only 250 watts at 8 ohms. Conversely, an amp
rated at 400 watts at 4 ohms would likely
shut down if it’s run at 2 ohms.
Your goal as a bassist is to keep your amp
happy by matching your cabs to your amp’s
rating. And you want to keep your cabs
happy, too. It’s perfectly safe to run a cab
rated at 250 watts RMS maximum with an
amp that can put out 400 watts—as long as
it’s not continuously putting out that much
power. In other words, there’s no need to
match cab and amp ratings exactly. In fact,
using an underpowered amp can cause more
problems since it will be straining just to put
out distorted sound waves that might be fine
for a guitar amp, but spelling doom for a
solid-state bass rig.
Stay tuned for more. In the next installment,
we’ll talk about cables, gain staging and EQs.
Dan is a professor by day and a bass player when the sun
goes down. He plays both electric and upright bass in
blues, jazz and pit settings.
Get To Know Your Bass Amp, Part 1
In the bass world,
there are 8-ohm cabs
and 4-ohm cabs, and a
bassist is often mixing
and matching cabs for
a specific gig setting ...
Your goal as a bassist is
to keep your amp happy
by matching your cabs
to your amp’s rating.