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Last month began a discussion on speakers explaining how
they function and how the enclosures work that they are
used in. July's article can be found in the Metro Music
Mayhem archives and will help you better understand this
month's article if you read it first. Assuming now that
we have a basic understanding of speakers and cabinets,
let's discuss their practical usage. Of course you find
speakers everywhere- your TV, cell phone, home stereo etc.
I am going to limit this discussion to categories relevant
to the direct reproduction of musical instruments and professional
sound reinforcement.
GUITAR AMPS
The first thing I can say about a speaker used in a guitar
amp is it is responsible for fifty percent of the tone
you hear. The easiest way your guitar sound can be varied
is to use a different speaker in your amp. I once, long
ago, took a 4 x 12 cab and loaded it with four different
speakers. I then wired in a rotary switch to select one
speaker at a time (don't do this, it can damage a tube
amp, I didn't know better at the time but was lucky). While
I played, I had someone rotate the switch and listened
to the way the sound changed from speaker to speaker. It
was a dramatic difference to say the least.
Everything about a speaker, its magnet type and weight,
cone paper type and thickness, surround style, voice coil
type etc, all have influence on the way a speaker sounds.
When this is added to how the same speaker can react differently
with different amplifiers and cabinet styles, there is
a seemingly infinite range of possibilities. How then,
do you go about selecting a speaker? There is certainly
plenty of talk on the Internet. The problem with that is
many people talking about this subject are presenting facts
based upon no real data. They may have come up with something
they think sounds good but don't know enough to really
state why. Therefore following their advice can be an expensive
mistake. Also, don't rule out your original speakers. In
higher end production and boutique amplifiers they were
likely present during the design process and played an
integral part in voicing the amp, so it may be hard to
improve on this and it may be a better solution to look
at other aspects of your guitar system for tonal improvement.
In lower end amps, the selection of a speaker by a manufacturer
may be more economics driven so the product meets a certain
price point. Replacing the speaker here can be a good idea
if you are not happy with its tone. It really comes down
to empirical testing in the selection of a speaker. You
can research all you want, but until you put the speaker
in your amp and use it, there is no way of knowing if you
think you have improved your tone. This is the same trial
and error testing that is needed with guitar pickups.
CABINETS
Cabinet style is the next speaker related item on the
list to influence guitar tone. With guitar amps we generally
see two types of cabinets employed.
COMBO CABINETS
This style has a few pros and cons. It does create a
small portable package that is fairly easy to move around
and fit in a car or onto a tight stage. But there are three
trade offs to this:
First, especially if it is a tube combo, although small
,the combination of the speakers and amplifier chassis
in the same cabinet can end up being quite heavy.
Second, the combination of the electronics and speaker
in one cabinet enables the speaker to directly vibrate
every electrical connection in the amp. And if it is a
tube amp, the tubes usually are right in the vibrational
field of the speakers exacerbating any microphonic problems
the tubes have. As opposed to mounting the electronics
in a separate head, the combo environment can be significantly
more problematic for the amp's electronics, even in comparison
to those who place a separate head right on top of their
speaker cab, because there is still some measure of vibrational
decoupling gained by using an independent head and speaker
cabinet.
Third and most important is the combo amp open back factor.
As we explained last month, since the speaker has both
a front and back waveform in a combo cabinet nothing is
there to prevent the waves from mixing and canceling each
other. This is especially true of the lower frequencies
which propagate more easily in an omni- directional fashion
as opposed to higher frequencies that move more in a straight
beam. This can lead to a boxy thin tone depending on the
speakers used and how much was accommodated in the design
of the amps voicing to offset this. Sometimes nothing is
done and the sound is thin. Plugging the amplifier into
an extension closed back cabinet and not using the combo's
internal speaker can make a world of difference in your
tone. A good example of this is the Fender Blues JR combo.
A great feeling amp that sounds a bit boxy. Plug it into
a closed back cab and it is a whole new amplifier.
But it is also possible to design the EQ circuit of an
amp to accommodate an open back cabinet. I recently repaired
a Dr. Z amp head and open back speaker cabinet combination.
After finishing the repair on the head, I plugged it into
my shop 4 x 12 cabinet for a play test. It was immediately
apparent that there was a tremendous excess of low end.
Even with the bass control turned off it was excessive.
I stopped and plugged it into its mating Dr. Z open back
speaker cabinet. The low end now sounded fine and the bass
control was completely usable. And remember, this is not
really a combo amp. It is a head and an open back speaker
cabinet. It seemed strange to me that a head would be EQ
voiced for an open back cabinet.
And a final word on combo cabs. Don't be tempted to close
up the back of your combo cab. It will impair the ability
of the amplifier to dissipate the heat it creates, especially
in the case of a tube amp where this can create a fire
hazard.
CLOSED BACK CABINETS
These cabinets come in several configurations,
generally one, two or four speakers. For guitar it is
usually a twelve inch speaker or sometimes a ten. Stevie
Ray Vaughn notwithstanding, a fifteen inch speaker is
not generally acceptable for guitar. The Marshall style
4 x 12 is probably the most popular and iconic cabinet.
Although it has diminished greatly in popularity due
to today's trend of smaller more portable (I need something
I can fit in my car) amplifiers. Time was that a guitar
player would use one to four 4 x 12's according to the
size of the venue. The closed back cabinet,according
to the speakers used gives a good balance of frequency
response. It has more lows than an open back, but not
as much as a ported cabinet which possibly could make
the guitar muddy. The more speakers used in a closed
back, the more tight low end you get as the surface area
of each speaker combines together with the other speakers
in the cabinet to act as if it is one larger speaker. The
benefit of this is that you can get more low end without
using an actual larger speaker as in a 15" or 18" where
there is more mass to the cone and therefore harder for
the voice coil to control it causing muddiness.One drawback
of this type of cabinet is that it has a beaming effect.
The dispersion of sound is very directional and does not
disperse much to the sides. If you are playing loud in
a small venue there will be a hot spot of too loud guitar
right in front of the cab and too little guitar everywhere
else.
IMPEDANCE
Most guitar players have only a vague understanding of
impedance. They know it is measured in ohms and generally
speakers are four eight or sixteen ohms. But what does
this mean? Impedance is similar to resistance, which is
the ability to resist the flow of electrons in a circuit.
In a DC circuit, which has no frequency, circuit resistance
is measured in ohms and the combination of resistance,
voltage and amps is an easily manipulated equation (Ohm's
Law) that will yield one answer. This is different with
audio. An audio signal is AC instead of DC and varies in
frequency. When the frequency varies so can the impedance
of the speaker at a particular frequency. I like to call
impedance - frequency dependent resistance. Therefore any
two speakers of the same impedance can have a very different
impedance graph. One speaker may have a lower impedance
at the same frequency than the other making it louder at
that frequency. To confuse the situation further speaker
impedances are stated at a nominal single frequency (usually
1K) which doesn't tell you much about how the speaker will
perform at the other frequencies. A guitar low E string
is 82 HZ, a bass guitar E is 41 HZ. Most of the guitar
energy fed into the speaker is below 1K so we are missing
the info on how the speaker will actually perform if it's
impedance curve varies from being flat. This is why (AMONG
MANY OTHER REASONS) different speakers of the same impedance
sound different than each other.
SPEAKER IMPEDANCE MATCHING
Here again, an extremely misunderstood concept. The question
often asked is how to impedance match speakers to an amplifier.
There are two answers to this one for tube amps, the other
for solid state.
TUBE
In a tube amp there is an output transformer. This transformer
takes the high voltage and low current the output tubes
operate on and transforms it to a high current low voltage
signal that can drive a speaker. A transformer is two coils
of wire wrapped around an iron core. It works off the ratio
of the primary coil to the secondary coil. if one coil
sees X impedance, the other coil will see whatever impedance
the ratio of the coils dictates. Vary the input or output
impedance to the transformer and the other value will follow
it. Obviously by design a tube amp transformer is set to
use with the speakers the taps are labeled for. Use a 8
ohm speaker with an 8 ohm tap and all should be well. There
is much misinformation on the Internet concerning this.
One guy claims that the highest impedance tap should be
used or damage could occur, so if your amp has an 8 ohm
speaker, but also a 16 ohm tap you should be using a 16
ohm speaker instead. This is completely untrue. Suspiciously,
this guy also sells speakers. Matching the speaker impedance
to the tap will give you maximum power transfer and tone.
But there is also the possibility of using a slight mismatch
to achieve different tones. A 100% mismatch especially
upwards (16 instead of eight) will throw away some of your
power. In a situation where you are trying to get output
tube distortion at a lower volume, this may help. The impedance
mismatch will also vary the tone of the system for better
or worse. I would try this on higher power amps where the
transformers are hardier. The possibility of damage always
exists when you try something like this especially if it
is in an amp with a substandard transformer, and the mismatch
also drives the output tubes harder so know going in there
is some degree of risk involved with mismatching.
SOLID STATE
It is not necessary to exactly match the impedance of
the speakers to a solid state amp. There is no transformer
here so the output circuitry of the amp is already set
up to drive speakers directly. What does apply here is
Ohm's Law. According to the speaker impedance used the
amps power output will be different. Therefore the lower
the speaker impedance the more power the amp can output.
Think of it this way- four ohms is a lower resistance than
eight so it will enable the amp to pass twice as much current
through a four ohm load. Since voltage times current equals
power (watts) the amp has doubled it's power output. Of
course this is if the amp is rated to put out this much
power. There are several limiting factors in the design
of an amp. The size of the power transformer is one. All
power going to the amp passes through it and past a certain
point it may overheat or possibly sag down and reduce its
ability to pass power. The output devices are also capable
of passing only so much current. This is also directly
tied to the square inches of heat sinking used and any
fans present to dissipate heat. Running transistors past
there safe operating area will trigger the amps thermal
protection circuitry which will shut the amp down until
it cools off or worst case the transistors will fail. Therefore
it is necessary to use the manufacturer's minimum rated
impedance or higher on a solid state amplifier
.
I knew a guy who did not grasp this concept. He was a home
theater installer. He decided that he had to match the
speakers impedance to the solid state amp so he installed
load resistors along with the speakers to "match" the
load to the amp. This had two downsides. First he was throwing
away amplifier power and headroom as waste heat. Second
these resistor banks were installed in people's attics
so it was a potential fire hazard. I gave up arguing with
him about this after a while.
EXTENSION SPEAKER JACKS
Sometimes I get a request to add an external speaker jack
to a tube amp that doesn't originally have one. For instance
if the amp has an original 8 ohm speaker and you add an
8 ohm extension cabinet the net impedance of the two speakers
working together will equal four ohms. You will have to
choose to live with this mismatch or replace both speakers
with 16 ohm models to make the net impedance equal 8 ohms.
For solid state I would be a little more cautious. As
we said above lowering the impedance load on an amp will
cause it to deliver more power. If the amp was designed
(due to heat sink area or output devices) so that the minimum
allowable impedance is equal to the onboard speaker impedance,
then adding another speaker will drop the impedance below
this amount and possibly cause problems. This is why the
manufacturer did not include an external speaker capability.
What you can do though is add an external; speaker jack
wired so that adding another speaker puts it in series
with the onboard speaker raising the impedance instead
of lowering it. Of course two eight ohm speakers will now
have a net impedance of sixteen ohms, drawing only half
the wattage the amp is capable of producing at a lower
impedance. But it is not as bad as you would think. Half
power is not equal to half volume (10 db), it is 3 db less
volume. Adding a speaker increases the efficiency of the
system by 3 db so you are back where you started volume
wise. So why do it if it results in the same thing? Because
it is not exactly the same. Adding an additional speaker
will allow it to be placed or aimed differently than the
first giving more coverage of the sound. The increased
surface area of the two speaker cones working together
will raise the low frequency response. The lowered demand
on the amplifier will increase its peak handling capability
making a cleaner sound. This last thing may be not desirable
for guitar sound, but helps with bass guitar or PA applications.
Next month will continue the discussion on speakers, moving
on to bass guitar speaker systems. Anyone with a question
on speaker systems please post a comment or email me and
I will include it in an upcoming article.
Steve
Weiss is the owner and main technician of Steve Weiss
Electronics Inc. He is experienced in the repair
of analog and digital musical equipment. This includes
everything from Vintage Tube Amps and Pro Audio equipment
to Digital Keyboards There is also a guitar repair
shop staffed by some of the areas top guitar repair
techs. He is authorized for warranty work on most
major brands. Steve Weiss Electronics is located
inside of Sam Ash Music at 5460 West Sample Road
Margate, FL 33073 954-975-3390 Ext 272. Steve has
also spent 25 years on the road as a performing guitarist
and is the designer of Primal Guitar amps that can
be seen at Primal
Audio.com Steve can also be reached at stevew@metromusicmayhem.com |
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