ohms?
This page is intended to help
people understand "ohms" in more detail. It also addresses parallel / series
wiring configurations along with bridging amplifiers. Dual voice speakers
are mentioned in the notes as well. I have a BS in Electrical Engineering
so I do not know how much of this the average Joe is going to understand. I am
also human so there may be mistakes below.
Understanding what ohms are and how they relate
to car audio is most helpful when trying to determine optimal wiring
configurations for connecting multiple speakers to an amp. There are some
basic electrical engineering formulas I will use on this page. Fortunately
they are fairly simple. Here they are:
Ohm's law: Current = Voltage /
Resistance or I = V / R
You can rearrange this formula
in a couple ways: V = I * R
and R =
V / I
Power equation: Power =
Voltage^2 / Resistance or P = V^2 / R
Rearranging this formula gives us: R = V^2 /
P and V = SQRT(P * R)
Using Ohm's law we can derive some more power
equations: P = I^2 * R
Rearranging gives us:
I = SQRT(P / R)
Abbreviations used on this page
are:
I : current, measured in amps
V: voltage, measured in volts
R: resistance, measured in ohms
P:
power, measured in watts
With that out of the way we can get
down to business. As stated earlier, ohms are a measure of electrical
resistance. It helps in this discussion if we consider amplifiers to be
perfect voltage sources. Consider this 25 watt amp:
P = V^2 / R
Most amps are rated into 4 ohm loads so
we now have:
25 watts = V^2 / 4 ohms
Solving for voltage we get: V =
SQRT(25 * 4) = 10 volts
So our 25 watt amp can be considered
here to have the ability to produce up to 10 volts output.
Now let us consider what happens when we connect two
speakers in parallel to the amp. At this point I am going to introduce
some more formulas. We are considering speakers to just be simple 4 ohm
resistors for this discussion. There are formulas which dictate what
happens when you combine resistors in various ways. Before that we need to
explain what is meant by parallel and series connections.
Each channel of an amplifier has a positive (+) and
negative (-) connection. The amp develops a voltage between these two
terminals and this voltage is what drives the speakers. The equivalent
resistance of what you connect to the amp is referred to as the load.
Normally when connecting a single speaker to a single
amp channel you merely connect the "+" terminal of the speaker to the "+"
terminal of the amp. Then do the same thing for the "-" terminals.
Things get more complicated when you are connecting multiple speakers to a
single amp channel. In a parallel configuration you connect both "+" terminals of the speakers to
the "+" terminal of the amp. Then do the same thing for the "-"
terminals.
A series connection is a little more
complicated. First, you connect the "+" terminal of the amp to the "+"
terminal of one of the speakers (let us call it speaker A). The next thing
you do is connect the "-" terminal of speaker A to the "+" terminal of the other
speaker (speaker B). Lastly, you connect the "-" terminal of speaker B to
the "-" terminal of the amp. You can see in this connection that power
from the amp goes through both speakers one after the other, hence the name
"series."
Finally, here are the formulas that
tell you what resistance load you end up with when wiring multiple
speakers:
For two speakers in parallel:
1 / Rt = 1 / Ra + 1 /
Rb
where Rt is the total equivalent
resistance or load and Ra and Rb are the resistances of the two speakers.
You can see that you can use speakers of different resistances but there other
implications of doing that which are usually undesirable because the power will
not be spread evenly between the speakers.
Working through the math if you put two
4 ohm speakers in parallel you get:
1 / Rt = 1 / 4 + 1 /
4 = 1 / 2, Rt = 2 ohms
The equivalent resistance is exactly
half of what we started with. We will look at the implications of this a
little later. Let us do another example first with three 4 ohm speakers in
parallel:
1 / Rt = 1 / 4 + 1 / 4
+ 1 / 4 = 3 / 4, Rt = 4 /3 = 1.33 ohms
You can see that as you put more and
more speakers in parallel the equivalent resistance will drop further.
Next lets look at the equivalent
resistance for speakers in series:
Rt = Ra + Rb
Now that is easy! You just add
the resistances for each speaker so putting two 4 ohm speakers in series will
you give a single load of 8 ohms.
With that background out of the way we can look at
what effect these different wiring combinations have on the amplifier.
Going back our 25 watt (10 volt) amplifier with a single 4 ohm speaker we
have:
Current = Voltage / Resistance = 10 / 4 = 2.5
amps
So when this amp is producing maximum power (25
watts) into a 4 ohm load, the load will draw 2.5 amps from the amp.
Now let us look at what happens when we connect two 4
ohm speakers in parallel (which gives us a 2 ohm equivalent load) to this
amp:
Power = Voltage^2 / Resistance = 10^2 / 2 = 50 watts
This is seems great! Our 25 watt
amp is now producing 50 watts but there are some complications. Let us see
the current now:
Current = Voltage / Resistance = 10 / 2
= 5 amps
Even though our voltage is still the
same (10 volts) our current has now doubled from 2.5 amps to 5 amps. If
the amp has the capability to produce this much current and dissipate the heat
that this will generate then everything will be fine. One way to determine
if your amp is capable of this is to look for power ratings that are given into
2 ohms in addition to the normal 4 ohm rating. Further if the power
doubles into the 2 ohm rating then the amp has ample current capacity.
Another clue to tell whether the amp will work with 2 ohm loads is look for the
phrase "2 ohm stable." Being 2 ohm stable only means that the amp will
function with 2 ohm loads; it does not necessarily mean that the amp will
produce more power into 2 ohms. If you attempt to use a 2 ohm load with an
amp that cannot handle it a well designed amp will shut itself off or blow a
fuse and a poor one could be permanently damaged.
Next let us look at a series connection with two 4
ohm speakers. This will give us an 8 ohm load and we will use our 25 watt
(10 volt) amplifier again:
P = V^2 / R = 10^2 / 8 = 12.5 watts
Our amp is now producing only half its power
rating! And the current is:
I = V / R = 10 / 8 = 1.25 amps
The current is 1/2 its original value as well.
Series connections are not used as often as parallel connections because they
reduce power. However, they are easier for your amp to drive since they
draw less current.
Another option that is often available is to bridge
an amplifier. This process takes 2 amp channels and combines them to act
as a single more powerful amp channel. How to do this to an amp and wire
everything varies so please do not ask me how to bridge your amp. I can
explain the effects of it though.
What typically happens when you bridge an amp is that
the voltage it can produce doubles. Our 25 watt (10 volt) amp can now
produce 20 volts. Let us look at how that affects power:
P = V^2 / R = 20^2 / 4 = 100 watts
This is 4 times the original power of the amp but let
us look at the current situation:
I = V / R = 20 / 4 = 5 amps
So now we see even with a regular 4 ohm load the
current is already double what the normal value (2.5 amps) was. For other
reasons the power usually does not usually quadruple when you bridge an amp but
will typically at least double. Connecting a 2 ohm load to a bridged amp
raises the current requirement even more. However, if your amp can handle
it then you will be squeezing a lot of power out of the amp.
In fact, many people use that kind of setup in
competition where the classes are judged by the power rating of the amps in the
system. A 25 watt amp can produce many times more power when bridged and
driving low resistance loads so the competitor gets more power than what appears
on the surface. Zapco and Phoenix Gold make amps which are able to drive
such low resistance loads (sometimes as low as 0.5 ohm!)
Some final notes:
On this page I have considered amplifiers to be
perfect voltage sources. They are not though and they have some internal
resistance which lowers power output slightly.
I have also considered speakers to be perfect 4
ohm resistors. In actuality the resistance of the speaker depends on the
frequency the speaker is playing. For example, a speaker may have a 3
ohm resistance at 80 Hz and a 9 ohm resistance at 300 Hz. If you were to
make a plot of resistance versus frequency you would get what is called the
impedance curve of the speaker. Also, speakers act in some ways as
inductors and capacitors so a true model of a speaker must include those
components as well. How does this affect the sound you ask? Well
if you have an amp that has very weak current capability it may work fine into
perfect 4 ohm loads but when you connect our real speaker which has an
impedance dip at 80 Hz the amp may have difficulty and smear sounds that have
80 Hz components. These are minor but audible effects. This is why
it is good to get a 2 ohm stable amp even if you never plan on running 2 ohm
loads.
Placing speakers in parallel and bridging amp
channels are effective methods for increasing the power in your system
assuming your amp can handle the increased demand.
Use a series configuration when you need to raise
the effective resistance of the load. This occurs more often when you
are using dual voice coil speakers.
Dual voice speakers have two speaker connections
on them. This typically increases power handling capability and gives
you more wiring options. For example, if you have two dual 4 ohm voice
coil speakers you can get a single 4 ohm load which is suitable for connecting
to a bridged amp. You would do this by connecting the voice coils on
each speaker to each other in series. This would give you two 8 ohm
speakers. Next you put those two 8 ohm speakers in parallel and this
will give you a single 4 ohm equivalent load.
Be aware that using lower resistance loads and
bridging produces a greater load on the amp. Well designed amps that
cannot handle the demand will either shut themselves off or blow a fuse.
A poorer designed amp can permanently damage itself. Also, even if an
amp works in these configurations it will probably generate more heat so
ventilation is even more important.
As mentioned earlier ohms are a measure of
electrical resistance. You should be able to understand why
resistance changes affect the amp power as it does. If you raise the
resistance the amp is not able to drive as much current through the load and
thus you get less power. If you lower the resistance of the load math
says that more current will be drawn from the amp. Assuming the amp can
handle this you get more power.
To understand why series and parallel
configurations have the effect on resistance that they do consider this.
When you connect speakers in series current must flow through both speakers
and so it hits the resistance of both speakers. When you have speakers
in parallel, the current has multiple paths since it can go through either
speaker so the equivalent resistance is always lower than that of either
speaker alone.
|
