If you are really serious about this, you will probably want a 70-Volt speaker system. But if you rather save a couple of bucks and do a lot of hand tuning...
This type of amplifier is most common in home stereos. They have a low impedance output of around 8 ohms or 4 ohms.
This type of amplifier is commonly used for commercial "distributed" audio systems with speakers in numerous locations. These amps have a high impedance output identified by a voltage, like "25 Volt", "70 Volt".
The actual speaker impedance varies with frequency and is difficult to measure outside a lab. For convenience, we deal with the nominal impedance. The nominal impedance will approximate the DC resistance of the voice coil in the speaker. Impedance is measured in "ohms", often indicated by the uppercase Greek letter Omega (Ω).
You don't need to rip apart a speaker cabinet to examine the individual speakers inside. If you plan to hook the speaker cabinet to the amplifier as a unit, the impedance of the whole unit is what matters.
The general tendency is:
| car speakers | 4 ohms |
| home audio speakers | 8 ohms |
| pro-sound single woofer PA cabinets | 8 ohms |
| dual woofer PA cabinets | 4 ohms or 8 ohms |
| "monitor" speakers | 8 or 16 ohms |
The best way to find out the impedance of a speaker is to read it off a label on the speaker. If the speaker is unlabeled, look for manufacturer's literature or spec sheets. If all else fails, you can attempt to measure speaker impedance with an ohm meter set for 20 ohms full-scale. The ohm meter will read slightly low:
| 3 to 3.5 ohms | 4-ohm speaker |
| 6 to 7.5 ohms | 8-ohm speaker |
| 13 to 15 ohms | 16-ohm speaker |
There is no practical way to measure the output impedance of an amplifier. Read the value off the back of the machine, or look it up in manufacturer's literature. Impedance is measured in "ohms", often indicated by the Greek letter Omega which looks like a horse shoe.
The general tendency is:
| car amps | between 1/2 and 4 ohms |
| home audio amps | either 4 or 8 ohms |
The speaker output should be labeled with either
Be aware that the output isn't always a single value:
Note: Even if you could measure the impedance of your amplifier, you wouldn't want to do so. Maximum power is delivered to the load if the source and load have the same impedance. But when this is true, both the load and the source are dissipating the same amount of power. So, if you want to send 100 Watts out to the speakers, you must generate 200 Watts, half of which is wasted as heat in the amplifier. The trick is to have a carefully calculated impedance mismatch, so that most of the power goes out the speakers. This is taken into account by the designers of the amplifier. So an amp that says "8 Ohms" doesn't necessarily have an 8 Ohm output. That's just that the amplifier was designed to drive an 8 Ohm load. If it helps, think of an amp's rating as the "target impedance".
Amplifiers, intended to drive speakers, have ratings for the same characteristics that speakers have. They are designed to drive a particular impedance, and they put out a certain amount of power.
[There are other characteristics of a sound system, such as the frequency response, distortion, etc. Those things matter when you sit down in a quiet room and immerse yourself in music or other sound with a fine pattern. Haunts are usually less concerned with such details. They just want "BOO!" to be loud and fairly clear.]
If your speakers are the wrong impedance, or you have hooked several speakers up in such a way that the combination presents the wrong impedance to the amplifier, you expose yourself to numerous problems. Impedance matching is necessary to:
Everything just works better if it's all matched.
NOTE:
According to legend and conventional wisdom, solid state (transistor) amplifiers can drive higher impedances than their rating demands. So a transistor amp with a label saying "4-ohm" will drive 4, 8, and 16 ohms with no troubles. You can go up but not down. Don't try 2 ohms.
Older legends say that tube amplifiers can drive lower impedances than their rating demands. So a tube amp with a label saying "8-ohm" may drive 4 and 8 ohms. You can go down but not up. Don't try 16 ohms. And never run a tube amplifier without speakers attached. Other legends of the tube age say that any attempt to use a tube amp with speakers of different impedance may cause damage to the amplifier.
I have not tried either of these combinations, so don't blame me if it causes problems.
Honestly, you are better off with a proper match. Just because your tube amp with 8-ohm output survives driving a 4-ohm speaker doesn't mean that it likes driving that load. You are probably reducing the life time of the amp.
Now, 4-ohm speakers and amplifiers are common. I have seen 2-ohm components, and even amplifiers that claim "1-ohm stable". The lower impedances are popular with automotive audiophiles. Since they are limited by the car's 12-volt power system, they attempt to boost power by using lower impedances.
Warning - The lower the impedance, the heavier the wire you need. Wire has resistance, which acts like impedance. If you wire up a 1-ohm speaker, with wire that has one ohm of resistance, you are wasting half of your power in the wire! And if you crank up the volume on that speaker, that wire is going to get hot. The solution is a thicker wire, which exhibits a lower resistance.
So, before your start, take an inventory of what speakers you have, and what impedance and wattage they are rated for. Then read the plate on the back of your amplifier, and find out what impedance load it likes to drive.
Our goal is to string the speakers together in such a way that they present a combined impedance that matches the amplifier rating.
The problem with this is that high quality transformers are very expensive. And even the best transformers distort the signal somewhat.
This is simple in theory, but problematic in practice. You need to use a high power resistor (in the 4+4=8 example, the resistor will dissipate as much power as the speaker). Also, sound quality will suffer, because the speaker impedance changes with frequency, but the same is not true for the resistor.
The problem with this is that high quality transformers are very expensive. And even the best transformers distort the signal somewhat.
The short answer is that there are several choices: series, parallel, series-parallel, and more complex.
I have seen web sites that prefer parallel, so long as the resulting impedance mismatch isn't too bad. In all cases, you are much safer if all of the speakers are identical.
--------(8 ohms)----+
|
--------(8 ohms)----+
Is the same as
--------+
|
(16 ohms)
|
--------+
The schematic looks like this:
The math is pretty simple:
R = R1 + R2You can take this as far as you want:
R = R1 + R2 + R3 + R4 ...
If you have a bunch of identical speakers with an impedance lower than what the amplifier likes, a series circuit can raise the impedance to keep the amplifier happy.
Benefits:
Drawbacks:
--------+-----------+
| |
(8 ohms) (8 ohms)
| |
--------+-----------+
Is the same as
--------+
|
(4 ohms)
|
--------+
The schematic looks like this:
The math here is a bit more complex:
1/R = 1/R1 + 1/R2 + 1/R3 + 1/R4 ...For two resistors, this simplifies to:
R = (R1 x R2) / (R1 + R2)And if the resistors (speakers) have exactly the same value:
R = R1 / 2
If you have a bunch of identical speakers with an impedance higher than what the amplifier likes, a parallel circuit can lower the impedance to keep the amplifier happy.
Benefits:
Drawbacks:
--------+-----------+
| |
(8 ohms) (8 ohms)
| |
| |
(8 ohms) (8 ohms)
| |
--------+-----------+
Is the same as
--------+
|
(8 ohms)
|
--------+
The schematic looks like this:
The math here is just a combination of the math for series and parallel circuits:
The schematic looks like this:
Benefits:
Drawbacks:
"I have a pair of 8-ohm speakers rated 50 Watts; three more 8-ohm speakers rated 25 Watts; a 4-ohm job that can take 75..."
Getting them set up so that the impedance presented to the amplifier is correct, is only the beginning of the problem. Then you have do deal with the power allocation to each speaker and the volume you get out of it.
That's when you'll really want a 70-Volt speaker system.
In theory, this can be done. Some amps are even designed to operate either in stereo, or as one huge combined ("bridged") amplifier. But unless your equipment is well matched and adjusted - and you really know what you are doing - attempting this is dangerous. If you have multiple amps, your best bet is to have each one driving a different set of speakers.
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