8 Ohms or 70/100 Volt Sys

Just like how we have 110v and 220v in electricity, we also have two main kind of systems in the audio world.

(1) 8 ohms -or- Low Impedence System

(2) Commercial -or- Constant Voltage -or- Distributed System -or- 25/70/100 volt system

In a masjid environment constant voltage system is used for alleys, outside, classrooms, reception, offices etc whereas the low impedence (8 ohm) system is used for the main prayer halls. You cannot mix and match a 70 volt speaker with a low impedence only amplifier. It will not work or work very inefficiently. We find this is a very common mistake in many masajid. You can use both at the same time provided you have separate amplifier and their respective speakers for each system. Everything before the amplifier in the audio chain can remain common.

The main purpose of both the systems is to deliver the right amount of power to the speakers to function properly and give the desired sound.

Power(watts) = voltage(volts) * current(amps)

8 ohms -or- Low Impedence System


This is the system that is used in home theaters, music shows etc where you see large speakers on stands and huge hanging array of speakers. They are very expensive and give the best fidelity and bass output. This is by far the most clear sounding system out of the two. Fidelity / clarity and detail is very prominent in this system. We use this in most of our masjid installations at least for the main prayer halls where we want the best sound so that Tajweed is not compromised.

One drawback of this system is that you can only connect few big speakers per channel on the amplifier. Also we have to calculate aggregate impedence (resistance) of the speakers per channel and make sure that they fall within the impedence range of the amplifier. One way to do this is connect the speakers in a combination of series and parallel connection to get within the right impedence range of the amplifier. If the aggregate impedence of speakers falls then the amplifier spits out more and more power (wattage). When this impedence gets really low then the amplifier gives out lot of wattage destroying the speakers and overheating itself in the process. This can eventually lead to the destruction of both the speakers and amplifiers.

The speakers get destroyed as they get more wattage/power than they are rated for and the amplifier gets destroyed because of overheating caused by constantly working to increase the output power / wattage to more than what it is rated for.

For example ... The amplifier has an impedence range of 4 ohms to 14 ohms per channel. All our speakers are rated at 8 ohms and we have six speakers to connect to provide sound in the entire area. Here is how we can proceed.


Rules ...


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Parallel connection of speakers
- positive to positive and negative to negative connection ...

Total impedence of parallel connection can be calculated as follows

1/R = 1/R1 + 1/R2 + 1/R3 ...

where 1/R is the aggregate impedence (resistance) and 1/R1, 1/R2 etc is the impedence of individual speakers. In our case they all will be 8 ohms.

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Series connection of speakers
- positive to negative and negative to positive connection ...

Total impedence can be calculated as follows

R = R1 + R2 + R3 ...

where R is the aggregate impedence (resistance) and R1, R2, R3 etc is the impedence of individual speakers. In our case they all will be 8 ohms. 

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Please note that impedence or resistance is calculated in ohms.

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So if we have to connect three 8 ohm speakers per channel on the amp and we connect all of them in parallel we get ...

1/R = 1/8 + 1/8 + 1/8

then R = 2.4 ohms which is out of our amplifier impedence range of 4 to 14 ohms


If we connect all of them in series we get

R = 8 + 8 + 8

R = 24 ohms which again is out of our amplifier impedence range of 4 to 14 ohms

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The solution lies in connecting two speakers in parallel and the third in series in which case the impedence will be as follows

two speakers in parallel

1/8 + 1/8 = 4 ohms

one speaker in series

8 ohms

Aggregate impedence will be 4 ohms + 8 ohms = 12 ohms and this happens to be in the range of the amplifier impedence of 4 to 14 ohms.

Please note that most modern Class D amplifiers can go as low as 2 ohms so in our example we can also connect all three speakers in parallel as the aggregate impedence will be 2.4 ohms and still within the impedence range of amplifier.

Now how many watts of power will my speakers get when they are connected in parallel or series or any combination. Let us take an example to understand this. Some numbers first. Every amplifier manufacturer has output power listed at various impedence ratings. For example a Crown XLS2500 amplifier gives ...

440 watts per channel at 8 ohms
825 watts per channel at 4 ohms
1200 watts per channel at 2 ohms

So if we connect one 8 ohm speaker on one channel of this amplifer, that speaker will get 440 watts of clean amplifier power. If we connect two speakers in parallel to one channel on this amplifer then the amplifier will give out 825 watts as the aggregate impedence will now be 4 ohms. So each speaker will get 825/ 2 = 412 watts of power. Now if we connect three speakers in parallel to one channel on this amplifier then the amplifier will give out 1200 watts as the aggregate impedence will now be around 2 ohms. So each speaker will get 1200 / 3 = 400 watts of power.

So essentially wherether you connect one speaker or three speakers in parallel, the wattage that each speaker will get is essentially the same at around 400 watts. But since you cannot go below 2 ohms per channel as per amplifier specifications, we cannot connect more than three speakers in parallel on each channel. You can connect more than three speakers in a combination of parallel or series to match the impedence range but in this case the speakers will not get enough power from amplifier. For example ...

If we connect three speakers in parallel and one speaker in series (total of 4) on one channel of the same amplifier above its aggregate impedence will be 2.4 ohms + 8 ohms = 10.4 ohms. If the amplifier gives 400 watts at 10 ohms, then we need to divide this 400 watts across 4 speakers which will be 100 watts each and will be significantly lower than 400 watts that each speaker was getting when we connected three speakers in parallel.

So in a 8 ohms or low impedence system, you need to worry about two things. Final impedence that an amplifer is seeing when you connect speakers in series and parallel and also what is the wattage that each speaker will get when they are all connected.


Commercial -or- Constant Voltage -or- Distributed System -or- 25/70/100 volt system

We mentioned earlier Power(watts) = voltage(volts) * current(amps)

There can be two ways we can deliver this power. Say if we were to deliver 200 watts of aggregate power to one end we can do the following.

low voltage and high current

200 watts = 10 v * 20 amps

But you need a large guage (diameter of conductors) cable to carry this high current. Now you know why the car battery has such thick cables.

-OR-

high voltage and low current

200 watts = 50 v * 4 amps

For this you will need a small guage cable as amperage is low.

In both cases end result is 200 watts of power. This rule of electricity is applied in audio engineering as well. After all it is electricity of some sort that is flowing through the audio cables. The second option of high voltage and low current is used in constant voltage system. Constant voltage system can be 25 volts or 70 volts or 100 volts and sometimes even 200 volts. 70 volt is the most common system used in north america and 100 volts is used in europe and asia. 100 volts system is commonly used when very long cable runs are required to minimize line loss. 

            

Constant voltage system can drive hundreds of speakers over long cable lengths. The aggregate wattage of all the speakers must be lower than the output wattage of the amplifier channel to which they are connected. All speakers in this system are connected in parallel, meaning +ve to positive and -ve to negative all the way through. You do not have to worry about impedence matching. Nowadays we can also get amplifiers which drive low impedence on one channel and constant voltage on the other channel.


We are skipping minor details to avoid all the math and keep the concept to simple basics.

In a constant voltage system the amplifier has a built in step up transformer that raises or maintains a voltage of 70 volts in the wire. Remember this wire now carries low current roughly over 1 amp so the wire does not get heated and hence there is minimal power loss in the form of heat. We can also use smaller guage cable even for very long runs. The most common size cable used is 18 guage. You can use an external step up transformer if the amplifier does not already have one. There are also now transformerless amplifiers that can safely drive high voltage 70 volt lines.

At the other end each speaker has a step down transformer that matches the 70 volt line to the speaker's impedence. The speakers/transformers in this system has something called primary taps which are like 1w, 5w, 10w, 15w, 25w etc. The higher the primary tap value that you connect to the louder the speaker will sound. The tap values determine the amount of power to be delivered to the speaker. Usually doubling the tap value will make the speaker sound 3 db louder. You can connect 100 speakers tapped at 1w each to a 100 watts per channel amplifier or 4 speakers tapped at 25 watts each to the 100 watts per channel amplifier or any combination in between. The idea is that the total of primary tapped wattage of all speakers/transformers must be equal or less than the power output of the amplifier channel they are connected to. The ideal solution is to keep a 20-25% cushion to account for transformer and line loss and another 25% to add more speakers in future if needed. So if the aggregate for 4 speakers is 100 watts then it makes sense to start with an amplifier that gives out 150 watts per channel. There is no limit for the upper value meaning you can have a 70 volt amplifier with like 500 watts per channel connected to four 70 volt speakers rated 25 watts each. This will not damage the speakers as the step down transformer on them will take care of the additional power.

Note : When we say speaker we mean the transformer on the speaker. Most of the commercial speakers come with built in transformers. A speaker is connected to the secondary tap on the transformer with matching impedence. The primary tap on transformer is finally connected to the 70 volt line going to amplifier.

A 70 volt system does not always run at 70 volt but at lesser voltage. Also most of the step down transformers that are built into cheap ceiling speakers distort sound a lot. They also have very low quality transient response. In a nutshell with a 70 volt system you will get an airport type of sound where as a low impedence system will give you music quality sound. But of course at a higher price. In some cities the building code may require to install 70 volt lines in conduit.

You can also add attenuators (volume controls) on one speaker or a group of speakers. Each attenuator is rated in wattage like 25 watts or 50 watts or 100 watts etc. What is means is that you can use one 25 watt rated attenuator for one speaker tapped at 25 watts or for a group of 5 speakers tapped at 5 watts each. The sum of the primary taps of the speakers for which the attenuator will be used must be equal or less than the wattage the attenuator is rated for.

All the speakers after the attenuator in the 70 volt line will be effected by volume settings of the attenuator. Any speaker before attenuator or in parallel 70 volt line will not be effected.