The two most popular ways of referring to a PAVA amplifier’s output are:
- 100V line and
- Constant voltage.
For a long time these terms were a complete mystery to me, no explanation seemed complete.
The two I heard the most, and still do, are:
- It’s the same theory as used to distribute electricity. The high voltage means more volts and therefore power gets to the destination. This occurs because the cable loses fewer volts. But this needs transformers and when I did the maths their inefficiency seemed to equal the losses of running at lower voltages.
- Simplified cabling: that is definitely a benefit but not really an explanation.
100 volts didn’t seem to be enough of an increase to get the efficiency benefits of a high voltage distribution system.
And unless I broadcast a continuous tone, the voltage reading was anything but constant.
So after much digging I finally got it.
Like all mysteries, once you know how it’s done you won’t believe it’s simplicity.
To start, I need you to forget about 100 volts and the absurdity of volts and current ever being constant when dealing with music and speech.
OK, with those banished from your head let’s unravel this mystery.
Whatever impedes current from completing the circuit, puts that current to work.
Connect a loudspeaker to the amplifier’s output and that current can only get round the circuit by passing through the loudspeaker. The loudspeaker then puts that current to work by making it move the loudspeaker’s cone.
Of course, what gets forgotten is the thing that connects the amplifier to the loudspeaker and that’s…
This useful and age-old way of delivering electricity is not perfect, it too obstructs the current’s journey.
The cable, by obstructing the current’s journey, also puts current to work by heating itself up.
If the above makes sense, then here’s the simple truth:
The amplifier shares its power with everything connected to its output.
And the amplifier shares its power in a certain way that relates to ohms.
Ohms is what we use to measure the amount that current is obstructed by something.
When the voltage and current never change polarity, the ohms measurement is resistance: when it does change polarity, the ohms measurement is impedance.
Once we know the number of ohms of each thing on the circuit, the amplifier will share its power in the same proportion.
Stay with me if that sounds confusing as it’s quite simple.
An example might help explain this better.
Let’s pretend someone decides to use a CAT5e cable to connect a loudspeaker to an amplifier; no one would do that, would they?
They use 1 of the 4 pairs for this.
To find out the cable’s resistance, they short together the wires at one end, then with their multimeter at the other end, they read 4 ohms.
They then measure the loudspeaker and that reads 6 ohms.
So the whole circuit obstructs the current’s journey by 10 ohms.
The amplifier distributes power in proportion to these readings.
So a whopping 40% of the amplifier’s power is consumed by the cable – not good.
If this was a 100 watt amplifier that means the loudspeaker only gets to use 60 watts.
But with 100V line this all changes
The secret is adding a transformer to the loudspeaker. This raises the loudspeaker’s reading from 6 ohms to say 996 ohms.
As the CAT5e stays the same at 4 ohms, we now have a total of 1000 ohms for the circuit.
The amplifier will still distribute its power in proportion to the impedances of everything connected. This means the cable now only gets to consume a measly 0.4% of the amplifier’s power with the loudspeaker getting the rest.
So that’s why 100 volt line is used, but why that name?
The alternative to a 100V line circuit is a low impedance circuit, so maybe we should call it a high impedance circuit.
This high impedance system also simplifies the loudspeaker cabling, as all the loudspeakers connect in parallel.
Is there a reason why it’s called 100V line?
Yes and it also relates to the constant voltage name.
To understand why we need some maths, well one formula.
The formula we use for the calculations includes power, resistance (or impedance) and voltage.
We need to have numbers for two of these to work out the other, so it would be quite useful if we could fix one of the three.
There’s a huge variety of amplifier and loudspeaker powers and every circuit will have a different resistance. But standardising on the voltage is possible so in the maths world they call that a constant.
Then we must decide on the value of that constant voltage: Europe chose 100; America went for 70.
The formula is then:
With the voltage fixed, we only need to measure the circuit resistance to find out the power, you’d best call this the circuit load.
Here’s a quick example (I’m using 100V line for this).
You measure the circuit resistance to be 50 ohms so…
That’s a circuit load of 200 W.
Now all you need is an amplifier that’s 200 W or bigger. Just remember that 200 W needs to be delivered from the amplifier’s 100V line output.
Essentially, if everything is based on 100V line, then all you need to do is add up all the loudspeakers’ powers. You then need to connect an amplifier that’s power is at least that or more and you’ll be OK.
Having trouble with the above formula? Try this very fancy calculator:
Before I finish, I better clear up something, otherwise I’ll get a beating from you tech heads out there.
I didn’t make it clear how you measure the loudspeaker. In fact, I misled you to think you can do this with a multimeter and look at the resistance.
In reality you would need to use an impedance meter not your multimeter and then you would measure the loudspeaker’s impedance not resistance.
You could also use your impedance meter to measure the cable impedance too, but most of the impedance is going to be resistance so your multimeter will be close enough.
My defence is I didn’t want you to start worrying about not having an impedance meter before you understood the principal behind it all.
But now you need to tell your boss you need an impedance meter. Which one you might ask? Go here next.