G'day, Gary from Simple Audio Tips.
Did you know that you can't test the true resistance or impedance of a 70 volt speaker system
with a standard multimeter like this one?
Stick around and I'll prove it with a simple test
and show you the tool you'll need.
Without getting too complicated, and increasing the 'yawn' factor to about ten,
I'm gonna go over some basic differences between these two meters.
A standard multimeter checks the resistance in a circuit by applying a 'DC' or a direct current.
and displaying the result on the meter here.
An impedance meter, on the other hand, tests the circuit with the same conditions
as an audio signal.
And that's by applying an alternating current.
With a simple resistive circuit like a resister inline,
the results between these two meters are going to be exactly the same.
But when you introduce speakers coils and transformers into the works,
weird things start to happen with the resistance in the circuit,
especially when you're applying an alternating current.
Temporary magnetic fields produced in the coils
resists current flow
and in turn produces resistance.
I don't bore you with any more detail than that but put simply
a standard multimeter like this one is unable to give you a true load reading
because it doesn't apply the same signal to the circuit
as the impedance meter.
Now the audio amplifier gives alternating current, the impedance meter does the same.
Now let's look at this in practice
we've got an audio impedance meter and have got a standard multimeter.
I'm going to connect the audio impedance meter to a standard resistive circuit
that's just a little resistor
that is labelled at 470 ohms.
we going to connect that up to the leads and I'm gonna turn the meter on
and it's set to the 100 scale.
It reads just below 5
That's 5 x 100 so that's just below 500 ohms.
Now, let's have a look at the normal multimeter, and we'll connect that up.
Connecting that up, it's just under 500 ohms, as well or 0.469k
So, as far as these two meters a concerned,
applying it to a standard resistive circuit
they see the same amount of resistance
Now what happens when you test a speaker system
that's got transformers and voice coils and things like that inline?
Let's have a look at that.
Firstly what I'm going to do is
connect the impedance meter to the feedline,
and remember, this works on a scale, and is a multiplication scale.
So whatever reading we have here, is multiplied by 100
You'll hear a tone because that's the alternating current going through the system.
and the reading is just under 450 ohms.
Now let's look and see what the multimeter gives
Connecting the multimeter to it,
0.028 k which is 28 ohms,
A big difference between the two
We've got 28 ohms here and when we test with the
audio impedance meter, we've got 450 ohms,
a big difference between the two.
Does it really matter that the two readings a different?
Well amplifier manufacturers state the minimum impedance
that can be connected before the amplifier overloads.
If you overload the amplifier,
you can damage it or even burn it out.
Now let's imagine an installation
where you've got 20 or 30 speakers in a large area
or even down a corridor or something like that.
Without an impedance meter, you can only make a calculation
by adding each of the speaking outputs together that's written on the labels.
But this doesn't take into consideration any wall attenuators
in separate rooms.
Maybe line faults caused by damaged cables
or incorrectly marked speaker outputs.
There are so many different variables to take into consideration.
An impedance meter tests the total load
at the point where it's connected to the amplifier
and checks that it is between the amplifiers tolerances
In the next video I'm gonna take a closer look at two
popular audio impedance meters on the market in Australia
the "Australian Monitor" and also the "Altronics Red-back" unit which is a little more automatic
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