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Saber Audio – Part I

One source of confusion is the speaker impedance (da ohms).  this is caused mainly because of many blown amplifiers. This post is intended to explain the meaning of speaker impedance and relations to the amp.

Ω ohms
The Ω is the unit of measure for impedance, which is the property of a speaker that restricts the flow of electrical current through it. 

Typical speakers have impedance ratings of 4 ohms, 8 ohms or 16 ohms.  The impedance of a speaker is a physical property that (ideally) does not change value, although from an engineering standpoint, there are many complex characteristics that make up speaker impedance    For this reason, the rating of a speaker is called its ‘nominal’ value, which pretty much means “in name only”.   For the average audio user, the nominal impedance is the dominant characteristic and for the purposes of this discussion, we will use the nominal value of the speaker’s impedance.

Why are ohms important?
Two reasons:

  • If you connect your amplifier to the wrong speaker impedance, you risk damaging the amp. In solid state amps, if  the speaker impedance is too low, the amplifier will tend to overheat and more power is used up in the amplifier than is delivered to the speaker.  Too many speakers on a solid state amp can burn up the power output section.
  • The amplifier will deliver maximum power (volume) to the speaker when the speaker impedance matches (is equal to) the internal impedance (called the OUTPUT IMPEDANCE) of the amplifier.  Too low an impedance will result in weak output and poor tone.  If the speaker impedance is higher than that of the amplifier, its power output will again be less than it is capable of.

Understanding Ohms and Impedance:
In order to understand the reasons for the rules for speaker connection, we need a bit of electrical theory.  In order to relate it to something you are more familiar with, let’s consider the ordinary garden hose.  The flow of water through the hose  is similar to electric current, which is usually described as the flow of electrons through the wire and is measured in Amps.

Now put your thumb over the end of the hose and try to stop the flow of water.  Feel the pressure? This pressure is similar to Voltage. It is the force of electricity that pushes the electrons through the wire.  Notice that if you succeed in plugging the water flow, (no current) the pressure is still there.  This is like an amplifier with no speakers attached, or an AC outlet with nothing plugged in.  Voltage is present, but there is no current flow.

Now, move your thumb a bit to allow some water to spray. By varying the position of your thumb, you can control how much water comes out of the hose. Your thumb is restricting the flow of water.  In an electrical circuit, things that restrict or control the flow of current are said to impede current flow, and are described as having impedance.  In a hose, we use a nozzle to restrict the flow.  In an electrical circuit, the device that uses electrical energy and has impedance is called the LOAD.

Ohm’s Law states: In an electrical circuit, current flow is directly proportional to voltage and inversely proportional to impedance.  Mathematically, this becomes: Current (in amperes) equals voltage (in volts) divided by impedance (in ohms).

As an example, if a (solid state) amplifier is producing 10 volts AC to an 8 ohm speaker, the current in the speaker will be 10 volts / 8 ohms or 1.25 amperes. If the amplifier output is increased to 20 volts to that 8 ohm speaker, the current becomes 20 Volts / 8 ohms or 2.5 amperes. So increasing the voltage increased the current. If the voltage decreases back to 10 volts, the current will decrease back to 1.25 amperes.

Now, if our amplifier with 10 volts output is connected to a 4 ohm speaker, the lower impedance will allow more current to flow. The amount will be found by 10 volts / 4 ohms = 2.5 amperes.  If we use a 2 ohm speaker, even more current flows: 10V/2 ohms = 5 amperes.


Next, let’s connect another 8 ohm speaker to the amplifier terminals in the same way, so you have two wires from the amp’s red terminal going to the ‘+’ terminals of the speakers, and two wires from the amp’s black  terminal to the speaker ‘-‘ terminals.  This is called a PARALLEL connection, because of the way it looks in an electrical schematic diagram.

The first thing to understand is that the voltage output from the amplifier does not change. (In reality, it might drop just a hair, but for this discussion let’s assume a perfect amplifier.)  So it’s still 10 volts AC.  And since each speaker is connected directly to the amp’s output terminals, each speaker will receive 10 volts from the amplifier. As we saw earlier, if 10 volts is applied to an 8 ohm speaker, it will draw a current of 1.25 amperes from the amplifier.  And if each speaker needs 1.25 amperes, then the amplifier must supply a total of 2.5 amperes to the two speakers. If you add a third speaker, it will also draw another 1.25 amperes, (total 3.75 amperes) as will a fourth (which would total 5 amperes).  If you keep adding speakers, at some point the speakers will demand more current than the amplifier can deliver, and it gives up its smoke and dies.  Too many loads is an overload. (See importance #1, above.)

ADDING SPEAKERS DECREASES THE TOTAL OHMS IMPEDANCE.

If you are mixing speakers with different impedance ratings, be sure to check the total impedance using the rules above to be certain the total is within the limits of the amplifier.  Solid state amps typically have a ‘minimum load impedance’ and the total speaker impedance must be equal to or greater than that value.

The ohmmeter function of a digital multimeter can help identify what the impedance of the speaker itself should be.  Generally, the reading given by an ohmmeter will be about 2/3  to 3/4 of the impedance of the speaker. So, a 4 ohm speaker will typically measure about 2.5 – 3 ohms, and an 8 ohm speaker will typically read about 5-6 ohms, while a 16 ohm speaker will measure around 12 ohms.

The used Amplifiers

The infamous Crystal Focus boards use simple d-class amp by Texas instruments, more specifically the model TPA0211

Those chips are quite dynamic and can handle alot. But two configurations are mainly suggested in the datasheet:

TPA0211

  •  2 W Into 4 Ω From 5-V Supply
  •  0.6 W Into 4 Ω From 3-V Supply

 

Capture tpa0211

As you see from the second graphic  the relation between load resistance and output power is exponential.

 

 

The MAX98304 mono 3.2W Class D amplifier is a really nice alternative, although acquires a bit more external components.

You can read more here: http://www.engineersgarage.com/tutorials/circuit-design-how-measure-impedance-loudspeaker

 

To be continued….