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Tri-Mode Power Dissipation

On this page you will see how an amplifier's power is distributed when the speakers are connected in a tri-mode configuration.

  • For this example, we will assume...
  • The amplifier is stable into 2 ohms stereo and 4 ohms mono
  • The amplifier will produce 100 watts per channel into 4 ohm stereo, 200 watts per channel into 2 ohms or 400 watts into 4 ohm mono. Read the Amplifier Bridging page if you don't understand why the output power is 4 times as high when bridged as it is for each channel in 4 ohm stereo.
  • When it is said that the load on the amplifier is increased, it means that the impedance of the speaker load is decreased and there fore putting more stress on the amplifier.

This image is a review of the signals present on each of the speaker output terminals of a bridgeable amplifier. All of the lines of the same color are connected together. You can see that the left positive output has the 'normal' signal and the right negative has the 'inverted' signal. The left negative and the right positive are connected to a reference (ground) and have no signal on them.

In this diagram, you can see that the voltage across the subwoofer is twice the voltage of the stereo speakers. The little 'probes' show the points where the voltage would be measured/read and the gray display shows the voltage measured by the probes above each display.

If we use the Ohm's Law formula: P=E^2/R
If the small speakers are 4 ohms each and the sub is an 8 ohm speaker...

We can see that the smaller speakers will dissipate:

P=20^2/4
P=400/4
P=100 watts per channel
.

The 8 ohm sub will dissipate:

P=40^2/8
P=1600/8
P=200 watts into the bridged speaker.

The total power dissipation is 100 watts for each of the 4 ohm stereo speakers plus 200 watts into the bridged 8 ohm speaker. The total power output/dissipation is 400 watts. The same total power as if you were running the amplifier into a 4 ohm bridged OR a 2 ohm stereo load.


Tri-mode with different types of speakers:
In the previous diagrams, it was assumed that all of the drivers were woofers. All of the speakers were directly connected to the amplifier (no crossover components). If you use passive crossovers (capacitors or coils), the system will be able to drive more speakers without increasing the effective load on the amplifier. If the 'stereo' speakers in the previous diagram are 4 ohms each and the bridged speaker is an 8 ohm speaker, the amplifier will be loaded to its maximum safe load (the amp is 2 ohm stereo or 4 ohm mono stable). Using passive crossovers, the 'stereo' speakers could be 2 ohms and the bridged speaker could be a 4 ohm speaker and the load would still be a safe load. Without the crossover components, the amp would 'see' a 1 ohm stereo/2 ohm mono total load.

For this next example... If the passive crossover has a crossover point of 150 hertz and the crossover components are connected as follows:

The high frequency speakers will not load the amplifier below 150 hertz and the low frequency speakers will not load the amplifier above 150 hertz. In the real world, the point where the high and low pass speakers stop being a load on the amplifier is not as abrupt as this and there is actually an overlap of frequencies where both high and low frequency drivers are a load (albeit an ever decreasing load) on the amplifier. There is no single point in the audio spectrum where both the high frequency and low frequency drivers present their rated impedance to the amplifier.

On the following graph, the violet line is the effective impedance of the high frequency speakers. You can see how the impedance keeps rising.

  • At point 'A', the effective impedance of the speakers is equal to the rated impedance of the speakers. When the line transitions from the flat area to the knee of the slope, the impedance presented to the amplifier starts to increase (the amplifier 'sees' a lighter load).
  • At point 'B', the effective impedance of the driver and series crossover component is 1.4 times the impedance of the driver. This is the 3dB down point of the 150hz crossover.
  • At point 'C', the effective impedance is twice the impedance of the driver.
  • At point 'D', the effective impedance is 2.8 times the impedance of the driver.

This is the impedance plot of the low frequency drivers. The description from the previous graph also applies here.


Losing Headroom in Trimode Operation

Loss of Headroom:
When using an amplifier without an electronic crossover, you lose some head room. In the following 2 diagrams, you can see two sine waves that have plenty of headroom. This is a 100hz sine wave.

And here you can see a 2000hz sine wave.

You can see (in the previous pictures) that neither of the sine waves are near the top or bottom of the window (black area). In the next picture you can see that the higher frequency is 'riding' on top of the lower frequency. When both of the frequencies are mixed together (below), you can see that they are very close to reaching the top and bottom of the window (we will consider the upper and lower borders of the window to be 'clipping'). You should also notice that the individual signals are exactly the same levels that they were before they were mixed. If the volume were only slightly increased (above the present volume setting), the amplifier would start to clip. When using an electronic crossover, the signal in third picture is separated back into the signals in the first and second pictures (if the crossover point was set approximately in the center of 100 and 2000hz).

More info on clipping:
Read the Too Little Power page.

Don't Panic!:
This does not mean that a tri-mode configuration can't sound good. It just means that you'll have less power available to the highs and lows when the audio has both high and low frequency content. If the audio is a pure low frequency sine wave, you'll have approximately the same headroom as with an electronic crossover. If there are multiple frequencies (as you have with normal music), the higher frequencies will be 'riding' on top of the lower frequency waves and your dynamic headroom will be reduced.


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