Too bridge or not too bridge....

[Tasmaniak]

What does “bridging an amp” mean?

“Bridging” refers to taking two channels of an amplifier and combining them to turn the amplifier into a one channel amplifier.


Why should I bridge my amp?

For increased power. If your amp can handle the load, it will put out more power through a bridged channel than it would into through a non-bridged channel. Theoretically, a “perfect” amplifier that puts out X watts into Y impedance into each of two channels will put out 4X watts into Y impedance into one bridged channel. Be aware that some amps more closely approximate that perfect amp than others, and some manufacturers build current limiters into their amps to allow them to remain stable into difficult loads at the expense of power gains.


Why shouldn't I bridge my amp?

There are several reasons: you might need those extra channels; your amp might not be stable into the load your speakers present if the amp is bridged; you might be a hyper-perfectionist that can't stand the thought of an small increase in distortion; or perhaps you just don't need more power. Car audio power is relatively cheap, and if you are not trying to make a mega-gonzo system, you may not need to double your power.


What happens when an amp is bridged?

Basically, one channel's signal is inverted, and then the two channels are combined to form one channel with twice the voltage of either of the original channels.

Ohm's Law for Alternating Current states that I = V/Z where I is current, V is voltage, and Z is impedance. We also know that P = IV, where P is power. If we use Ohm's Law and substitute into the power equation, we get P = V(V/Z), which can be rewritten as P = (V^2)/Z. Therefore, power is the square of voltage divided by impedance.

Now, why do we care about all that? Because it explains precisely what happens when an amp is bridged. I'll give a practical example and explain the theoretical basis of that example.

Imagine you have a two-channel amp that puts out 50 watts into each channel when driven into a load of 4 ohms per channel. Since we know P and Z, we can plug these numbers back into our power equation and find V. 50 = V^2/4 -> V = sqrt(200). So, we're seeing a voltage of 14.1 volts across each channel.

Now, imagine we bridge this amp, and use it to push just one of those 4 ohms loads. When the amp is bridged, the voltage is doubled. Since we know the voltage (2*14.1 volts), and the impedance (4 ohms), we can calculate power. Remember that P = V*V/Z. That means P = (28.2)^2/4, which is 198.1 watts. It should be clear by now that the new power is approximately 200 watts - quadruple the power of a single, unbridged channel!

You can probably see that should be the case, especially if you look back at the power equation. Since P = V*V/Z, if you double V, you quadruple power, since V is squared in the power equation.

Now, all this assumes the amp is stable into 4 ohms mono. The mono channel is putting out four times as much power as a single unbridged channel, so it must be putting out twice as much as the two single channels combined. Since the voltage on the supply side of the amp is dependent on the car's electrical system, it doesn't change (OK, the increased current might cause a voltage drop, but let's not worry about that now). Looking at the first power equation, at the supply side of the amp, we see P = IV. Now, then we bridged the amp, we doubled the power, but the input voltage stayed the same. So, if we hold V constant, the only way to double the power is to double the current.

That means the amp is now drawing twice as much current when it's running at a given impedance mono than it would be running two stereo channels at the same impedance. There are only two ways the amp can do that - it can simply draw more through it's circuits, and dissipate the extra heat, or it can utilize a current limiter, to prevent the
increase in current. Of course, using the current limiter means you don't get the power gains, either! So, if the amp can't handle the extra current, and it doesn't limit the current in some way, kiss it goodbye. For that reason, an amp is typically considered mono stable
into twice the impedance it is considered stereo stable.

Does bridging an amp halve the impedance of the speakers?

Impedance is a characteristic of the speakers. The speakers don't give a flip how the amp is configured: they have a given impedance curve, and that's that. It should be clear that when you bridge an amp, you are changing the amp. The speaker's impedance is not a function of the amp, but the amp's tolerance to a given impedance depends completely on the way the amp is configured. If you'll remember from section 4, an amp bridged into a given impedance draws twice as much current as it would if it were driving two separate channels, each at that impedance. So, a four ohm speaker stays a four ohm speaker, if it's hooked to one channel, a bridged channel, a toaster, or the wall socket. But, it is more stressful for the amp to drive any impedance bridged than unbridged.

So, why do people talk about the impedance halving? Well, it's a simple model that isn't correct but is easy to explain to people who don't know what's really going on. It goes like this: When you bridge the amp, each channel is “seeing” half the load presented to the amp. So, if you bridge an amp to 4 ohms, each channel “sees” 2 ohms.
Therefore, each channel puts out twice as much power, and the combined output is quadruple a single channel at 4 ohms.

Why is that still wrong? Because each channel isn't really used as a single channel. You've used part of one channel, and an inverted part of another channel to create a totally new channel, the bridged channel. Also, there's no way for a channel to “see” only part of a circuit. If it's “seeing” half the speaker, it's “seeing” it all.

Second, it makes it awkward if people believe that the impedance is really, literally, changing. If you use that model, is it safe to run a 4 ohm mono stable amp into a 4 ohm speaker? It should be, but we just said the impedance halves, so that's now a 2 ohm speaker, and you can't use it. That's wrong, and confusing, and it makes people think
they can't do things they really can.


Can I bridge my 4 channel head unit?

Generally, NO. Unless the manuals that came with your head unit specifically state that your head unit can be bridged, then do NOT attempt it - this could destroy the head unit's internal amplifier, and possibly void your warranty.

[dephilile]

Since we know the voltage (2*14.1 volts)

Why doesn’t running the speaker on double the voltage damage it?

Also, while I think of it. what does the voltage actually do? Is it what controls how far the speaker moves in and out or does it just control how fast it moves (i.e. frequency)?

[Tasmaniak]

The wire wrapped around a voice coil is actually quite durable. It's quite possible for the wire filament to cope with around 100-150Volts. Take a look st the size of the filament wire in lightbulb thats having 240V shoved up it's backside...now, albeit, it does glow through resistance, but that resistance is caused my the little tiny coil it's wrapped up in.

The voltage controls how far the cone is pushed forwards and backwards. As an audio signal is AC, how fast the cone moves is controlled by the frequency of that current.

[Phreddy]

Great post King!

Hey dephilile - voltage can be equated to "electrical pressure", pushing the current through the load. In other words, if the load impedance remains constant and the Voltage increases, so too does the current flowing in the load. The power disippated in the load is the product of the Voltage and Current (V x I), or as the King said the load impedance divided into the square of the Voltage. Running the speaker on "double the Voltage" is quite OK, as long as the speaker itself can handle the power that the extra Voltage and Current are going to disippate in it.

Cheerz!

[bikerpunker]

could someone please give a how to on bridging?

thanks heaps.

[Tasmaniak]

rather simple mate...hook the postive of the speaker to the positive terminal on one channel of the amp and then hook the negative of the speaker on the negative of the OTHER channel on the amp.

[bikerpunker]

cheers 12volt king..and i take it that its exactly the same for bridging to a sub?

[Tasmaniak]

Yup

I think I use the Winking smiley too much...oh well.

[dephilile]

Ok, just bear with me for a minute. I understand how the speaker coil is able to handle the higher voltage. But if the voltage is what pushes it forwards and backwards wouldn’t that then cause the speaker to move further than it was designed to causing physical damage to the rubber around the edge and the cone itself? I would have thought that when bridging the two outputs on an amp you would effectively be putting the two AC voltage sources in parallel, not series which would add the currents together, not the voltages.

[Tasmaniak]

Yes, it would cause the speaker to move then it's capable IF you exceed the capability of the speaker. It's called OVERPOWER. Simple. You don't feed a 1Kw into a speaker thats only capable of 500w.

Pull an amplifier apart and have a look at what actually happens when you bridge it. Look! theres a whole other section of circuitry designed to flip polarities over!