# what does bridged wattage mean?

Discussion in 'Amps and Cabs [BG]' started by nunk6, Sep 8, 2000.

1. ### nunk6

Jul 29, 2000
i saw on the carvin site that one amp is 350w 4 ohms but then 100w bridged 4 ohms

2. ### jazzbo

Aug 25, 2000
San Francisco, CA
I was just about to ask this question. I'm wondering the same thing myself!

3. ### nunk6

Jul 29, 2000
cool i got some back up yea really though i thought i was almost well educated in the ways of bass then this.....bridged wattage i wish i knew

[Edited by nunk6 on 09-08-2000 at 08:08 PM]

4. ### White_Knight

Mar 19, 2000
USA
I don't know about Carvin, but bridging usually has to do with running a stereo amplifier as a single unit (i.e. mono mode). Thus running the two power amps together as one. Most often you'll see something like 100 watts per side at 2 ohms stereo, 200 watts at 4 ohms bridged mono. Basically it means that you're running the two amps in a stereo power amp in mono mode to produce the exact same signal as each other.

5. ### MikeyD

Sep 9, 2000
From what I've seen, the bridging of a stereo amplifier allows its output voltage to be doubled. This has the potential of increasing the available power to the speakers by a factor of *four*. How? Power = V*V/Z = voltage squared divided by impedance (resistance actually in this case). So if one could double the voltage across the speaker terminals, the power could be quadrupled to the same load - IF the amplifiers (now bridged together) are capable of delivering the increased current. That's a BIG "if": you may, in many situations, not see a quadrupling of one-channel power when bridged, due to current limitations, etc. in the amplifier.

From a schematic I've viewed, it looks like the positive output side of one amp is connected to the negative side of the other amp, and one amp's input polarity is reversed. In effect, you have a signal that's twice the voltage that either amp can deliver - analogous to the difference between a 120-volt house circuit and a 240-volt circuit. 240 is made up of two 120-volt circuits that are 180 degrees out of phase (reverse polarity).

One last point is: be very careful when using an amp in bridged mode. Be sure you exactly follow the manufacturer's recommendations for speaker loading (impedance) and power handling. You may also note with some very high power amps (e.g., the 2000+ watt) that they get into "shock hazard" territory when bridged.

Mike

6. ### piercefreethinker

May 25, 2000
San Francisco, Ca
7. ### throbbinnut

This comes from a previous thread:

Here's some of the science and math behind bridging an amp:

A standard solid-state amp usually has a power supply that is +/- with respect to ground. For this example, we'll use plus and minus 40V, a total swing of 80V possible. During normal operation, one leg of the speaker is connected to ground (common), and the other is connected to the output transistors. The output transistors put out a voltage that is between the "top" voltage (+40 V) and the "bottom voltage" (-40V), depending on the instantaneous value of the input signal, they pull the output voltage up or down. So the most voltage the speaker can see across it is magnitude 40 volts, either positive or negative.

When you bridge a stereo (2-channel) amp, you hook one side of the speaker voice-coil to the transistor output of one channel, and the other side of the speaker voice coil to the transistor output of the other channel. Special circuitry then drives one channel 180 degrees out of phase with the other, so when one output is going up towards +40V, the other output is going down towards -40V. This allows all 80 volts of the power supply to be used as the voltage swing for the speaker.

But the output transistors have a current limit, and this current limit still holds true even in bridging. So even though the voltage has doubled, the current remains the same (Wasn't that a Led Zeppelin movie?) So Power, which is voltage times current, is doubled, since the voltage across the speaker is doubled. But to have the same current at 2x the voltage, the impedance has to double as well.

On the amp above,

40volts peak is equal to 28.28 volts RMS, so If the amp is rated for 4 Ohms per side, then
Vsquared / R = power >>> 800/4=200Watts and Current=7.07Amps RMS

So now doubling the voltage to the speaker, 28.28 * 2 = 56.56 V RMS, times the same 7.07 amps, gives 400 Watts, as expected. V=I*R, so if V=56.56, and I=7.07, then R has to equal 8 Ohms. If you have a single 4 Ohm speaker bridged from ch1 positive to ch2 positive, the amp may fry, since it will try to put out too much current. Over-current is one of the main things that kills transistors.

It makes sense, since you are electrically "stacking" the two channels of the amp, then you need to elecrically "stack" the 2 speakers they each want to see. 4 ohms + 4 ohms = 8 ohms. 200 Watts + 200 Watts = 400 Watts.

This is just a general explanation, specific cases are slightly different, depending on the amp in question.

Chris