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Bridged Mono OK, everyone talks about it but almost nobody knows what it really means. So here ya go... Most professional audio power amps have two channels: channels 1 and 2. There is a line level input for channel 1 and another for channel 2. There is a speaker level output for channel 1 and another for channel 2. So far so good. Everyone knows this. No problems. Stereo mode: This is where channels 1 and 2 have different inputs from different cables, and outputs 1 and 2 feed separate speakers with separate cables: left main and right main, or mains and monitors, etc. This is the most common configuration.
Parallel mono mode: In most amps, this is where inputs 1 and 2 are tied together, or sometimes input 2 is not used at all - only input 1. Regardless, one signal is fed (usually from input 1), to BOTH internal amp channels. This means that output 1 and output 2 are always putting out the same thing (depending of course on their separate volume controls).
One notable exception to this is Crown Macrotech amps. When Crown says "parallel mono" they are talking about something completely different and much more complicated. It's a very cool way to drive extremely low impedance loads. I'll just stop right there. If you don't work with Macrotech amps, then just move on. If you're interested, then go to the Crown website and check out one of the owners manuals for the Macrotech series. Bridge mono mode: OK, first let's quickly discuss the basics... The output of all amps varies depending on the input. Ideally, the output is just a bigger more powerful exact copy of the input signal. So what do we mean by bigger and more powerful? Voltage. Let me first say that you don't need to understand voltage to understand bridged amps. Just realize that the voltage is really just the speaker level output of the amp. Most line level signals have a voltage below 5 volts, and even that's pretty freakin' hot for a line level signal. Voltage from the output of a power amp can reach up to 100 volts sometimes (or more). The CRITICAL thing to remember here is that voltage is measured between two points. One of those points, 99 times out of 100 is GROUND. The other is what we usually call the HOT signal. You see, GROUND never changes. It's a wire that eventually goes into the dirt (i.e. ground). It's always at the same electrical level and it never changes. That's what makes it such a good reference point. Remember what was just said - voltage is ALWAYS measured between two points. The output stage on all audio power amps consists of two wires (per channel). Sometimes they use a banana connector, sometimes a Speakon connector, sometimes a 1/4" connector, and sometimes it's just hard-wired with screw terminals. But it's always just two wires. One of these wires is ground. The other is the HOT signal - until we bridge the amp. Then we're using both HOTs without the GROUNDs. Don't panic - keep reading. Hopefully you're familiar with the concept of phase. If not, here's a quick lesson. When you have two audio signals that are the same, they are usually either "in phase" with each other, or they are "out of phase" with each other. "In phase" means that when signal 1 goes up, signal 2 goes up. When signal 1 goes back down, signal 2 goes back down. They always stay together because they are "in phase" with each other. "Out of phase" is like this: When signal 1 goes up, signal 2 goes down. When signal 1 goes back down, signal 2 goes up. They are always mirror images of each other - always doing the opposite - "out of phase" with each other. The only time they are the same is when they are both zero. Here's the really important stuff: Remember how the output of amp channel 1 is just a bigger copy of the input of channel 1 ? Therefore, the output of channel 1 is "in phase" with the input of channel 1. When the input signal goes up, the output signal goes up, etc. Granted the output is a much bigger copy than the input, but it's still "in phase". Likewise, the output of amp channel 2 is "in phase" with the input of channel 2. Remember about voltage being measured between two points ? Well, GROUND isn't moving, but the HOT signal IS moving right along with the audio. We're about to tie all of this together... When you bridge an amp, it is no longer a two channel amp. You are using both channels together to give you double the power of a single channel. Thus it is ONE double-power amp channel instead of two regular-power amp channels. The trick is this: When bridged, there is only one input (on channel 1). Inside the amp, the input signal on channel 1 is split into two separate copies. One copy moves happily on its way, unchanged, to amp channel 1. The other copy is flipped "out of phase" from the first, and sent happily on its way to amp channel 2. If channel 1 is the positive signal, we'll call the "out of phase" copy on channel 2, the negative signal. Now since channel 1 is getting the positive signal, the output of channel 1 is a much bigger copy of the positive signal. Since amp channel 2 is receiving a negative (out of phase) signal, the output of channel 2 is also a much bigger negative (out of phase) copy of the input signal. Check out the diagram, then read on.
By the way, the GROUND on channel 1 is the same as the GROUND on channel 2. They're both just GROUND. When you bridge an amp, you take the output across the two HOT (usually red) leads, and you leave the two GROUND connectors unused. So now, we've taken one signal and amplified it positively on channel 1, and we've taken the same signal, flipped it "out of phase", and amplified it negatively on channel 2. Let's say for arguments sake that the HOT output of channel 1 is at 50 volts (measured to ground). Let's say that the HOT "out of phase" version in channel 2 is at -50 volts (measured to ground). Now you measure the voltage between the two HOT signals (one positive and one negative). Since one of them is "out of phase", the difference between them is 100 volts (between +50v and -50v). If the audio signal goes up, amp channel 1 goes up, and the phase-inverted channel 2 goes down the same amount. If the change is 6 volts, then channel 1 goes up 6 volts, and channel 2 goes down 6 volts. Now we have +56 volts, and -56 volts, and the voltage between the two points is 112 volts. That's how it works. Double the power in bridged mode. Power is the product of voltage and current. To simplify this explanation, we didn't talk about current. When you connect across the two HOT leads on the two amp channels, you're basically building a "bridge" between the two channels. That's why they call it "bridged mode". It's called "bridged mono" because you only have one output channel instead of two separates. What sits on that bridge is the speaker you're driving. Hope this helps. This explanation by Brian C. - chief engineer at Media Write Group. |
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