Amplifier bridging adapter - a tutorial, of sorts

whoaru99

Epic Member
A number of times I've read about fellow members wanting to bridge power amps but neither their pre amp nor the power amps had built-in bridging capability.

In days past some companies sold bridging adapters for this very purpose. However, bridging adapters, or at least adapters advertised as such at retail, seem to have effectively gone the way of the Dodo bird. Or have they? :)

There are caveats to bridging like the amplifier needs to be of common ground design to bridge. Good news is that many, dare I say most, amps are common ground. However, it's your responsibility to be sure your amp is suitable for bridging, both from perspective of common ground and load impedance. I'm not going to go through all the in and out details here as threads exist on AK and elsewhere about that sort of thing.

The purpose of this thread is NOT to debate the pros and cons of bridging.

The assumption is you've done your research and wish to proceed with bridging.

There are a couple special things you'll need (beyond the obvious amp(s) and pre amp / pre outs):
  • An unbalanced to balanced converter
  • XLR to dual RCA cable(s)
Below you'll see the gear I'm using for this example. Top to bottom:
  • Henry Engineering Matchbox unbalanced<>balanced converter
  • Crown XLS1000 amp
  • Yamaha C-40 pre amp
Note the Crown amp has built-in bridging capability but I've left the amp in 2-ch mode to show the external bridging setup. Also, to keep the cabling more clear, I'll be connecting only this one amplifier.

Gear1.jpg

Also key to the process is the XLR to dual RCA cable(s). You can buy these or you can make them. I made mine with the following pin outs:
  • Pin 2 of the XLR to one of the RCA plug center pins (red in this case)
  • Pin 3 of the XLR to the other RCA plug center pin (white in this case)
  • Pin 1 of the XLR connects to the shield wire/braid going to the shell contact of both RCAs
Gear4.jpg
Regardless if you buy or build it is important to know the pin out of the cable. You have to keep track of which RCA connector goes to which XLR pin as this factors into how the speaker will be connected to the amplifier. Standard XLR wiring convention is Pin 2 normal or "hot", Pin 3 inverted or "cold". Confirm the pin out on your converter. Thus, in the case of my adapter, the red RCA carries normal signal and the white RCA carries inverted signal.

The connection we'll start with is from the Right pre out to the Right unbalanced input of the converter using a plain old RCA-style cable. If your converter uses a different naming convention for inputs and outputs just keep the connections in relative order.
Gear2.jpg

Next connection is from the converter Right balanced output to the amplifier inputs. In this case, the amp inputs/outputs are CH1 and CH2 rather than Left and Right. Remember which RCA of our adapter cable is carrying normal and which RCA is carrying inverted?

I've connected the red RCA to CH1 and the white RCA to CH2. This means (in this example) CH1 of the amp is handling the normal signal and CH2 is handling the inverted signal.
Gear5.jpg

Lastly, connect the speaker. Remember before we said the normal signal is connected to CH1 and the inverted signal is connected to CH2. We need to keep this straight when connecting the speaker.

Connect the speaker (+) to CH1 (+) of the amp. Connect the speaker (-) to CH2 (+) of the amp. Since CH2 is carrying an inverted signal, CH2 (+) is actually the (-) output of the amp in bridged mode. With this cable I'm using, white for (+) and black for (-), you see the wires connected accordingly to CH1 (+) and CH2 (+).
Gear6.jpg

The principle behind bridged mode is that the normal channel swings a relative signal value of 1 and the inverted channel swings a relative signal value -1. The difference between 1 and -1 is 2, which represents the doubling (2x) of the voltage swing, hence the increased power output of bridged mode.
Bridged.png


That's it, easy peasy. Repeat for the other amp following the same connection methodology.

One should note the unbalanced/balanced converters often boost the signal level too. This can cause the sound to get loud with just a small rotation of the volume control. Most of the converter boxes have level controls on them so you can cut back there to regain good usable range of the volume control.

Hope you enjoy.
 
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well that was a long explanation of useing a power amp in mono all that's needed is two signal 180 degrees out of phase a transfomer is the easiest one that has two secondary one in one out the transfomer has to be able to Handel a line level and have good frequency response

LOL. It's always easy if you know how. But, the intended audience isn't people that already know how. :)

And, yes, there are a number of ways to skin the cat. This is an effective way to use commonly-available and inexpensive hardware. Many different mfgs. make unbalanced/balanced converters.

A pair of good transformers like Jensen will cost more to do it but it certainly could be done that way too.

As well, a pre amp having balanced outputs will accomplish the same thing as the converter box to supply the normal and inverted signals. You'd just need the XLR to dual RCA cables in that case.

FWIW, the signals are not 180-deg out of phase. It's just a matter of polarity in this case.
 
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No debate, just accuracy. Nothing wrong with that. I applaud whoaru99's fortitude and stick-with-it-ness in this matter.
 
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