# Mono bridging a (non)-bridgeable stereo amp via XLR cables



## Coilvoice (Aug 18, 2011)

Cv - I recently spoke with a friend about the potential of mono bridging a (non)-bridgable stereo amplifier via altered XLR cables from a fully balanced preamp and it sounds interesting /very useful to say the least. I have never heard of this before, so it is all new to me. I thought it would be interesting to get other people's opinions on this and while doing so, spread the wealth of this information. Out of respect for my friend who mentioned this theory, I will leave his name out of it, so the choice will be entirely up to him if he would like to get involved in this thread. I will label his comments Mr.Z to keep his identity private, not because he has asked me to or anything of the sort, but rather to respect his privacy. The following is what he wrote about this theory.

Mr. Z - "Ok, so what you're going to be doing is driving the right input of your amp with the non-inverted output of you preamp (one channel) and the left input of your amp with the inverted output. and you do this with both amplifiers. so the principal behind bridging an amp is to drive one side positive and one side negative. Luckily, this is the same principle behind balanced lines, we're just using them in a different way then they're intended. The positive speaker terminal on your amp on the right channel becomes positive and the positive terminal on the left channel becomes the negative for the speaker. With all bridged amps the ground speaker terminal is not used.

In principal, because this doubles your voltage output of your amp, theoretically your wattage output should increase by a factor of 4. but in the real world it usually only increases it by a factor of 2 to 3 because of power loss and other various factors. Remember power equals voltage times amperage. I use canare quadstar cable also to do this but you'll need your dmm to get it right because there's not enough color's in the cable to identify all the wires properly. Can you think of how this is accomplished with quad star cable and a XLR connector with 2 RCA connectors? I can outline it but it will take a bit of explaining. Just procedure really, it's not complicated.

I have used it many times especially to drive a big Adcom 555 amp for subwoofer duty. Just make sure that the preamp has a real balanced output. A good example is my preamp now has balanced outputs but they're just for show. When you look inside, the inverted terminal is just shorted to ground with a 50 k resistor and I have a really nice preamp, so you gotta know your equipment well to do it. Sometimes, it is truly balanced and sometimes it's just for show. What one pays for a preamp is not a good indicator. The last setup I did this with was with a Marchand balanced crossover and Adcom poweramps and it was awesome.

You should realize too that because your spreading the speaker load over two hot terminals each side of the amp will see only half the impedance. The impedance is spread across both halves so for an 8 ohm speaker, each hot terminal on your amp would only see 4 ohms, so you also need to be careful that the amps can handle it. If an amp can't do a 4 ohm load stably, you shouldn't bridge it and actually if your speaker is a 4 ohm speaker and you wish to use a bridged amp each side of your amp needs to be completely comfortable with a 2 ohm load. Purely on paper, you are effectively doubling current seen by each half and voltage doubling but like I said before you don't get a true quadrupling of power but usually 2 to 3 times.

I'm sorry if some of my explanations are a bit deep, I have been an electronics instructor since 1995 (gave it up a few years ago) as well as an avid DIY audiophile for most my life and sometimes I forget what is common to know and what is not. I've tried my best to speak in the plainest terms I know how to. One last caveat about bridging amps, the new class D amps and so forth cannot be bridged. They have some funky grounding technique that I'm not really up on, how you tell is in the manual. If it says that the outputs of an amp cannot be used for headphone output, you know the kind that plug into a converter from the terminals, then you know bridging is out of the question."

Cv- Ok, that's what Mr.Z wrote about this theory and obviously puts into practice. I would love to be able to do this with my amps which are Aragon 8008bb models and the preamp I would be using is the Aragon Aurum which Mr. Z and I have confirmed is a truly balanced design, not simply for show. Your thoughts and comments are appreciated. It is an amazing concept and if this works out as intended for my rig, it will open up a whole new world of possibilities for me. 

One more thing... I have a Behringer dcx2496 crossover that I may or may not incorperate into this multiamp 2 channel configuration I am doing at the moment and my question to all is... Instead of the modified XLR cables coming from the Aragon Aurum preamp, could I use them coming off the Behringer dcx2496 in this manner to my amps? 

In other words... Connecting the Aragon Aurum's main balanced outputs to the Behringer's main balanced inputs via standard Mogami XLR cables. Then coming out of the Behringer's main balanced outputs via MODIFIED XLR CABLES (as mentioned previously) to the Aragon 8008bb amps to mono bridge them.

I guess I'm asking... Does anyone know if the Behringer dcx2496 is a true balanced design that can be hooked up in this manner? This may work out even better for me if so. 

Thanks for your time and patience on the matter friends! :T


----------



## Kal Rubinson (Aug 3, 2006)

I do not know why you would refer to this as a "non-bridgeable" amp.  If the channels share a common ground and can handle the lower impedance load, it is bridgeable. Does the Aragon amp fit the criteria?


----------

