Mono-Output from 2-CH Power Amplifier for passive Subwoofer (no Dual Voice Coil)

A friend of me want to have a methody to get a passive low impedance mixer for a L+R summing signal, which should be suitable to feed a bass driver without dual voice coil. Moreover, no additional amplifier is to be used (at least temporarily).
Are there a solution like the topology under
https://brashleraudio.files.wordpress.com/2012/08/screenhunter_02-aug-27-19-35.gif?w=584&h=187
but instead the both 1K resistors two 5-10 mH inductors (maybe additional capacitors to avoid the impedance enhancing in direction to the high frequencies) ?
My friend purchase a bass horn similar like this under
http://www.abload.de/img/basshorn_detail_9x1686.jpg
but smaller (approximately 150cm height and 100cm width - and without the both subdivisions).
Inside is an passive 6-12db crossover-network, consisting of a very heavy inductor (arround 50kg) and more than twenty capacitors in parallel mode. Unfortunately the transducer isn't a dual voice coil version.
Thank you for your advives.
 
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An idea I posted somewhere in DIY in 2012.
 

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There are lots of problems with this approach. The main problem is this: the more wire you have between the amp and the subwoofer, the less power and control the amp has. At higher power levels, inductors get bulky and/or sloppy.

Your combiner is going to introduce crosstalk between the two channels. How bad it is depends on the amp driving your speakers. High damping factor is a good thing, but it will still mess with your soundstage. Phase cancellations and reflections between channels, impedance mischief and stuff happens. You can't get around this; your only recourse is to add more reactive parts. The whole thing turns into a "How good is good enough" optimization experiment.

Like Jon said, a dedicated subwoofer amp is the "professional" way to do it. In the end, an amplifier may be less expensive than all the time and parts you will put into a combiner and matching network. It just depends on what your time is worth.
 
tiefbassuebertr's friend set the criteria of "no additional amplifier is to be used".

JonSnell Electronic

Apparently it can be done. Is it perfect? The answer is probably not.
But will it work? The answer is it probably will. (IMHO)

jeff5may

Thank you for pointing out the shortcomings of a "no additional amplifier is to be used" solution. I agree with some of your comments.

tiefbassuebertr, please keep us posted if your friend will try this solution and how well (if at all) it works.
 
Another way to "cheat" and get what you want is to invert one channel of the power amp. This can be done with an isolation transformer on one channel of the amp output, or with any number of phase-inverting devices on the input. This would enable you to get a true summed-channel output to the single subwoofer without any crosstalk. Car amps refer to this "cheat" as tri-mode operation. It is a matter of endless controversy, but is commonly accepted without much question in the car audio realm. If your amp has the balls, it can send 4x the power to the subwoofer compared to a single-channel connection.
 
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