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Re: bridged-amp + series speaker vs se-amp + parallel speaker?

"Q" is variable denoting directionality of a given source." No, as much as I know, Q is the damping factor. In fact, there is 3 damping factors:
Qms the mechanical damping factor (mass of cone/voice coil against compiliance of the suspension, and loaded by the acoustic impedance of the air around)
Qes the electrical damping factor (as seen by the elctrical source, i.e the PA)
Qts the combined damping factor. Here the damping factor of the LP in their box.
Given identical LPs electrically in serial, and acoustically in parallel (in the same box ) and in ideal conditions (no energy loss in the box, perfectly adiabatic -no heat exchange with the exterior or box walls- and size small when compared to the wavelength of the sound), you should get no change in Qts .
Let me try to explain without maths.
In fact, each LP "sees" a source which impedance follows its own one, which means an higher impedance around the resonance,which means less current in this area, and so less energy, and so the resonance peak is weaker, which is another way to say that the damping factor is lowered.
BUT, at the same time, each LP of the pair is moved back and forth by the other LP. When the above conditions are met , when one LP tries to go forth, it pull the other LP back against its compliance and mass, and it creates a back EMF in its wiring, which is equivalent to lower its impedance. In perfect conditions, down to zero exactly at resonance.
(Apart from the resonance, the same phenomenon exists, but much harder to explain without maths, as it involves some phase delay between the source signal and the back EMF in each LP)
It means that each LP sees the source as if it were connected directly on it. Compared with the parallel connection, it sees the source voltage halved, that's all.
So, what about these "ideal conditions". Do you meet them? Topic by topic:
- a single box for both LPs
- same speakers. Bought together, so that manufacturing deviations are likely to be narrower than if built months apart. Both devirginalized (worked at strong SPL for at least 20 to 50 hours) : the LP compliance changes a few %, due to suspension stabilisation -glue, too rigid fibers which will have to break and will do-
- no energy loss. It means no turbulences. True as long as your box has no thin leaks like splits, has no too dense a damping material just behing the LPs, and has a wide enough vent if vented -no turbulence hiss heard near the vent when driven on high excursion: you can make this test with a 10-15Hz sine, yes, that low!, and hear near the vent-.
- box is adiabatic. Harder to get as the frequency lowers. Because there is more time to exchange heat. Under the damping material, it is maybe a good idea to use some thermal insulating layer with low thermal mass like polystyrene foam? Anyway, a raw calculation gives a few millijoules of heat exchange by air compressed by a 20Hz sine at 105dB SPL in a 0.5 m3 box. It should not be a problem.
- size small enough. For a box with its resonance at 30Hz, a distance of 50cm between LPs centers will phase lag EMF with 18°, which will give an impedance seen by one LP at resonance of about 1/10 the initial impedance. That's not zero, but it would be hard to hear any effect!
Note that at the crossover frequency (say 330Hz) the same distance of 50cm between LPs centers gives 180° phase lag. It just means that all the above explanation is fine for low frequencies, wrong for the upper ones. We don't care as we are far away from the resonance, so the Qts stays unchanged, and so the sound.
Incidently, it also means that it doesn't work for squackers (medium).
And what about your initial question, well, I would advice that, IF your PA is built to work bridged (I mean built, not externally tweaked, because a pro PA (well-) built for bridged operation has, among others, special protections against one PA "babbling" or "vomiting" in the other's "mouth" and uses special design tips for overall stability), you may use it bridged on the serial connected LPs.
It won't overheat, as the impedance is 16ohms, and each PA of the pair sees 8ohms, which won't overheat it.
---> Just never never use a bridged PA on parallel connected LPs, as each PA would see only 2ohms from 8ohms LPs. Even if marketing liars needs that 2 ohms be specified in PA datasheets, that's just bad engineering practice to use it. For it will heat, lowers the components reliability, and sounds bad : overroom is strongly reduced, ease to trigger thermal induced distorsion , longer times to recover from overload. It's just like running your car on a continuous basis at 6,000tpm because it's just the limit before the red area on the rpm dial.
OK, you got 1600W instead of 400W, that is 6dB. Use good efficiency LPs and save these 6dB for free! Furthermore, you got a good sounding PA that will last for years instead of a short lived saturatioon boom system. And, cherry above the cake,as we say in French, you save some tons of CO2 or some milligrams of U235.
An advantage of using serial connection on bridged PA is that the current is halved. It means 4 times less losses in the wire. You say your wire is 30ft long, I don't know what are ft (and don't want to). It will have the same losses than a parallel connected 7.5ft long wire (beauty of maths: I know it's true even without knowing the unit!)
The matching drawback (each advantage has a matching drawback!) is 4 times more capacitive losses, for the same reason. We just don't care of it for a bass box, forget it.
Good luck!


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