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Series or parallel connection of bass drivers?

ozziozzi

ozziozzi

New Member
I am planning OB speaker with two bass drivers in vertically stacked U frames with fullrange driver above on flat baffle and I have to decide if I should connect bass drivers in series or parallel. I have previously read on various forums that resistance in series with bass drivers may effect their bass extension and/or SPL and can produce results similar to a High Qts driver using a Low Qts driver. Does the resistance of one driver in series with another affect the behaviour of both?

Can someone please correct me if this idea is false and also advise if there is a difference in bass extension or SPL from two woffers connected either in series or parallel? (Asuming that passive XO is adjusted for either 4 or 16ohms nominal impedance.) What is the difference if the drivers are driven by SSamp (high damping factor) either in series or parallel connection. ((I realise more power output (6dB?) if amp can run with 4ohm parallel load.)) I realise I can experiment, but passive connection method requires purchase of large and expensive inductor and caps. Just being frugal and trying to avoid unecessary purchases.

Any comments or advice appreciated.
Ozziozzi
 
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HI Glen, not isobaric, just a U-frame open baffle with one driver above the other and both driven in phase. The main variable is whether to connect the two bsss drivers either in series or parallel. In series resistance/ impedance of pair is around 16ohm, in parallel 4ohm. This obviously affects the passive XO components required. In discussion at another forum it was agreed that putting drivers in series might reduce the damping because of increased resistance / impedance of the combination. It is like putting a resistor in series with one driver to make it "appear" to the system that the driver has a higher Qts. I believe that each driver in series would appear to have a higher Qts than each drivers in parallel as one acts as a resistance in series with the other one. On top of that is that two drivers in parallel would each appear as a shunt resistor across the other., increasing the damping of the pair and hence lowering the apparent Q of the system. The downside is that 16ohm impedance means higher value (read expensive) inductor in series for low pass filter in XO.

I COULD bi-amp with active XO, but would prefer passive XO so I can just use one amp and reduce complexity of system. Hope this further explains my questions in first post. Thanks for your interest. It looks as if this concept is in the "too hard" basket. I am just trying to be cheap and avoid expense of hi value coils.
Ozziozzi
 
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ozziozzi said:
HI Glen, not isobaric, just a U-frame open baffle with one driver above the other and both driven in phase. The main variable is whether to connect the two bsss drivers either in series or parallel. In series resistance/ impedance of pair is around 16ohm, in parallel 4ohm. This obviously affects the passive XO components required. In discussion at another forum it was agreed that putting drivers in series might reduce the damping because of increased resistance / impedance of the combination. It is like putting a resistor in series with one driver to make it "appear" to the system that the driver has a higher Qts. I believe that each driver in series would appear to have a higher Qts than each drivers in parallel as one acts as a resistance in series with the other one. On top of that is that two drivers in parallel would each appear as a shunt resistor across the other., increasing the damping of the pair and hence lowering the apparent Q of the system. The downside is that 16ohm impedance means higher value (read expensive) inductor in series for low pass filter in XO.

I COULD bi-amp with active XO, but would prefer passive XO so I can just use one amp and reduce complexity of system. Hope this further explains my questions in first post. Thanks for your interest. It looks as if this concept is in the "too hard" basket. I am just trying to be cheap and avoid expense of hi value coils.
Ozziozzi
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I would think the damping(and mass) would increase if the drivers were wired in series. Afaik, you Qts would not change if wiring in series since the first speaker doesnt see the second as a fixed resistor but rather its in flux...something like that. So both drivers are one in the circuit.

The main consideration id take seriously is if your amp can handle running a 4 ohm load since cost is your issue. I dont know if running ob woofers in the free air increases or decreases minimum impedence. Thats a question for a smarter man than I.
 
Two drivers reproducing the same material will cause cancellation and or combing. Sound will suffer. You want to wire the bass drivers in a 2.5 way config. One bass driver plays all the bass material - and the second, the helper woofer kicks in at around 100Hz. To do that, you need a choke tuned to that frequency. The size of the choke is impedance and frequency dependent. If you want the formula for your unique situation, PM me. Your bass helper woofer will give you 3dB more efficiency.
 
turnitdown said:
Two drivers reproducing the same material will cause cancellation
Click to expand...
Not if they are in phase and only a fraction of a wavelength apart in proximity.

turnitdown said:
and or combing.
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Again, wavelength is everything, and it's doubtful that the OP is going to locate his woofers far enough apart to even begin to see any combing effects.

Kept close enough together on a common baffle, wired in phase woofers "couple" and will benefit by about 3db of gain in sensitivity across a reasonable BW.
 
I connect my 8 ohm OB woofers in parallel so my bass amp sees a 4 ohm load which extracts it's full power.

Going passive results in huge problems to overcome. You should definitely bi-amp with an active crossover.
 
I seem to have trouble describing what I have in mind so the attached picture is of a similar layout. Just two bass drivers in shallow U-frames to increase the baffle width without getting too wide from the front. Each U-frame approx 13" square and 9" deep. Either a fullrange driver above or fullrange plus super tweeter. So two bass drivers 31Hz - 150Hz, fullrange / widerange 150-10,000, supertweeter 10,000-->.

Gadzatronik
Your opinion coincides with about 95% of the articles I have come across. My amp can handle 4ohm loads. Impedance of drivers in mH is pretty low so probably has little effect. Yes Qts of each driver will not change, BUT (I have it on good authority from a much wiser person than myself) that the SYSTEM of two drivers acts as one driver that APPEARS to have a higher Qts. As high Qts is supposed to be desirable for OB I am hoping two drivers in series would be better than two in parrallel.

Turnitdown
You are correct about cancellation and combing especially with line array speakers, but AFAIK this is not a problem or detectable by ear at 150Hz and below. What IS the advantage of a 2.5 system rather than as I suggest?) Each bass driver can run 31-~3kHz with a smooth rolloff. WIderange/fullrange I will be using run 50 - to 12k or above. Crossing at 150 is mainly designed to relieve the widerange/fullrange of large excursion and improve power handling and so reduce distortion from FR. All drivers are 8 ohm. Because of well behaved drivers I was thinking first-order 6dB/oct XO around 150Hz, perhaps Lpads on FR and tweeter if necessary. If there is an obvious peak caused by overlapping freq range of bass and FR I can handle this by asymmetrical xo. Eg Lopass 150 hipass 300. For a smooth transition bass to midrange.

Thanks for both your ideas
Ozziozzi
 

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While writing my long-winded reply to Gadz and Turnitdown I overlapped Bowtie and Poultry's replies. No arguement from me to either of your suggestions. I was just trying to make system as simple as possible and stop it looking like a small power station ;-) I can biamp and I have a 2-way active XO and a few amps that can handle 4ohm load. I will go active if passive becomes too hard.
 
ozziozzi said:
While writing my long-winded reply to Gadz and Turnitdown I overlapped Bowtie and Poultry's replies. No arguement from me to either of your suggestions. I was just trying to make system as simple as possible and stop it looking like a small power station ;-) I can biamp and I have a 2-way active XO and a few amps that can handle 4ohm load. I will go active if passive becomes too hard.
Click to expand...
If you wanted to do baffle frequency compensation, all you would really need to do is have the bottom woofer roll off lower than the top one does. Below a certain frequency it doesnt matter as much. But since it appears that your woofs are both crossed over under 250hz I wouldnt bother with trying a to do 2.5 way.
 
I've compared series and parallel woofer connections a couple times before. I've always opted for parallel. I even opted for this method when building a system with four woofers. Had to find an amp that was stable with 2ohm loads, but it was worth it.
 
Base efficiency (1 driver, 1W, 1m) = Base sensitivity (1 driver, 2.83V, 1m) + 10*log(nominal impedance / 8 ohms)

The effective efficiency and voltage sensitivity gains from running multiple drivers can be calculated as:

Efficiency gain = 10*log(Number Drivers)

Sensitivity gain = 10*log(8 ohms / nominal impedance of series-parallel combo)

Net efficiency of system = base efficiency + efficiency gain

Net sensitivity of system = base efficiency + efficiency gain + sensitivity gain

I stole all of that from another site when I was fiddling with some line arrays recently.

Not sure what the SPL rating of your driver is, but the two gains can be easily calculated. Efficiency of two drivers gives you +3 db no matter whether they are in series or parallel. Sensitivity gives you +3db if wired in parallel at 4 ohms, and -3db when wired in series at 16 ohms. Parallel gives you +6db total (at the expense of the load on the amp) and series gives a zero net change. I hope I did that right.
 
Here's the thread where I got that, and below this post is an attached spreadsheet which will give you those results. It's a little Excel file but apparently I can't attach it here.

http://www.htguide.com/forum/showthread.php?27677-Calculating-line-array-sensitivity

The calculation is relatively simple for a couple of drivers, but when you're considering series/parallel combinations of 16 or 24 drivers or whatever, it kinda helps to have a calculator. :p
 
I am not interested in line arrays at the moment, but will file away your calculator. The most drivers I have used is 4 in series/parallel as a short line array. Good power handling, especially with small drivers.

Amp I will use for bass duties if I go active is AMC 2445. This is 4 channel switch bridgeable for 2, 3 or 4 channels. Built for solid 2channels of 90W RMS/ch. (all day?). Probably Luxman L230 for upper frequencies. I firmly believe in the KISS principle, so prefer passive if possible with firstorder XO. asymetrical if too much overlap between bass and mid/treble. As many people before me have done, experiment with activeXO then convert to passive equivalent. Best of both worlds. I am certainly not interested in Seigfried Linkwitz speaker complexity and leaning more to Martin J King's approach to OB.
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