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Speakers in Parallel with Bridged center

 
Old 09-24-2017, 06:50 PM
  #1  
celiawessen
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Question Speakers in Parallel with Bridged center

This is a question for the audio pros and electric engineers.

I have a Cayenne with Bose speakers I'm rewiring to a JL XD600/6. It's been a while since high school and I am really iffy on the Ohm calculations.



In school I learned that when speakers are connected in parallel, I add the reciprocals of each speaker and the reciprocal of the sum would be the total resistance.

Three 2 Ohm speakers
Code:
1/2 + 1/2 + 1/2 = 3/2 --> 0.66 Ohm
Is this correct? If one looks at the Front Left or Front Right speaker bundles alone, that would be the case. That means the amp is seeing a 0.66 Ohm load on each channel... not good for a 2 Ohm stable amp like the XD600.

But then, there is the Center speaker that is bridged over both channels. I don't remember learning about super-positioned resistance in school and how it affects current draw on an amp.

So now, how many Ohms is the amp really seeing? And how badly is it going to throw off the Amp draw? Would it be underpowered (low sound) or overpowered (overheat)?
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Old 09-24-2017, 09:57 PM
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v10rick
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I am not a car audio pro but I have installed many commercial sound systems in a previous life.

The factory layout utilizes passive crossovers which is a poor mans (i.e. cheap) way to drive these speakers.

The xd600/6v2 provides 3 separate and adjustable outputs for each speaker, per channel. The impedance will not be an issue with this configuration but a separate line will be required for each speaker.

If I were investing this amount of money and time I would do it right. Re wire the speakers to accommodate xd600/6v2. Trash the factory speakers and utilize the active/adjustable crossover built into this amp.

If you decide to bridge the amp with a center speaker it looks like this amp is limited to 4 ohms. So that speaker will need to be changed or you could add a resistor in series to make the amp happy.

To answer your question the impedance of the factory arrangement looks to be .5 ohm...too low for the xd600.

Last edited by v10rick; 09-24-2017 at 10:22 PM.
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Old 09-25-2017, 03:03 PM
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I don't know many details yet, but the Bose audio system doesn't work like all/most other audio systems. Research the Bose system here... I seem to remember that you need some sort of "adapter" that allows you to use non-Bose speakers and amps with the Bose system but I don't remember any details. Also, your connection "map" with the center channel bridged around the other channels probably (operative word here, probably) should NOT be used with the Bose system. Your formula for impedance is correct. But your math is wrong. 3/2=1.66 not 0.66. Speakers and crossovers create complex loads and present a different load to the amplifier at different frequencies. The impedance formula is just a general reference, but you COULD end up with extremely low (or high) impedance at some frequencies that can be difficult for some amplifiers to deal with. I would suggest you search previous posts for people who have replaced individual drivers and/or revamped everything to get a better sense of the specific needs of the Bose system. I wish I had more details, but I decided to just leave the Bose system alone since the inside of a car is among the worst environments possible for listening to music. So I didn't dig into any depth... and what I read may not even apply to what you are doing... but better safe than sorry. BTW, you need a complex formula to calculate the impedance of the center channel as shown in the diagram you attached. The formula for the center channel would be impedance of left channel (all 3 speakers) divided into 1 PLUS the same formula for the right channel. You then take THAT result (call it Y for this example) and solve the formula 1/Y+1/center=1/total impedance of network. But you should check how the Bose system REALLY creates the center channel... is it JUST wiring or does the head unit synthesize the center channel BY ITSELF and send that signal to 1 amplifier channel so you would NOT bridge-wire the center channel in your diagram example. Hope this all makes sense.
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Old 09-27-2017, 12:28 AM
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Thank you very much for the replies.

To clarify, I am installing an Aftermarket Head Unit (Pioneer AVIC series) and bypassing (removing) the factory Bose amp. So I have 4 pre-amp channels going into a JL XD600 feeding 10 speakers - and a mono subwoofer from the HU to the factory Bose S/W. This is my third JL amp on a 9PA but with different speaker config.

As suggested, I will be using a 50W2RJ resistor for the front two channel and satellite speakers to maintain >2 Ohm/Ch.

I've updated the diagram:


As for the Passive crossovers, I think that only happens in the rear door speaker clusters? It does make the music sound really cheap in the back seat.

Also, does anyone know the maximum input ratings on the little Bose Subwoofer amp? It says "100W" on it, but there's no indication as to how much gain I can push to it. At zero gain from an Pioneer AVIC receiver, it goes into protection mode after 10 min and starts whining. I currently have it set to -25 (barely makes a sound).
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Old 09-27-2017, 06:03 PM
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Those resistors will be turning much of the amps output into heat. Assuming the front speakers will not be driven hard this may not be an issue.

BTW your impedance calculation is correct.

Two speakers in parallel will be a 1 ohm load on the amp. Adding a 3rd speaker in parallel lowers the impedance even more.

https://www.allaboutcircuits.com/too...ce-calculator/

The .66 ohm speaker load in series with the 2 ohm resistor results in 4 watts turned into heat for every 1.32 watts going to the speaker load.

You could eliminate the resistor for the 3 speakers by rewiring these in series, providing a 6 ohm load to the amp.

Using a resistor in series with the center speaker will result in a 50% power loss to that speaker. Or change that speaker to 4 ohm.

Last edited by v10rick; 09-28-2017 at 09:13 AM.
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Old 09-29-2017, 09:43 PM
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v10rick, Thanks again for the info! I didn't realize the power:heat ratio was that high.

I don't think I want to rewire the Lo/Mid/Hi in series b/c of the passive crossovers. The frequencies would be filtered out as the signal passes through each speaker before it gets to the next. So I'll keep it parallel with the power loss with the resistors for now.

I also realized that when a Pioneer AVIC head unit says the Level for the subwoofer is +0 (default), it really means it's at 50%. It actually goes down to -25 (0%). I now have it at -16 (18%) and it doesn't overload the Bose SWAMP to sleep.
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