Crossover Polarity Question

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Chuck Simon

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Jan 19, 2011
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I will preface my question by saying that I am no expert on passive crossovers, a fact that will be obvious to some of you after reading my question!

Just because I had them laying around, I thought I would put a couple of JBL 2206H's into a couple of JBL MPro 412 cabs. As expected, there was a nice improvement in the low end of the cab, especially at higher volume, but what puzzeled me is that switching the polarity of the woofer had no effect on the sound. Whether I wired the + to the red or black terminal of the JBL, I could hear no difference! Should I have heard a difference? How should switching the polarity of the woofer affect the performance of a passive crossover?
 
Re: Crossover Polarity Question

You were listening to the wrong part of the spectrum.

It's all about what happens at the *acoustic crossover*, which controls your on-axis AND off-axis response for that octave (approximately) of sound. Note that the electrical crossover point is likely to be a number that is different from the acoustic crossover point.

Have fun, good listening.

Tim Mc
 
Re: Crossover Polarity Question

I was trying to listen to the whole spectrum. What is it you think I should have been hearing?
 
Re: Crossover Polarity Question

I think that what Tim is saying is that inverting the polarity of the woofer won't affect the electronic crossover as you are inverting the polarity after the crossover, however it will affect the acoustic crossover and how the LF and HF drivers interact acoustically. Those effects would occur where both the woofer and tweeter are involved in reproducing the signal and may vary based on the relative location to the drivers. So try some frequency sweeps and listening both on axis and vertically and horizontally off axis.
 
Re: Crossover Polarity Question

One way it should be noticeably better than the other as you walk around. Standing in one place it might not be noticeable, but there could be pretty severe comb filtering in the 'wrong' polarity. Especially try walking the vertical coverage- or side to side if the cabinet is in monitor position.
 
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Re: Crossover Polarity Question

Chuck Simon said:
I will preface my question by saying that I am no expert on passive crossovers, a fact that will be obvious to some of you after reading my question!

Just because I had them laying around, I thought I would put a couple of JBL 2206H's into a couple of JBL MPro 412 cabs. As expected, there was a nice improvement in the low end of the cab, especially at higher volume, but what puzzeled me is that switching the polarity of the woofer had no effect on the sound. Whether I wired the + to the red or black terminal of the JBL, I could hear no difference! Should I have heard a difference? How should switching the polarity of the woofer affect the performance of a passive crossover?
Click to expand...
Proper phase using a passive crossover depends on the interaction of the voice coil, with all the capacitors, resistors and coils in the circuit.
Putting in a different speaker with different T.S. parameters as you did will affect the acoustic crossover frequency. The phase difference between the LF and HF at crossover may have shifted enough that now there is no correct polarity choice.
In other words, the phase response will be wrong using either polarity. Each polarity will be slightly different, but neither will cause a complete null at the crossover point as it would with a proper crossover alignment when polarity is reversed.

Even though the components in a passive crossover may suggest a second order (12 db per octave) slope, the actual acoustic crossover may be closer to 18 or 24 dB per octave, in which case the phase difference may occur over such a narrow band as to be difficult to detect. Even with a 1/3 octave RTA, the dip can occur between ISO centers, and still look like a flat response through the crossover region.
 
Re: Crossover Polarity Question

To clarify- if you reversed the polarity on the back of the amp you will hear no change (unless you have a FFT built into your brain). At this point you have flipped the HF driver with it. You would notice a change between the monitor and the PA or other wedges on the stage since you have reversed the Polarity (not phase) of the box.

If you flipped the polarity of a driver after the passive crossover than you are shifting the response of that driver backward a slight amount (think delayed). It's going to be such a small amount that you wouldn't hear it. You're only going to hear the change in the region where the drivers interact. So if you're passive xover is crossed at 1kHz then you should listen from 800 to 1200 cycles. That range of 400 is somewhat arbitrary because it's based on the slope filters used on the PCB. For instance if a 12dB per octave filter set was used than than the number would be larger than say a 48dB filter since that filter would be tighter.
 
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