Reverse Polarity?

[Tubedude]

Hey guys,
I got my Magnificents loaded with fresh 416s and new diaphragms from GPA... Bill is great and very helpful. I connected the first cab's drivers in accordance to the diagram on the N-500-G xover, both drivers in phase. Well, I opened the second cabinet and there was an installation page stapled to the inside of the cab. That diagram shows to connect the horns out-of-phase with the woofs. There are applications in which this is beneficial, but for ALTECs? What gives?

[amandarae]

The way I understand it;

The N500-G if I am not mistaken is a Second Order Butterworth which means that the sum of the signal at Fc(cut frequency) is +3dB more. That is why it is recommended to reverse the phase of the HF driver to cancel some of it because if not the signal at Fc will be more pronounce than the rest of the frequency spectrum.

If you open the HF driver, you will notice that L1 is actually connected to (-) of the voice coil and L2 to plus. Thus, if you follow the conventional Altec wiring with + signal from the output of the XO to L1, and Negative(-) signal from the XO output to L2, then what really is happening is that you are feeding the HF signal + to the negative of the voice coil and (-) to the positive of the voice coil. In short, with conventional Altec wiring, the polarity of the HF with respect to the LF is already reversed.

I could be wrong, but this concept worked for me even if I am using a 1st order XO where plus is plus and minus is minus.

Abe

[Tubedude]

The N-500-G has a stepped HF attenuator, so db reduction via reverse polarity might not be likely.

[Cal Weldon]

Hi Tubedude,

A 2nd order (12dB) crossover leaves the two drivers 180 degrees (1/2 wavelength) out of phase so you reverse the tweeter to bring them back in phase. a 24dB XO leaves them 360 degrees out of phase which is basically being back in phase but one wavelength out so in that case you would wire the tweeter in phase. 6db and 18 dB are 90 and 270 degrees respectively so often nothing is accomplished by changing the phase.

[whitebroncoii]

The phase change related to the crossover function, as well as the physical relationship of the acoustic centers of the two drivers, often make them closer to being in-phase when the polarity of the HF driver is reversed.

It is very common with original Altec Lansing crossovers to find the actual HF polarity reversed. This can be accomplished within the crossover circuit or the with the HF driver terminals.

[theophile]

Then,to really confuse things,some amps invert phase.