+/-/+ Phase Wiring Question

[Maelfactor]

Help the newb!

I've got a set of Sony (hush now, I was young) floor speakers and I just replaced a capacitor for the mid-range. While doing so I noticed that it was wired out of phase with the tweeter and woofer.

My other "WTH?" is that cabinet is rated 8 Ohms, but all of the speakers test as 9 Ohms and they're wired parallel. How the???

Thanks,

Mael

[Hybrid fourdoor]

Nothing wrong with Sony speakers (there affordable). But I'm sure you these are the ones you own...right???




Anyways the mid is wired out phase, so that at the crossover frequency, the two drivers cancel out. I think this gives the impression of a sharper crossover. But don't worry, once you get away from the crossover frequencies they stop affecting each other.

I'm sure others will give a better explanation.

[johninCR]

Originally posted by Hybrid fourdoor
Anyways the mid is wired out phase, so that at the crossover frequency, the two drivers cancel out.

?????

Crossovers cause phase shifts of varying degrees and it is quite common for some of the drivers to need to be wired out of phase to correct or offset an out of phase condition resulting from the crossover.

[Svante]

I suppose the filters are 2:nd order? Such filters *should* be connected out of phase to *avoid* a dip in the frequency response. The filters add a 180 begree phase shift and the reversal of the speaker polarity compensates for this.

How do you measure the speaker impedance? If you use a multimeter, you will only measure the DC impedance (=resistance) which mainly is the woofer plus any resistance of the filter. The mid and tweeter is effectively diconnected at DC by the filter.

[Maelfactor]

Ah hah. So then the amplifier as well will only "see" the woofer as load, impedence wise?

I have no idea what the filters are. They're 3.3microfarad electrolytic, non-polarized 50v capacitors. There's no coil like I've seen in most commercial live-sound "crossovers." They're just bass blockers, I suppose you'd call them. Well, the tweeter cap is a 1microfarad.

Those towers look pretty funky, Hybrid. Unfortunately that's not what I've got.

Thanks guys, info filed away for future reference.

Mael

[Svante]

For low frequencies (up to the first crossover frequency, including DC) the amp will mainly see the impedance of the woofer, for mid frequencies it will see the impedance of the midrange, and for high frequencies it will see the impedance of the tweeter. This is a slight simplification, around the crossover frequencies it will see a bit of both and sometimes, when the filter design is poor, the filter can make a negative impagt on the impedance.

If the filter only two components I guess the filter is first order HP and "0th order" LP. A coil in serier with the woofer, and cap in series with the mid and tweeter? If the design is carefully made, the natural rolloff of the drivers can have been used, and given that the voice coils have an inductance they can in principle keep the impedance above 6 ohms in spite of the lack of inductors.

Then again, this might not be the case, too.

[Maelfactor]

What confuses me is that I know that if I wire three 8 Ohm drivers in parallel, I should wind up with some crazy 3.x Ohms impedence, yet the cabinet is only reading as 8 Ohms. Now if that capacitors make the drivers invisible to the multimeter, does that make them invisible to the amplifier as far as it's concerned? I didn't test the speakers through the capacitors, only on the terminals and all three checked out at 9 Ohms.

These cabinets have no coils, only capacitors on the midrange and tweeter. My full range PA cabinets have a capacitor+coil setup, but as far as I know, the woofer is not low passed. It's connected straight to the input jack. I wonder why there would be a coil then on the horn? I know next to nothing about passive crossovers except that I can buy them premade.

And then if the midrange is wired reverse to account for the polarity change by the capacitor, why isn't the tweeter? Higher frequencies aren't as important as far as phase goes? Obviously the woofer isn't making anything above the 10K mark, but I'm sure the mid-range is as it's only a 3".

Originally posted by Svante
For low frequencies (up to the first crossover frequency, including DC) the amp will mainly see the impedance of the woofer, for mid frequencies it will see the impedance of the midrange, and for high frequencies it will see the impedance of the tweeter. This is a slight simplification, around the crossover frequencies it will see a bit of both and sometimes, when the filter design is poor, the filter can make a negative impagt on the impedance.

If the filter only two components I guess the filter is first order HP and "0th order" LP. A coil in serier with the woofer, and cap in series with the mid and tweeter? If the design is carefully made, the natural rolloff of the drivers can have been used, and given that the voice coils have an inductance they can in principle keep the impedance above 6 ohms in spite of the lack of inductors.

Then again, this might not be the case, too.

[phase_accurate]

And then if the midrange is wired reverse to account for the polarity change by the capacitor, why isn't the tweeter?

There is also phase-shift (caused by the crossover and the drivers own frequency response) between midrange and tweeter. If this is not properly taken into account you end up with a notch at the crossover frequency. This has to do with the phase-relations between the two adjacent ranges. When the tweeter is wired in phase with the woofer it is wired out of phase to the midrange at the same time, avoiding the notch at the mid/high crossover frequency. Simple as that.

Regards

Charles

[Maelfactor]

Okay, that'll take a minute to sink in, but it makes sense. I'd probably be able to put it into context better with a graph of some sort, but I underestand how reverse phase frequencies cancel eachother and create notches, but I wasn't aware that capacitors actually changed the polarity of the signal.

Or maybe I'm still misunderstanding?

[Svante]

Well, if you were to wire all 3 speakers in parallel without the caps, and measured them with the multimeter, you'd end up with 3 ohms. since the multimeter measures the DC resistance. Now, the impedance (kind of like the "resistance" for AC) of a typical loudspeaker is not the same as the DC resistance, in fact it is always higher, and depends on the frequency.

The typical driver has a peak of 20-40 ohms at the system resonance, a wide low dip down to something near the DC resistance in the middle of its frequency range, and a slowly increasing impedance towards high frequencies due to the voice coil inductance. At 20 kHz most woofers have an impedance of several tens of ohms.

All in all this means that the impedance of the three drivers in parallel, particularly if the series capacitors are included, will be higher than 3 ohms. Also, the nominal impedance of a speaker system is an average over a frequency range, which means that the typical 8 ohm speaker has impedance dips down to some 6 ohms and peaks up to 20-40 ohms at certain frequencies.