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Re: PPSL

Posted by Wayne Parham (M) on May 17, 2003 at 01:11:03

In Reply to: PPSL posted by djk on May 16, 2003 at 17:26:18:

Hi Dennis!

I agree with others on this thread - I'd like to see more discussion of this. Can you show some more detail and maybe link some references?

I can see how second harmonics would be suppressed if they were made to be out of phase. And so I can see some merit in the idea - Sort of like complementary-symmetry amps. But the thing I'm wondering about is how do you get the anomalous harmonics to be 180 ^o out of phase while having the fundamental in phase. In an amplifier, this is done in the coupling mechanism, where the two out-of-phase signals are combined to be in-phase. It creates a condition for common-mode rejection, where the artifacts cancel out. But in a loudspeaker, no similar mechanism can be employed that doesn't cancel the fundamental. At least, that's where I'm drawing a blank on this one.

Seems to me that if two similar motors are used, the phase relationship between these components would be the same. Even if you drive the two diaphragms in opposite directions, if you house them in a cabinet that combines their pressure waves for summation then the results are the same. And so then when you connect them so that the pressure wave of each creates summing pressure - as opposed to cancelling pressure - then it would seem that the same relationship would be maintained for any anomalous harmonics that were generated.

I mean, I think that if there are non-linearities that are made by the mechanical suspension - something that, in effect, causes a sort of rectification-like property whereby the cone actually goes further in one direction than it does in the other - I could see how this push-pull arrangement might reduce second harmonics. This sort of non-linearity is, after all, what causes second-order artifacts, so you might be able to cancel the effects of high-excursion mechanical suspension non-linearities with this configuration. But the thing I'm not understanding is that I would expect that sort of distortion to be onset at high levels, and not down low.

Distortion caused by the magnetic structure isn't something I would expect to be improved on by this configuration because it seems to include odd-orders harmonics as well as even-orders, and that's a different issue entirely, different causes and so requiring different solutions. But then again, the voice coil energy is polarized the same as the magnet on one half cycle, and opposite on the other, so I suppose that's a condition that tends to increase second-orders as well. I guess I need to do some reading about these kinds of systems, because I'm completely at a loss to this sort of arrangement. It's all speculation on my part.

So I'd appreciate any links you could share that I could peruse. I agree with you 100% on the big basshorn thing. It would be great to be able to have 'em, but horns for low frequencies must necessarily be large. So for deep bass from speakers that aren't as big as a house requires other technologies. Horns are great down to 40Hz, but below that they become pretty big. You just can't make a basshorn have 30Hz f3 in 20 cubic feet or less.

Wayne

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