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-   -   Push Pull vs Normal Distortion Compared (http://www.diyaudio.com/forums/subwoofers/191833-push-pull-vs-normal-distortion-compared.html)

weltersys 1st July 2011 03:29 AM

Push Pull vs Normal Distortion Compared
 
4 Attachment(s)
Having never seen an actual comparison of the same cabinet used as a push pull (PP) compared to a normal forward facing pair, decided to do it myself.

PP proponents claim large reductions in distortion due to the cancellation of the differences caused by non-symmetrical cone movement, but without a comparison of the same speaker in the same cabinet, other design features such as a front plenum or horn loading make the reductions hard to quantify.

A suitable sized rock was chosen after an extensive 30 second observational period and placed in the enclosure on top of some rags to keep the internal cabinet volume the same when the cone was reversed for PP testing. The speaker with the magnet facing forward has the polarity reversed, both cones move forward when positive voltage is applied.

We had a perfect day for testing,(other than smoke from the many nearby fires, and 100 degree temperature) ambient and wind noise were very low. Testing was done at 2 meters.

The Lab12 speaker is unusual in some respects, so the distortion figures below may not be representative of other speakers in other designs.

Although second, fourth, sixth order harmonics were greatly reduced by push pull loading, the second harmonic sometimes increased, resulting in the THD (total harmonic distortion) not always significantly changing, as can be seen in the 20 Hz 49 volt test results .
That said, distortion figures were reduced by 50% or more at some frequencies.

Since the frequency response and output of both the PP and standard cabinets were almost identical, an A/B listening test was simple, just swap the speaker cord and short out the unused cabinet.

At lower power levels, where the Lab12 is quite clean, no difference could be detected. When pushed at a power level where distortion could be noticed, the most distracting artifact was the audible vent noise from the reversed magnet structure of the PP. The lack of the second harmonic, an octave, made the now predominant third harmonic distortion more apparent in the PP cabinet. This is a music related thing, the third harmonic, being a perfect fifth, may sound OK with some compositions, while sounding “wrong” with others.

Although the technical side of me says reduction of distortion of any sort is a good thing as far as a reproduction system is concerned, both my old ears, and a college freshman apprentice thought the push pull cabinet sounded less “musical” when pushed hard.

Listening tests were not extensive, as we had to put the speakers away due to rain, the first in months. I put the speakers back to normal, and put the grill back on. Today I realized we had not done listening tests on the sealed versions of the cabinets. Also forgot to take an amplitude response for the PP ported cabinet, but judging from the response curves of the sealed PP cabinet compared to normal, looks like the frequency response would be little different below 250 Hz. The sharp dip around 300 Hz in the “Raw 2x12” is a port resonance.

The distortion screen shots are in posts 3 & 4.

Addendum:
The Lab 12 DC resistance and cabinet minimum impedance is almost the same, 4.29 ohms per speaker, 2.145 for a pair of Lab 12s at 49 volts is about 560 (rather than 400) watts per driver, 282 (rather than 200) watts per driver in the 38.4 volt test.

Art Welter

jbell 1st July 2011 03:35 AM

art:

This is not the first time I've seen you do this. The octave is the SECOND harmonic, as harmonics start with 2. The octave harmonic is the EVEN order one, not the odd.

PP reduces EVEN harmonics

see wiki:
Harmonic - Wikipedia, the free encyclopedia

weltersys 1st July 2011 03:37 AM

10 Attachment(s)
The distortion screen shots for the various tests from 20 to 40 Hz:

weltersys 1st July 2011 03:44 AM

8 Attachment(s)
The screen shots from 45 to 70 Hz.

I also tested at 80 and 100 Hz, but the distortion for both normal and PP is so low as to be of little interest.

Art Welter

weltersys 1st July 2011 03:53 AM

Quote:

Originally Posted by jbell (Post 2624903)
art:

This is not the first time I've seen you do this. The octave is the SECOND harmonic, as harmonics start with 2. The octave harmonic is the EVEN order one, not the odd.

PP reduces EVEN harmonics

Jim,

Do what:confused:?

You are correct, I have made that mistake before, corrected it in the previous post, thanks.

The second harmonic is the fundamental frequency X2, ie, a 40 Hz fundamental has a second harmonic of 80 Hz, an octave up.
Third harmonic of 40 Hz is Fx3, 120 Hz.
Fourth harmonic is Fx4, 160 Hz, another octave.

At any rate, the screen shots are up, so you can see what PP reduces and what it does not in the front loaded Lab 12 cabinets.

Most frequencies distortion do what one would expect, others not so much.

Been at this for 13 hours, best I sign off before I make any more errors.

Art Welter

18Hurts 1st July 2011 04:03 AM

I know that KEF did push-pull back in the 80's with the 104.2 and other speakers. M&K also did push-pull to lower distortion. As far as Isobarik face-to-face loading of woofers are also push-pull and the distortion drop has been noted.

Look forward to your results with sealed boxes, those should be much easier to measure and less to lug around in the 100F heat.

Radugazon 1st July 2011 05:28 AM

Very courageous job ! Maybe the most interesting is the listening observation that distorsion reduction has better to be harmonious and not only on the even orders.

weltersys 1st July 2011 01:57 PM

Quote:

Originally Posted by 18Hurts (Post 2624930)
I know that KEF did push-pull back in the 80's with the 104.2 and other speakers. M&K also did push-pull to lower distortion. As far as Isobarik face-to-face loading of woofers are also push-pull and the distortion drop has been noted.

Look forward to your results with sealed boxes, those should be much easier to measure and less to lug around in the 100F heat.

The results are in posts 3 and 4.

The pictures with four graphs have the 7.76 (gross) cubic foot Fb 36 ported cabinets on the left, sealed on the right, normal front load on the top, PP on the bottom, all tested with 35 volt sine waves, 400 watts in to the nominal 3 ohm load. The same cabinet and speakers were used for all the tests, ports were stuffed shut.

The pictures with two graphs are sealed cabinets run with 49 volt input, 800 watts. Again, the top cabinet graph is normal, the bottom PP.

18Hurts 1st July 2011 02:38 PM

Do you think the lowering of harmonic distortion with push-pull will lower the stress on the woofers during maximum drive levels? Every little bit helps to keep those things alive. Since horn loading really puts the pressure on drivers, the distortion reduction could increase longevity of the driver?

Thanks for posting your results, as with everything--it does work but it all depends on what you want. In the classic battle between size and reliability so would two drivers in the same box push-pull be more reliable than two individual boxes? Would lugging around the larger and heavier push-pull be worth it reliability wise VS two lighter and smaller boxes?

Thanks for throwing another variable in my quest to build a horn loaded subwoofer, Art! My subwoofer design is growing faster and larger than the national debt. ;)

weltersys 1st July 2011 02:50 PM

Quote:

Originally Posted by 18Hurts (Post 2624930)
I know that KEF did push-pull back in the 80's with the 104.2 and other speakers. M&K also did push-pull to lower distortion. As far as Isobarik face-to-face loading of woofers are also push-pull and the distortion drop has been noted.

Look forward to your results with sealed boxes, those should be much easier to measure and less to lug around in the 100F heat.

The results are in posts 3 and 4.

The pictures with four graphs have the 7.76 (gross) cubic foot Fb 36 ported cabinets on the left, sealed on the right, normal front load on the top, PP on the bottom, all tested with 35 volt sine waves, 400 watts in to the nominal 3 ohm load. The same cabinet and speakers were used for all the tests, ports were stuffed shut.

The pictures with two graphs are sealed cabinets run with 49 volt input, 800 watts. Again, the top cabinet graph is normal, the bottom PP.


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