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Author Topic: Push pull double 18  (Read 10106 times)

Mark Wilkinson

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Push pull double 18
« on: March 19, 2018, 01:32:36 pm »

Got this idea from a thread on the diyaudio forum http://www.diyaudio.com/forums/subwoofers/177905-thread-ppsl-enclosures.html

They call it a push-pull-slot-loaded sub.  The inverted driver in the slot is wired in reverse polarity, so both squeeze into the slot at the same time.
The slot is basically made as small as possible, and then you build whatever  kind of box you want around the slot/driver module.

I chose a bass reflex box, because I had already built a successful single 18 with the drivers, and could simply  double the net box size for two 18s, after accounting for the extra driver module volume inside the overall box.  Pretty easy design really. Ended up 49Hx21.75Wx31.25D

What's really cool about it IMO, is the wonderful reduction in cabinet vibration.  Opposed drivers really do reduce vibration it appears.
This was the primary reason I tried this design....I absolutely love the single 18's I built other than how much they can walk/jump around.
The single 18's are well braced, but it seems it's weight that matters.  I watch orbitshifters and th-118s want to walk around too....

Supposedly, the design reduces 2nd order harmonic distortion too.  We'll see....   
I can say it sounds very, very, clean, kinda like a labsub (which btw doesn't move much either...opposed drivers again)

Anyway, here's a pict and measurement taken this morning. 
Measurement is raw other than the 3 peq's shown in table.
The slot provides a heck of a boost in the high 100'sHz, that has to be knocked down.
I'd put efficiency at 101-102 @ 2.83v after the knockdown.

« Last Edit: March 19, 2018, 01:36:31 pm by Mark Wilkinson »
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Scott Holtzman

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Re: Push pull double 18
« Reply #1 on: March 19, 2018, 01:47:28 pm »

Got this idea from a thread on the diyaudio forum http://www.diyaudio.com/forums/subwoofers/177905-thread-ppsl-enclosures.html

They call it a push-pull-slot-loaded sub.  The inverted driver in the slot is wired in reverse polarity, so both squeeze into the slot at the same time.
The slot is basically made as small as possible, and then you build whatever  kind of box you want around the slot/driver module.

I chose a bass reflex box, because I had already built a successful single 18 with the drivers, and could simply  double the net box size for two 18s, after accounting for the extra driver module volume inside the overall box.  Pretty easy design really. Ended up 49Hx21.75Wx31.25D

What's really cool about it IMO, is the wonderful reduction in cabinet vibration.  Opposed drivers really do reduce vibration it appears.
This was the primary reason I tried this design....I absolutely love the single 18's I built other than how much they can walk/jump around.
The single 18's are well braced, but it seems it's weight that matters.  I watch orbitshifters and th-118s want to walk around too....

Supposedly, the design reduces 2nd order harmonic distortion too.  We'll see....   
I can say it sounds very, very, clean, kinda like a labsub (which btw doesn't move much either...opposed drivers again)

Anyway, here's a pict and measurement taken this morning. 
Measurement is raw other than the 3 peq's shown in table.
The slot provides a heck of a boost in the high 100'sHz, that has to be knocked down.
I'd put efficiency at 101-102 @ 2.83v after the knockdown.

That's interesting.  Is the vent in the back for the top sub or is there a waveguide down the back to the bottom vent?
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Mark Wilkinson

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Re: Push pull double 18
« Reply #2 on: March 19, 2018, 02:02:19 pm »

That's interesting.  Is the vent in the back for the top sub or is there a waveguide down the back to the bottom vent?

Hi Scott,  both drivers vent into the same back chamber, just like a regular double 18 BR.
So the port at the bottom is handling duty for both drivers. 

The port shelf on mine is currently just screwed in place, and without it's bottom brace tying it to the cabinet.
Needed to make sure of tuning before gluing and bracing it in.
It definitely began to flap some at 30Hz as i turned up the juice !
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Dave Garoutte

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Re: Push pull double 18
« Reply #3 on: March 19, 2018, 05:50:46 pm »

What's the advantage of having the both aimed up?
Compared to if they were aimed together and not reversed polarity?
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Art Welter

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Re: Push pull double 18
« Reply #4 on: March 19, 2018, 05:56:26 pm »

Supposedly, the design reduces 2nd order harmonic distortion too. 

The slot provides a heck of a boost in the high 100'sHz, that has to be knocked down.
Mark,

The extra upper boost the plenum (slot) provides helps make up for the reduced even order harmonics the push pull arrangement provides ;^).

Looks svelte!

Cheers,
Art
« Last Edit: March 19, 2018, 05:58:40 pm by Art Welter »
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Don T. Williams

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Re: Push pull double 18
« Reply #5 on: March 19, 2018, 06:02:03 pm »

What's the advantage of having the both aimed up?
Compared to if they were aimed together and not reversed polarity?

One explanation that I have seen is that differences in linearity in the cone movement "out" vs. movement "in" will be out of phase and thus cancel.  The result should be lower distortion.  That's the theory.  Has anyone out there measured and confirmed the theory? 
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Mark Wilkinson

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Re: Push pull double 18
« Reply #6 on: March 19, 2018, 06:32:11 pm »

One explanation that I have seen is that differences in linearity in the cone movement "out" vs. movement "in" will be out of phase and thus cancel.  The result should be lower distortion.  That's the theory.  Has anyone out there measured and confirmed the theory?

Art, thank you !

Dave, yep, what Don is saying...

Don, I think Art has done more homework on this than anywhere I've seen.
I have a pair of single 18's, same drivers, and very close to same bass-reflex build in terms of volume and ports. 
I hope to measure distortion side by side and compare directly.

Below is a distortion test I ran with REW's swept sine this morning, but only at 110dB @ 1 meter equivalent, because my port isn't glued in yet and rattled alot at 120dB.
This was at 10 meters, so +20dB
Looks great so far i think....
 
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Jeremy Young

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Re: Push pull double 18
« Reply #7 on: March 19, 2018, 07:30:17 pm »

Very cool Mark!


My EAW FR250z 2x15" subwoofer is another example of a push/pull arrangement design, but in this case it's in two separate sealed portions of the same front-loaded reflex cabinet. 

In this case, a positive voltage on the input terminals makes both cones (forward and reverse facing) move forward, so it doesn't have the same advantages as the OP's design where the two cones move in different directions and therefore reduce cabinet vibration.

I can't say as I've ever unplugged one woofer and measured to see whether distortion changes, but the concept seems sound to me and it does sound good for its price/class.   

This past summer I A/B'd that sub against an Orbit Shifter Pro during an impromptu outdoor shootout and well..... let's say they are in different classes.  The OS Pro was much more transparent, but let's not forget it cost me about 20% of what the OS Pro did, and it's considerably smaller in size.

Sorry no measurements of that, it was far too casual for any measurements to mean anything in a formal post.  Up until that day I always thought it sounded pretty good, but I only ever use it in small-ish rooms where positioning and room-modes can have just as much affect.
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Ivan Beaver

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Re: Push pull double 18
« Reply #8 on: March 20, 2018, 07:51:23 am »


Supposedly, the design reduces 2nd order harmonic distortion too.  We'll see....   
I can say it sounds very, very, clean, kinda like a labsub (which btw doesn't move much either...opposed drivers again)


The lab sub does not use opposed drivers.

The reason for the minimal movement of the lab sub is the horn loading, and small back volume.
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Mark Wilkinson

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Re: Push pull double 18
« Reply #9 on: March 20, 2018, 09:12:45 am »

Very cool Mark!


My EAW FR250z 2x15" subwoofer is another example of a push/pull arrangement design, but in this case it's in two separate sealed portions of the same front-loaded reflex cabinet. 

In this case, a positive voltage on the input terminals makes both cones (forward and reverse facing) move forward, so it doesn't have the same advantages as the OP's design where the two cones move in different directions and therefore reduce cabinet vibration.



This past summer I A/B'd that sub against an Orbit Shifter Pro during an impromptu outdoor shootout and well..... let's say they are in different classes.  The OS Pro was much more transparent, but let's not forget it cost me about 20% of what the OS Pro did, and it's considerably smaller in size.



Thanks Jeremy, 
Yeah, I saw the FR250z mentioned in the diyaudio thread, but like you say, it is considered more of a regular dual-driver bass reflex, only with one driver mounted backwards. I couldn't glean any clear consensus on what folks thought EAW was trying to accomplish...

Interesting you compared your sub against an OS.  I have a pair of OS's and use one to compare against the diy 18's I'm making.
Here's a past thread, 1 OS  vs 2 diy 18's.  http://forums.prosoundweb.com/index.php/topic,163594.0.html
Having used the boxes for a while, I've really come to consider 2 of the single 18's as an equivalent to 1 OS  in terms of output.

If all continues to turn out as designed with the new dual 18 PPSL in this thread, it should directly equal a pair of my single 18's....
which in my mind, means this new box should equal an OS.   When I'm certain it does, I'm most likely putting my JTR rig up for sale....going totally DIY i think :)
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