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Author Topic: Push pull double 18  (Read 22032 times)

Mark Wilkinson

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Re: Push pull double 18
« Reply #110 on: February 07, 2020, 01:36:40 pm »

Mark -

"The first is designed to go as low as reasonable without worry.  It is based on a 63V drive voltage. It has a 25Hz BW4 hpf.
The second trades a little low end extension for a little more overall SPL.  It it based on 77V. It has a 31Hz LR2 hpf."

You mean you're not "driving with Watts"? Huh?  /sarcasm

Rolando - the above is why I've been obliquely trying to get you to do some homework.  WATTS are relevant as an expression of potential (volts) turned into work (motion or heat), and as a way of referencing the amount of work that can be produced.  It's important that you understand the difference.

Tim   ;)

Yeah, i thought i'd try to help your noble cause,
by posting the Driver Electrical Input Power (watts) graphs to show so it how much it changes by freq.
I guess the beginning step for folks to think in volts is to realize amps are constant volt, not constant power devices. (Trying to aid the cause :)

Rolando, if you skip the PEQ notch, check out the increased power need at 100Hz as shown below.
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Tim McCulloch

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Re: Push pull double 18
« Reply #111 on: February 07, 2020, 05:12:20 pm »

Mark,
but if we implement a 120Hz LP, why you need the 178hz PEQ
anything above 120Hz will be cut
so we do not need the PEQ
do we ??

Out of band EQ is used to accomplish phase alignment - keep the phase response "flat" throughout the acoustic crossover and, ideally, for an octave on either side of it.  That's easier to do when adjacent pass bands exhibit flat phase response.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Rolando Saenz

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Re: Push pull double 18
« Reply #112 on: February 08, 2020, 05:15:19 am »

Out of band EQ is used to accomplish phase alignment - keep the phase response "flat" throughout the acoustic crossover and, ideally, for an octave on either side of it.  That's easier to do when adjacent pass bands exhibit flat phase response.
:o
darn!! i have never imagined that
so even that on my  case I like to cut my
tops at 120 , my subs will only go as high
as 120 or even 100, if there is peak @170+ hz
that will affect my sub power draw??
the graph says that, but how!!
I have setted my LP filter at 100 .so anything
above is removed from the frq response.
am I missing something? a Newton, Einstein or hawking theory
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David Morison

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Re: Push pull double 18
« Reply #113 on: February 08, 2020, 08:14:36 am »

:o
Darn!! I have never imagined that
So even that on my case I like to cut my tops at 120, my subs will only go as high as 120 or even 100, if there is peak @170+ hz that will affect my sub power draw??
The graph says that, but how!!
I have set my LP filter at 100, so anything above is removed from the frq response.
Am I missing something? A Newton, Einstein or Hawking theory

Not quite Newton or Einstein level, but the key thing to remember here is that crossovers do not cut everything off dead like a guillotine.
Rather, they apply a downward slope that takes some bandwidth to push the out of band level down far enough before it can be ignored.
24dB per octave is what Mark was modelling and even though that sounds quite steep, it still lets through enough content in the first half to one octave above the nominal crossover frequency to have the effects Tim & Mark are describing/showing.
HTH,
David.
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Steve-White

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Re: Push pull double 18
« Reply #114 on: February 08, 2020, 10:29:40 am »

^^^ I'm still leaning a little towards "Einstein" for Rolando. (not Einstein level over Rolando's head - Rolando as Einstein)

DISCLAIMER:  This is light hearted humor and chiding and not intended to insult.

Rolando, you're a Wildman with all the questions - your brain must be smoking with all the input - it'll cool down, and this stuff will all begin to integrate and make sense.

It's still 3-D chess and there is a ton to learn - just remember, EVERYTHING is a compromise somewhere in the final equation - EVERYTHING.  Cost, size, weight, performance (bandwidth, distortion, tone), lifespan, resale value, etc etc etc.
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Chris Grimshaw

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Re: Push pull double 18
« Reply #115 on: February 09, 2020, 06:46:43 am »

Hey Mark,

Do you remember what your port area is, and how does it hold up at high power levels?

Cheers,
Chris
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Mark Wilkinson

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Re: Push pull double 18
« Reply #116 on: February 09, 2020, 10:32:54 am »

Hey Mark,

Do you remember what your port area is, and how does it hold up at high power levels?

Cheers,
Chris

Here you go Chris,

160 sq inches, 24 inch long. 
Input screen,  displacement, and port output velocity at 126V pmax below. (i used Ricci's T/S not bms)

It's held up fine.  No chuffing really.
In fact, I found the motor noise of a driver hanging inverted in the plenum to be much more noticeable than any port noise.
That's why i run the drivers push-push (clamshell), instead of push-pull.....to keep the driver motors inside the box.
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Chris Grimshaw

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Re: Push pull double 18
« Reply #117 on: February 09, 2020, 11:17:48 am »

Here you go Chris,

You're a star, thanks Mark.

I have a pair of Faital 18XL1800 drivers here which I'd like to build into a reasonably meaty 2x18" box, hopefully replacing 4-6 of my current 15" designs which use Faital 15HP1060 drivers.

I like the idea of opposed drivers to cancel vibration and indeed, the last bespoke cabinet I did included that feature - that was a 4th order BP 2x18" with both magnets in the central ported chamber. Forced air cooling and sealed back chambers meant it was difficult to abuse without dropping a lot of power in there.

The 2x18" cab I'm planning this time will probably look a bit like yours, except I'm planning on having the magnets of both drivers in the plenum (probably offset so one is nearer to the exit than the other) and also running the port(s) through there to ensure forced air cooling at all frequencies.

I'll test the drivers for motor noise before committing to this build, though - I'd hate for that to ruin an otherwise good design.

Cheers,
Chris
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Rolando Saenz

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Re: Push pull double 18
« Reply #118 on: February 11, 2020, 12:23:10 pm »

Not quite Newton or Einstein level, but the key thing to remember here is that crossovers do not cut everything off dead like a guillotine.
Rather, they apply a downward slope that takes some bandwidth to push the out of band level down far enough before it can be ignored.
24dB per octave is what Mark was modelling and even though that sounds quite steep, it still lets through enough content in the first half to one octave above the nominal crossover frequency to have the effects Tim & Mark are describing/showing.
HTH,
David.

David,
can you stack to 48Db high pass so you can "decapitate" the rest of the audio spectrum, so you do not need to add an extra corrective PEQ
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David Morison

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Re: Push pull double 18
« Reply #119 on: February 11, 2020, 02:07:45 pm »

David,
can you stack to 48Db high pass so you can "decapitate" the rest of the audio spectrum, so you do not need to add an extra corrective PEQ

Sort of..... but there's a "But"...

Normal filters have an effect on phase, which can introduce other artefacts into the system response, and these often get worse the steeper the slope of the filter.
If the phase curve is steeper, (as it will be with a steeper cutoff) it may be harder to line up that phase with the phase for the tops, so you could end up with an uneven transition there, possibly even to the point of an unexpected cancellation occurring - do you have the measurement gear and experience to correct for that?
Secondly, the changed phase behaviour can sometimes be audible itself, often referred to as "ringing".
In short, stick with Mark's processing unless you have the measurement gear and experience to do better yourself - in which case, you probably wouldn't be asking these questions in the first place.
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Re: Push pull double 18
« Reply #119 on: February 11, 2020, 02:07:45 pm »


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