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Mortimer - Audible Physics 6.5" and North D25

edited October 2017 in DIY
Get the name out of the way - Mortimer was a mutt I had when I was a kid. He was hit by a few cars, and always came back - until the last time. Much like these old cabinets and the drivers, he is loved, remembered, but long gone. Well, the 6.5" woofer may make a come back at some point I imagine. Anyways, some squigglies etc.

Here is the woofer distortion at around 96db/1M, in-box (cabinet is the Cabrini Redux - these cast frame AP woofers drop right into a standard stamped frame cutout, by the way):


As you can see, very admirable distortion levels considering how fracking loud this sweep was. 

Here is the tweeter at the same amplifier setting:


Again, very low distortion above 1.8K or so. 

Both drivers have very low distortion, actually. 

~4.5ms in-box measurements of both drivers at 1M (I scaled level considerably to get it to display properly):


Overall, not too bad. I like to think if the cheesy metal dustcap were ditched on the AP, that breakup would be far more benign. Still not bad, though. 

My drivers did not measure all that well for making the bass - so currently I have been enjoying them sealed. 

Woofer T/S after 5 minute break-in at Xmax using 50Hz tone (and considerably manual exercising):

Loudspeaker parameters:

Fs  = 53.68 Hz
Re  = 6.34 ohms[dc]
Le  = 104.42 uH
L2  = 654.35 uH
R2  = 8.38 ohms
Qt  = 0.47
Qes = 0.57
Qms = 2.67
Mms = 15.63 grams
Rms = 1.978132 kg/s
Cms = 0.000562 m/N
Vas = 14.35 liters
Sd= 134.78 cm^2
Bl  = 7.646634 Tm
ETA = 0.37 %
Lp(2.83V/1m) = 88.84 dB

Added Mass Method:
Added mass = 25.00 grams
Diameter= 13.10 cm

However, I do expect these to take considerable more break-in than a lot of drivers - they ship with a stiiiiiiif suspension. Until they get more than the several minutes of break-in and what amounts to less than four hours of voicing/measuring I am not going to comment too much on their bass other than to say even in the sealed box I have them in currently they are very clean and, to my ears, accurate if somewhat lean. Additionally, sealed speakers have a way of fooling you into thinking they are digging deeper than they are.One thing that makes me think this is about as good as they are going to get on the bass, though, is that sensitivity of nearly 89db for an 8 ohm 6.5" - that is fairly sensitive.

Crossover is absurdly simple and resulted in this:


Well within +/-3db and impedance is pretty benign - it never exceeds 30 degrees either way, with the exception of a mild escape in the bass region where it hits ~-40 degrees. This is a solid 8 ohm speaker, and should be easy to drive with pretty much any amp:


Here is the crossover - as I mentioned, absurdly simple for the performance:


Nothing unusual there, very basic - almost universal TM crossover network (the network, not the values). Nothing expensive needed, and will fit on a small board. I used about 4.5db of BSC, give or take so a little livelier than most of my work but I figure most people are A: not going to build these and B: if they did, they would be within a foot or two of a boundary or three. If the woofers do not loosed up after extended use, I will keep them sealed as vented models in this box do not offer a lot of advantage of the sealed:



In the above, that is the same box tuned to 60Hz, what I get out of is somewhat more output across the same passband, but no significant difference in extension. In my opinion, if this is how the driver is going to be I am better off with the excursion limiter that a sealed box becomes down low than I am with a few db of extra output in the passband. 

Finally, distortion of the system at about 90db/1M:


I did not bring these to Iowa as I was not close to finishing the crossovers and wanted to spend at least one more morning listening and measuring. It makes me wish North would swing back into business, they are pretty damn nice dome tweeters. The woofer is remarkable, as well. All in all, this is not a bad design to stick on a desk or in a shop. For most people (read: people not reading this forum) this design would represent a significant step up over quite a few commercial speakers - but that is the blessing of DIY. 

The other blessing is this: the woofers ended up being something like $11 at MCM, and I picked up the NOS North tweeters for $20 shipped. The crossover can be built very inexpensively as well, using components from PE: *edit: add $3 for the two 0.33uf caps*



Personally, I would step up to a laminate core woofer coil - those powdered core have weird saturation characteristics. Whether that happens at levels we use them or not is probably worthy of an investigation, but powdered cores do exhibit a "soft saturation" characteristic. Look it up if you don't believe me ;)

Going with a laminate core would probably mean someone needs to possess the means to unwind a larger value coil - this day and age, though, we should all have that already. 

Just because:


The North required some very slight enlarging of the thru-hole than the print shows, but it is pretty much a drop in otherwise. The woofer required a spacer to be flush mounted, I made mine from a pile of cardboard and wood glue. 

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greywardenSilver1omo

Comments

  •    Thanks for that.  Have a few of the mcm buyout 6.5" AP's waiting their turn.  Have you considered other tweeters that would be a good match? 
  • Nice design.  That is a very smooth response using only 6 parts!
  • Kornbread said:
       Thanks for that.  Have a few of the mcm buyout 6.5" AP's waiting their turn.  Have you considered other tweeters that would be a good match? 
    I bet any number of Vifa tweeters would be excellent. I have a pair of those new fangled DX25 with the cast face plate and rear chamber I could throw in there. 
    I have a signature.
  • Looks like a fun listen Johnny!

    Yes, P-cores saturate earlier, but softer than a laminate or somid core, just like air cores of large values can pose problems due to excessive widing length. If harmonic distortion vs. Drive level is measured with a spectrum analyzer, inductors can exhibit a spectrum based harmonic profile that can represent 0.3% distortion.......before it even hits the drivers! 
    greywardenPWRRYD
  • jr@mac said:
    Kornbread said:
       Thanks for that.  Have a few of the mcm buyout 6.5" AP's waiting their turn.  Have you considered other tweeters that would be a good match? 
    I bet any number of Vifa tweeters would be excellent. I have a pair of those new fangled DX25 with the cast face plate and rear chamber I could throw in there. 


       Do it. 
  • Very nice JR. I have my mltl cabinets essentially finished sans baffles. Still deciding on which tweet to use. I do have a pair of the NorthCreeks I could use. 

  • edited September 23




    After a lot of searching, got this from mobilediy.  Seems to be some differences.  Whatdayathink?  Is this just due to stiff suspension or are they different animals? 

    Jr, yours are the 6.5" APs from the MCM closure, right?         



     
  • Yes, and my measurements are very close to those. 
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  • edited September 23
    Then it appears we have a model number of 635-cp2 to add to the MCM 55-5615 part number.

    Just playing around with the sims looks like both this and the Peerless buyout build are plausible.  Like to get cutting some wood on both projects this week as the UAW strike has me temporarily unemployed.  Funny thing is that some new positions just came open at work and it appears I am earmarked for one of them.  There is also another local company who is doubling their output and pay is better than my current position.  I'd venture to say major changes are on their way in the next couple of days, or weeks.  If that happens, I'll be too involved with learning a new job to make any headway on these projects. 

    The future job situation has me excited and scared to death at the same time.                 
    jr@mac
  • Ive used this exact combo, built into a MLTL. Really nice!
  • I should have snagged some of those AP drivers during the buyout.  :'(
    Don, Donno, or "Hey you" all work for me, But never "Mr Johnson"
  • Why no pics yet?
  • This was two years ago, cabinets have been repurposed already.
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  • Yeah it was a nice design, sent the drivers to Chuck I believe. 
    I have a signature.
  • Yep, sitting in my stash now. Might become a surround system for a friend and his family. 
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