FS: New Old Stock DIYSG Concentric-8 Coaxial Drivers

Discussion in 'Classified Section' started by Matt Grant, Oct 13, 2023.

  1. #1 Matt Grant, Oct 13, 2023
    Last edited: Feb 11, 2024
    Buried in the DIYSG warehouse were several dozen new Concentric-8 drivers, Erich and I dug them out on my recent visit and I've been slowly giving each a QC check and now have many tested and available for purchase. I also grabbed a smaller number of Concentric-6 drivers too.

    These were higher end coaxials with better dispersion characteristics then the Volt-8, much improved frequency response both on and off axis and lower distortion as well. That puts these much closer to the Vortex coaxials in terms of sound quality then the Volt line.

    They would be great for surrounds, LCR speakers in living room sized home theater setups, or just used as a stereo pair for two channel especially if you go with a larger ported cabinet. They would also be great if you want a set of coaxial studio / near-field monitors that you can really crank up.

    The HF side uses a 1.75" damped titanium diaphragm with a copper pole pole piece to lower inductance and distortion.

    Drivers Available:
    I have two different batches of these drivers, the first batch has a black HF cover and the dust cap is installed. The second got a custom machined aluminum back cover but the manufacture did not install the dust caps, You can use these without the dust cap or I can install it for you if requested.

    $200 each + shipping. (A free crossover PCB will be included with each driver).
    $10 off each driver if buying 4+.

    Fully assembled crossovers (built to order 2-3 week lead time):
    $80 for the standard crossover,
    $70 for the In-wall / IB crossover

    Remaining Stock (Updated February 10th, 2024):
    1 - Concentric-8 driver with the black back plate (dust cap installed).
    1 - Concentric-8 driver with the black back plate (dust cap installed). (B-stock, HF output slightly low $150, fixable with crossover mod).

    Here are the T/S specs of the woofer:
    * Piston Diameter = 165.1 mm
    * f(s)= 72.00 Hz
    * R(e)= 5.18 Ohms
    * Z(max)= 75.35 Ohms
    * Q(ms)= 8.750
    * Q(es)= 0.646
    * Q(ts)= 0.602
    * V(as)= 14.350 liters (0.507 cubic feet)
    * L(e)= 0.46 mH
    * n(0)= 0.79 %
    * SPL= 91.08 1W/1m
    * M(ms)= 21.92 grams
    * C(ms)= 0.22 mm/N
    * BL= 8.92
    Power Handling = 250w

    Cabinet Design:
    I just did a new refresh on the crossover design in a sealed cabinet that measures 10" wide x 13" high x 10" deep which matched the original few sample cabinets Erich had assembled from Bamboo and Corian. In that size sealed cabinet it will just about hit 80Hz and should be usable with either an 80-100hz crossover.

    A larger ported cabinet could be made 10" wide x 16" tall x 12" deep tuned to 55Hz for better bass extension, that should give an f3 in the upper 50's. Additionally a ported tower cabinet could be used as well something around 1.5cuft tuned to 45-50Hz should yield an f3 in the low 40's.

    Like the Volt-10 these could also be used in an in-wall/in-ceiling infinite baffle installation and crossed at 80Hz without needing a backer box. Though you can certainly use an enclosure if wanting to increase sound isolation for the room or port it for additional bass extension/output. I have a alternate crossover design developed if using them In wall/IB like this which eliminates much of the baffle step compensation on the woofer and removes some padding on the HF driver to give a flatter frequency response and greater sensitivity in that kind of install.

    Photos of the original Bamboo/Corian Concentric-8 cabinets:


    Photos of the drivers:


  2. #2 Matt Grant, Oct 13, 2023
    Last edited: Nov 10, 2023
    Crossover Info:

    Like I said above I retested the design and made some slight adjustments to the crossover largely focused on improving midrange linearity. Here is the schematic for use in a standard cabinet, recommended baffle width of 10":
    Concentric-8 v1 Crossover Schematic (2023).png
    Recommended parts for use with the PCB:
    C1, C2, C3, C4 and C6 should be poly caps, C5 and C7 should be NPE.
    L1 and L2 should be 20 gauge air cores, L3 and L5 should be 18 gauge I-cores, L4 should be an 18 gauge air core.
    R1, 2, 3, 4 and 5 should be 10w while R6 and R7 were designed to be 5 watt to save space on the PCB since little power is dissipated by those two.

    Optional - The resistor R1 can be adjusted up or down to fine tune treble output, making it 12 Ohm will reduce output by about 1.5dB, or 15 ohm will reduce by 3dB if you like more laid back highs. Making it 8 Ohm will increase it by about 1dB, 7Ohm will be 2dB and 6 Ohm is about 3dB if you wanted a sharper sound or to give a flatter response off axis say if the MLP is off axis from these when used as surrounds.

    10, 12 Ohm and 7, 6, 5 Ohm are optional values that give similar results on the In-wall version of the crossover.

    Concentric-8 v1 (2023)-3.jpg
    Concentric-8 v1 (2023)-4.jpg
    Concentric-8 v1 (2023)-1.jpg

    In-Wall / In-Ceiling / IB install Crossover:
    Pretty self explanatory, this crossover should only be used if installing the cabinet or bare driver flush in a wall, column, or ceiling. It eliminates much of the baffle step compensation that's needed in a normal cabinet resulting in a more even response and greater sensitivity.
    Concentric-8 v1 In-Wall Crossover Schematic (2023).png


    It's been pretty poor here weather wise for outdoor measurements the past couple weeks so I have not had a chance to measure the speaker on my larger turntable. But here are some measurements from my indoor turntable which just offers reduced resolution (1 meter distance, 5ms gate, 1/6th octave smoothing) Concentric-8 tested in the sealed 10" x 13" x 10" cabinet:

    VituixCAD Power+DI.png
    Concentric-8 v1 Crossover (v1.00) 2023 Directivity (hor).png
    VituixCAD Directivity (hor).png

    I still need to gather a few more measurements, namely distortion, compression and impeadance. I'll post those when I have them.
    taloyd likes this.
  3. Hi Matt, i sent you a PM about getting some of these. Thanks!
  4. Similar question as the one I asked in the concentric 6 thread: what eminence woofer is closest to the LF driver used in the concentric 8? I ask because I’m assuming these are eminence-made and perhaps based (in part) off an existing eminence woofer/coaxial.
  5. These coaxials aren't built by eminence, that said the Alpha-8A has very close T/S specs to the woofer on this.

    For the Concentric-6 I can't really find anything with close T/S specs to the woofer even from other manufactures.
    Brinkman likes this.
  6. Thanks Matt!

    I’ll look at the 8A; thanks for the suggestion.
  7. Any recommendation on running these open baffle? Would some of the recommended crossover configurations work? I’m of course considering the open baffle would reduce bass, that’s a given. Like the idea of these as a large desktop near field setup, but don’t want huge boxes on my desk.
  8. I really don't know how they will behave/sound open baffle, I ran a quick sim and it looks like the standard crossover would work if you build the baffle itself similar in size to the original. But be prepared for additional EQ to get things sounding right in the lower midrange and bass and much more directional sound in those ranges as well. Also they would not work well if you desk is up against a wall as a straight open baffle design like that needs more space behind the speaker then a conventional cabinet.
  9. Good point on the proximity to the wall, I’ll have some room but not oodles, approx 18”. And thanks for taking the time to do some simulations, appreciate advice on the lower mid and bass EQ. Im kinda looking for tighter directivity TBH, that’s in part why I'm considering a coaxial like this, which has somewhat of a waveguide/horn loading. Figured it would cause less reflections off the desk vs something with very wide dispersion. Although by going open baffle, I feel like I’m moving in the opposite direction in terms of reflected sound? Overall though I like the idea of a compact speaker that can play loud and clean, and be pretty compact, while also being good in a near field situation (and have somewhat controlled directivity!). I’ve been on the hunt for a setup like this and it’s been a tricky set of requirements to combine. Any other suggestions within the DIYSG catalogue? Kinda thought HT-6 might work…
  10. I’m curious how far a passive cardioid implementation with these would work towards increasing the directivity even lower in frequency.

    For reference see this thread, which although not coaxial, the woofer side-port concept is applicable:
    2-way: Waveguide + Cardioid-like
  11. I've been wanting to experiment with some kind of passive cardioid implementation on a speaker like that. I'm sure it would work well with the Concentric-8 though bass response would be reduced. From what I understand it just takes a lot of time/effort in experimentation to optimize the slots and damping to get the ideal polar pattern. I have to figure out some kind of modular cabinet so I can change things around without building a new box each time.

    On the active front I've had promising results from a smaller experimental cabinet I designed though the Dayton Coax was a bit of a letdown.
    CX120 3.5way build indoor sim Directivity (hor) polar1.png
    Box Sketch1.png
    Brinkman likes this.
  12. #12 Brinkman, Jan 25, 2024
    Last edited: Jan 25, 2024
    I still have two pair of the hemp 16-ohm 8” woofers used in the 88-Special. I was thinking of utilizing them to help with the bass response, maybe side mounting them.
  13. My current understanding is inadequate but is the efficiency hit of passive cardioid a result of the cardioid cancellation eating into baffle step compensation resulting in a necessity for increased shelving EQ (if that makes sense)?
  14. #14 Matt Grant, Jan 26, 2024
    Last edited: Jan 26, 2024
    It's sort of similar to an open baffle in that you use contributions from the rear wave of the driver. Though in this case you are trying to restrict how much of the rear wave is allowed to leak through openings in the cabinet, the size and position of those openings along with the type of damping used control where and how much of that leakage occur. When designed right the leakage in the midrange or low midrange can add constructively towards the front of the speaker increasing output, then as you move off to the sides and rear that output begins to shift to out of phase and causing a reduction in output in those directions resulting in that cardioid pattern. However this can only be achieved over a limited frequency range and at some point the wavelengths become long enough that the rear wave leakage is out of phase with the front wave no matter which direction you are and thus resulting in an increase in the bass rolloff.

    In an active setup you can use delay in the DSP with additional side and/or rear woofers to similarly aim a lobe forward for cardioid pattern control, the difference being that at low frequencies it can be setup so that the drivers will all combine in phase and you loose no bass extension or output.
    Brinkman likes this.
  15. I’d like to order some of these, sent you a Pm! Thanks
  16. Dang, this whole cardioid concept has been really intriguing. Given the coax driver that brought this up, along with doodling with the idea of using something close to a wall and a desk, the cardioid pattern would make a lot of sense (less reflected energy of the rear wall/better frequency response on the forward lobe). Dare I say this would be an ideal desktop or bookshelf speaker attribute at least in terms of friendliness to a close rear wall. It'd work great with this driver given the inherent good pattern control.

    Given I've now done a little research, it seems pretty clear that a passive cardioid design that works over a wide frequency range is not easy to design... From what I've seen, there's really only a few commercial companies who've done so: Dutch & Dutch and Fulcrum Acoustics (I'm sure I'm wrong, there's prolly more). Fulcrum seems to have figured out how to do this over the widest frequency range (appears they use multiple passageways and ports on the rear of the cabinet to broaden the frequency range of the cancellation). They seem to have also figured out how to create a more "rounded" rear cancelation shape vs. one with a sharp null right at 180 degrees. Seems like they do this by creating a consistently altered phase/delay range (115-230 degrees for instance) vs it being strictly at 180.

    Anyways, I find this fascinating. I have a lot of questions/comments but I don't think the purpose of this thread is to discuss this topic in detail unless anyone else doesn't care (trying to be respectful). There are admittedly countless other forums to bring this up.

    If no one minds though, I'd like to entertain this:
    Conceptually speaking, for a passive design, the goal is to delay the back wave response of the driver so that its close to or exactly 180 degrees from the front wave? If so, couldn't you considering using a different material for the sides of the cabinet instead of getting the openings perfect? What if you used cardboard, or two layers of cardboard laminated with green glue, or some other cheap/easy laminated material? Couldn't that also delay the back wave in way that allowed you to experiment w/o having to fab up cabinets?
  17. #17 Matt Grant, Feb 7, 2024
    Last edited: Feb 8, 2024
    Update, I can install the dust caps on the newer batch of drivers with the custom silver back-plates if requested. I just did 6 of them and the glue process was simple enough and went without issue, looks no different then the factory installed ones.

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