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Bamboozled

This was originally posted at PETT, but I thought I'd add it here as well.

I'd like to introduce my latest project. This is my most ambitious and expensive project to date. I had obtained several high end drivers on sale, and always wanted to build a dipole, and this is the result.

Design:
I always wanted to try and build a full dipole system, similar to what John K has done, but the cost of a fully active system and needing 4 large woofers (pair per side) had me at a disadvantage. After some research I decided to build a dipole system that utilized a monopole woofer, needing only a pair of woofers (1 per side) to make the desired SPL. I also decided to use a DSP plate amp, utilizing 1 channel for the mid and tweeter, and 1 channel for the bass range. A basic passive crossover is utilized between the midrange and tweeter, with response shaping applied by the DSP.

Driver Selection:
The woofers are Peerless XXLS 830847 10" subwoofer. These were chosen as I purchased the last few available at Solen, which were on a significant discount when the driver model was discontinued. They offer low distortion, relatively high sensitivity considering the xmax, and a healthy xmax and power handling.

The midrange are Peerless HDS Exclusive 6.5" 830883. These were purchased from the swap meet forum at DIYAudio several years back. I had used them in a centre channel for a few years, which was later dismantled. They are a excellent driver and found a very nice home in this speaker.

The tweeter is the Dayton AMTPRO-4. These were purchased from a forum member via the classifieds here. They are the model with a removable face plate, different that the faceplates currently sold. I originally had the thought to remove the face plate and mount the tweeter directly to the baffle, but eventually decided against it. These came with an ugly "veil" over the tweeter which was removed. These are a truly high end product, and are a fully dipole capable tweeter, the rear wave can be easily damped as well by adding felt (included with the tweeter).

Baffle Detail:
The baffle is a 1.5" thick bamboo counter top from Lowes. Originally I was going to use baltic birch plywood, but I liked the look of these boards more and the price was right so here we are. The baffle shape is optimized for dipole operation, and the shape was cut by CNC. I was fairly disappointed with these boards after cutting them, as there were significant gaps inside that needed to be filled. The gaps were filled with sawdust, then epoxied over top and sanded down. The round-overs were completed by hand.

Finish is Circa 1850 Tung oil. As far as I know this is straight tung oil with only thinner added (no varnish). I really liked using it for another project, and I am not disappointed here either. 4 Coats were applied, with a rub down using super fine steel wool in between coats. What is left is the natural wood lustre with a nice sheen. Some scratches and dings are visible up close, but I am real happy with how these turned out.

If I was doing this again, I would add a small brace in the middle of the port, as the bottom of the baffle does have some flex to it.

The baffle is 14" wide by 42" tall, which puts the tweeter at face height when sitting down. The baffle will not be glued to the cabinet, it is held on by steel angle brackets (standard hardware store materials) and a foam gasket prevents air leaks. The brackets are screwed in from the inside of the cabinet, it is awkward but they are all accessible through the woofer hole to remove the baffle if needed, or reaching my arm through the back by removing the plate amp.

Cabinet Detail:
The cabinet is simple 3/4" MDF with 1/2" round-overs. The finish here is White Duratex, applied with a foam finishing roller. This created a very fine texture, much finer than when using the standard Duratex rollers. It was tougher than the normal application to get the roller marks to not show, but I settled after 4 coats. It looks great next to the natural Bamboo colour.

The cabinet if I recall is 60 Litres, tuned to 30Hz.

Crossover detail:
The crossover is a hybrid design, utilizing both DSP and a passive crossover. I got this idea after looking at another project doing just the same, using a Seas Subwoofer and a Seas Excel coax (Seas KingRO4Y).

A simple passive crossover is used between the midrange and tweeter, that provides the necessary transfer function to cross over as well as attenuation of the tweeter. The passive crossover does not apply any other response shaping such as BSC or dealing with the rising response of the tweeter. This was tamed using the DSP as well as any other response shaping to suit the room and listener. The passive crossover is electrically a 2nd order on the midrange, a 3rd order on the tweeter, and a single 3 Ohm resistor in series with the tweeter provides attenuation. Crossover frequency is 1500Hz.

The DSP crossover is used between the midrange and woofer drivers. This arrangement makes the best use of the available amplifier power, and the crossover here is 300Hz. The availability of the DSP allows any further system tweaking to by applied on the fly.

The DSP amp is a Hypex AS2.100D purchased from another forum member in the classifieds here.


Completion
So far I am absolutely amazed with the sound. The clariy through the entire midrange up to the trebel is astounding, and I felt the impact of the bass was very nice, it should need only small adjustments with the EQ. There is something special about the dipole arrangement I think, the stereo separation and spaciousness is like no other speaker I've built.

Photo album is available on Google Photos below. There is a description available in most photos, just press the little "i" button.

https://goo.gl/photos/ACvpPpDDFTTUMypM9
My opinions are 100% factual
Nicholas_23jr@macBryan@MACkennykgreywardenJasonPPWRRYDR-Carpenter

Comments

  • What are your thoughts on the top end on those amt's?
  • Its the best sounding tweeter I've ever heard, and I think you could push these down to 1kHz or lower if you really wanted. The downside is the physical size, which of course limits the vertical listening window, so plan to aim them directly at you.

    They also have a very wide horizontal pattern, as a result I found the best sound from them was to leave some of the tweeters natural rise in place, with a bit of a dip in the on-axis response in the 2-4khz region to help even out the power response.
    My opinions are 100% factual
  • The limited vertical dispersion does provide the benefit of avoiding reflections from the floor and ceiling. These were easy to measure for me since my nearest reflection didn't show through in the response.
    My opinions are 100% factual
  • See, I had to add the AMT mini to my 442 project because the PRO just didn't have the top end to keep me happy, otherwise it is a fantastic sounding AMT. I'm crossing mine at 1k
  • I don't have a problem with the top end as long as I have them faced at me, but I also leave a bit of the top end rising response, and added some extra EQ in the top octave as well. DSP got me there, and I'm not disappointed in the least.

    I did consider the extra super-tweeter, but I really wanted to avoid another polar lobe in the top end response. Where did you end up crossing to the mini-AMT?

    My opinions are 100% factual
  • To give you some idea, I pulled some photos from my album. I've tweaked things a bit further since these screenshots were taken, but you'll get the idea.

    Here's the DSP curve, keeping in mind that my passive crossover does not flatten the response at all:


    In this response measurement the black line is the on-axis response, and the red line is a horizontal off-axis response average where I weighted the forward facing response greater than the wider angles. I think this red line gives a better picture of the presentation than the on-axis alone.



    The top octave has been improved further since this measurement. The response below 100Hz can be disregarded as room response, these were far away from the back wall which is why you see a big null at 60-70Hz.

    If I hadn't mentioned it previously, my passive crossover between the mid and tweeter is at 1500Hz simply because that's what worked best for the driver interaction with a basic 2nd order filter.
    My opinions are 100% factual
  • I think this is a more recent measurement after further tweaking. Here I applied an EQ curve based on the weighted average response above, rather than simply applying EQ to the ox-axis result. The difference looks quite small, but I found this result to sound a bit more natural than the previous attempt, mostly in the upper midrange.



    kennyk
    My opinions are 100% factual
  • I would love to hear these!  Thanks for sharing.  I maybe shooting some questions to you as I want to build a bigger and better hybrid OB like you have done here.  I love the little ones I did and want to scale up.
    dcibel
  • Happy to help where I can.
    My opinions are 100% factual
  • I crossed around 10k, simple 6db cap. You can see that even eq can't make that AMT play up top. It prolly helps, but to me being used to the top end of a ribbon, they just didn't cut it for me, hence the mini
    dcibelJasonP
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