My open baffle dipole with Beyma TPL-150

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Me and a friend started out with two large pine boards, and three hours later we had two working dipole speaker prototypes. The fastest speakers I've ever build!

We wanted to have a speaker that is true dipole, without using two tweeters like the Linkwitz Orion. The Beyma TPL-150 with the rear chamber and damper pad removed proved to be perfect! It extends more or less flat to 1.5 kHz. Removing the damper pad (which is glued to the rear side of the driver) produced a 5-6 dB peak around 9 kHz, but that was easy to get rid of with parametric EQ.

The midrange is the same Seas W22EX001 used in the Linkwitz Orion.

For the bottom octaves we are going to use a new 21" from Beyma, which has not yet arrived. This woofer named 21SW1600nd was presented at Prolight and Sound in Frankfurt a month or so ago, and production drivers are expected around the end of May. While we wait, a Beyma 18G50 acts as stand-in. The longthrow 21" will produce at least 6 dB more SPL than the 18G50. Not that the 18" is anyway close to bad... there is plenty of bass down to approx 30 Hz.

XO and EQ is run digitally in my computer. There's a long thread on this forum about that: "A how to for a PC XO". XO frequencies are 150 and 1500 Hz, 4th order slope.


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Disabled Account
Joined 2008
Unfortunately, Seas dont have a factory outlet.... well, they do... but you will have to buy at least one pallet of drivers. I dont need fifty W22 woofers right now...

The retail price here in Norway is a bit higher (would you believe that...) than in the US - NOK 1961, which is approx USD 300. At Madisound they are USD 230 or so.

The W22 is however the cheapest driver in this speaker. The TPL150 costs twice the W22, and the Beyma 21" is expected to be around 700 USD at least. Yes - its a lot of money in drivers, but there's no large cabinets to waste your money on... :)
Looking good! I am considering something very similar to this myself. I was thinking of using the Raal 140, so no dipole highs, but after such great success with the TPL here I am considering making the Beyma open backed also. Since you have experience with both styles now, what can you say about the perceived differences? Also, do you have any direct experience with the Raals?

We have a dealership for Beyma, so would it be cost effective for you to purchase drivers from us and post them overseas to you? Just a thought. Will help you out if we can.


PS. Am I correct in thinking that you worked on the Griffin studio monitors?
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Joined 2008
Yes - I designed the original Griffin Model 1, Model 2 and Model 2A.

After this little experiment, I would strongly recommend running the tweeter dipole as well, so you should look for somthing that can be modified for true dipole operation. Most AMT's will work, the B&G Neo3 and Neo8 (although the latter is not a tweeter) will also work, and they sound great too.

I think the TPL150 is absolutely wonderful when modified for dipole. I've tried it with the rear chamber also of course, but I feel that it sounds more open, stress-free and naturally detailed when the rear chamber comes off.

Have no experience with the RAAL ribbons.

I have a Beyma dealership as well, thats how I can get hold of not yet released drivers like the 21SW1600nd.

This experiment proved to be very successful, so we will of course develop it further! The current baffle is not very stiff, dead or heavy at all, being made of 18 mm pine board... Current ideas are 60 mm MDF for the lower frame (around the woofer), and maybe 30 mm MDF for the upper parts. We are also considering a very heavy base, possibly using a thick steel plate. I think we will keep the "wings" around the edge to further increase stiffness.
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Joined 2008
This is not a replacement for my big Gjallarhorn 418. I'm builing the prototype with a good friend of mine - it will be his main speakers. I will build a pair for myself also, just because its fun to have a pair.....

Today I have done some more measurements. The tweeter behaves very well, the only problem is a quite large peak at 1400 Hz, which correlates well to the baffle width at the tweeter height. Thats easy to control with parametric EQ, so I dont worry.

Whats worse, is a series of nulls and peaks in the midrange. There are nulls/dips at 300 and 900 Hz, and a rather large peak at 600. This also correlates well with the rear to front distance around the baffle. There is a bit too much of these peaks and dips, I would like to avoid that. Is the only solution a wider baffle? I did not see this that obvious when I measured the W22 before we cut of the edges to get the pyramid shaped baffle (we started with a rectangular board).

Any ideas regarding the midrange problems?

The bass works fine, much better than expected. There is however no point in adding EQ lower than 30 Hz, as we run out of cone excursion very very fast.
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Joined 2008
Found the source for the 300 and 900 Hz dips...: the floor bounce... :eek:

The large 600 Hz peak remains, but thats easy to control with EQ. Actually its 500 Hz, and the cure is a 9 dB dip there. Yes 9 dB. The speaker is unlistenable without EQ here.

And now - with a pair of absorbers on the floor, it measures a lot better, and its starting to sound really good!

Could you post your measurements of the TPL-150 with open back? It might work in my dipole design. Currently I use the BG Neo3pdr open backed but it is rather low in sensitivity and the PHL 1120 mid I'm using needs to be padded down quite a bit. I'd thought of using two Neo3's stacked in series but maybe the TPL-150 would be better. Do you think the TPL-150 sounds better than BG Neo3pdr?
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Joined 2008
Sure, here it is. Not as flat as 10$ domes though, will need some EQ. The peak at 1.5 kHz might be a bit alarming, but I cross at 1.5 kHz without any obvious problems.

I've used the Neo3 in quite a few designs. I like it, but it cant match the sound quality of a good AMT like Mundorf or Beyma.


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On my design, we initially used the Neo3 and Neo8 just experimenting, and they do sound great when used within their volume limits. We needed quite a bit more volume and better dynamics, so the TPL-150 was the way to go. I meant to try them dipole before the studio monitors were finished, but never got around to it.

Thanks StigErik, The BG Neo3 had the opposite problem. It needed some propping up in the 2k area. I put it in a shallow waveguide which helped a bit. It sounds very good but I'd like to try the Beyma.
Glad I happened upon this thread. I wasn't aware of this one.

Greg, you are a Beyma dealer?
Removal of TPL-150 rear cover

StigErik, I have a pair of the Beyma TPL-150 and I would like to remove the rear cover and give them a try as dipoles.

Could you tell me a little about your experience with removing the back covers of yours?

I obviously absolve anyone here of any responsibility if I accidentally murder one of my TPL-150's...


Thanks in advance.

Disabled Account
Joined 2008
Removing the rear cover is safe and easy. Just remove the four screws, and lift the cover off. To fully modify the driver to become dipole, you will have to cut away the read damper pad as well. This is not very tricky either. Use a sharp knife to cut between the pad and the magnet. Be aware that the magnets are very strong.

Whatever you do, make sure you don't loose any magnetic things like screws into the driver, as these will destroy the driver! The older TPL150 did not have a wire mesh grille in front, the latter ones do have this for protection. On the rear side there is no grille, so if you take off the rear damper, you might consider installing a wire mesh grille here as well.

Below is a picture of the driver with the rear cover removed.


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If you decide to rebuild the cabinets, you might try putting the mid off center of the baffle - this helps smooth the dipole response a bit. Have you played with Tolvan's Edge software? Its good for predicting these sorts of things.

BTW, it looks good. I wish I could hear them!

Would you post some off axis FR curves sometime? I'm currently doing an OB design, and am particularly interested in this.


I'm working on (and listening to) an OB with AMT for some time now and would like to backup StigEric's experience that an AMT used in dipole configuration sounds very very good (well, its been partly his recommendation I was following - after all).

Regarding FR I found that small OB's – even for an AMT design – are much more to my taste than wide ones - though you might give up on some max SPL - which should be of no concern with the Beyma anyway.

Below the polar diagram of an older status of my project with a roughly 30cm wide OB – my latest version OB should look even better, but I don't find time to measure

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

Crossover to the Jantzen is roughly 2.5kHz PC XO.

Please don't care about the irregularities below 1kHz as they are measuring artefacts.
In the lower plot on axis FR is set to be linear to focus on directivity only.

StigEric, good luck and hope to hear even more as the project develops...

Ahead of the curve

Hi StigErik,

I would just like to say congratulations on your TPL project(s), you are ahead of the curve big time with this driver, especially when used in Open Baffle.
I am a lifetime speaker builder addict and for the last 10 years have had a huge amount of fun (and frustration!) with the Manger driver, which in my opinion has been king of the hill as far as any single driver goes.
But the Beyma TPL on the scene the competition is now blazing hot!
I have just thrown to together a hardboard and MDF VPL open baffle for the TPL (back & damper removed) and the fantastic Volt BM251.4 (the 10 inch Pro midrange one) and the bass is the Beyma 15 inch SM115 K in a VPL open baffle.
I use a battery powered DEQX crossover with six channels of ESS DAC's instead of the DEQX internal DAC's.
I reckon I need 6 months to play around with the TPL to really do it justice.
Once again full respect to your goodself for being the first to bring this driver to the DIY market.

All the best

Disabled Account
Joined 2008
Hi Derek
Glad you liked the TPL too! It really shines open-backed.

This dipole is not my first speaker using the TPL. I made a 2-1/2 way with two Beyma 12P1000nd and passive XO, 98 dB sensitivity. It was magic driven by an Audio Note 300B amp... but not as good as this dipole is already in its very early stage of development.

Currently I'm without a high-end DAC, using a RME Fireface 800 now. Its not bad, but not the last word in resolution. In a couple of week I hopefully will receive a DAD AX-24. :D

Love your speakers by the way, have heard one of them in Bergen last year. Great stuff.
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