High efficiency dipole

[oneoldude]

A friend has just bot one of those puny single ended tube amps. He has asked me to help develop a speaker for it. He has a powered woofer to use on the bottom and an active crossover for the W to TM cross. So I thought a high efficiency dipole would do the trick for the mids on up.

Current plans are to use a Fostex 208E Sigma as the mid and perhaps a Ribbon on the very top.

He has ordered Olsher's Basszilla plans to see his suggestions and they are in route. In the meantime does anyone have any suggestions as to a good value tweeter (ribbon or otherwise) that would be a good match? By good value I mean reasonable performance for a low price.

Also, if anyone has some suitable parallel crossover and voicing suggestions, I would love to see them. I am told Olsher uses series crossovers and I find them very hard to work with.

Any other suggestions re: high efficiency dipoles for the described purpose will be greatly appreciated.

[JonMarsh]

Either parallel or series crossovers can have very low insertion loss.

LspCAD6 works much better than the older version for series crossover design- witness my "quasi-series" Elaine Marie, which wouldn't be possible to model in LspCAD 6. No experience personally with the Fostex.

Might look at Fountek NeoCD2, 98 dB/2.83 VRMS. Similar to the JP2's I have for the Saint-Saens.

[Saurav]

I'm using an Audax PR170M0 midrange and a Fountek JP-3.0 ribbon tweeter on an open baffle. Not the best choices if you were starting from scratch, but the Audax sounds pretty good to me, even though it looks somewhat nasty on paper. But my speakers are definitely hacked together, I wouldn't call it a 'design' by any stretch. It sounds good to me though, and I had a 3.5W amp on it in a 15x25 room and it went loud enough for me. My crossover was 2nd order on the woofer and 3rd order on the tweeter (electrical), shooting for 4th order LR acoustic slopes. I'd say I was at 93-95dB overall efficiency even after that crossover.

I want to figure out a dipole woofer solution. My speakers are in a much smaller room now, and I'm wondering if there's anything I can build that would be good up to 500Hz and could run off of a 25W chip amp (I know, Jon ), so about mid 80's sensitivity. Would the Extremis work in such an application?

And I think you might want to pay closer attention to the impedance if designing an XO for a high Zo tube amp.

[Mark Seaton]

With Parts Express now carrying B&C, you might want to look through some of their mids/mid-woofers at 6-8" diameter which have some quite significant sensitivity with efforts put forth for lowered distortion and better HF extension. Their larger woofers tend to have very low Qts making them most suitable for vented or very high cut-off sealed boxes, but the stuff is of extremely good build quality. Some of the small blips I see in certain drivers suggests either there are dimension-related blips, or there is something in the test setup.

Just a rather different option to consider.

[Saurav]

OK, I'll look into those (if your post was directed at me ). I really don't know anything about any of these drivers. I mentioned the Extremis only because I believe they go down pretty low, so I was wondering if I could get away with keeping a small profile, not using a sub, and still getting decent bass response in my room. I have no idea if it can be used as a dipole from 500Hz down to ... I dunno, wherever I hit dipole rolloff, which depends on the baffle ... and that's as far as my thoughts have gone so far.

oneoldude, I had looked at some of the Fostex drivers when I was building my speakers too. Unless I'm looking at the wrong driver, the 208EZ has a Qts of 0.18. I'm not sure how well that will work on an open baffle. Everything I've read says that you want a driver with a highish Q to work on an OB. Though it also has an Fs of 42Hz, so maybe the Q doesn't matter if you cross to a woofer at a few hundred Hz? I remember that I hadn't found anything in the Fostex catalog that was a largish midrange/fullrange, mid-90's or higher efficiency, 0.5 or higher Qts, and within my budget (which was lower than yours). On other forums I've heard PHL mentioned often as a choice for hi-eff midrange units. Don't remember the model numbers off the top of my head.

[oneoldude]

Saurav - I think you are right on the low Qts issue. If you are in the fat part of the response above Fs you don't need to worry. And keep in mind that speaks like the Fostex go for very strong bl products that will result in a low Qts. Supposedly the strong motor will reduce distortion etc. while the cone is very light and breaking up all over the place. :roll:

A couple of drivers that look quite good from a paper spec standpoint are the Eminence Beta 8A and APT-80. The Beta sports a sensitivity of about 100 dB, Xmax 3mm,Fs 58 Hz, Qts .34 and a smooth response. The APT-80 sports a sensitivity of about 105 dB, Fs of 2,500 and a smooth response. Or so says Eminence. :B

These are high efficiency pro audio drivers that are made to work together and would seem to be quite simple to match. But for all I know they may be distortion/breakup monsters and sound like %$#^%#! :M

Anybody here have any experience with them? Any suggestions where I can go to get some practical info and/or advice on how they sound or perform?

After all, if they are close to Fostex and Lowther in performance (paper cones and all), it sure would be great to buy American, save some cash and keep the trade deficit down. :T

[oneoldude]

Hey all,

Another way to look at the Eminence drivers is that the 8" M is rated at 225 W and the T is rated at 45 W. That means in their mid bands and above, at full rated power, they should produce around 120 dB SPL! :E

Now if they are being driven by my friend's 6 W amp, they surely should not be strained in any way. Aside from the fact that you cannot escape horn distortion (T), might they be worth a try? Hmmm.

Advice, thoughts and suggestions are surely welcome. :T

[Saurav]

I've heard the APT-50. I've owned the Adire HE12.1, which is an Eminence Beta 12CX (the coax) with an Adire modified APT-50 mounted in the middle. You can see the plans on the Adire website, especially the XO that was needed to get the two to work. It sounds pretty good... my previous speakers were (very good) small monitors with 5.25" woofers, so the change in dynamics was an eye/ear opener.

Having said that, I think the Fountek ribbon I'm using now is a better tweeter. I've also tried a Selenium DT150 compression tweeter in my HE12.1s. That got me the increased HF extension over the APT50 that I was looking for, but it had a nasty peak at around 4-5kHz that I could never get rid of. But back then my only tools were an RS SPL meter, so I'm sure someone else could have done a better job. At the same time, I could see the peak on the SPL meter with test tones spaced 1kHz apart, so I wouldn't recommend the Selenium either. I finally gave up when I heard someone else's speakers and realized that my treble was harsh and distorted in comparison. The ribbon has better extension and a sweeter sound. I don't know how much of this applies to the APT-80. I think the PSD series are supposed to be better tweeters, but that's all hearsay.