Compact, low cost, active 3-way speaker


I haven't worked with the SB26STAC, mainly because I wanted to avoid 100mm faceplates. But that isn't a concern (or indeed an advantage) for a 3 way, it's arguably better suited that the SB29SDAC.

The 29SDAC can certainly cross lower, and but top octave is definitely in breakup, which is why the response is the way it is:

Here's the 26STAC, taken from SB's datasheet:




When I was looking at the datasheet some years ago, I didn't pay much attention to that rise in the top octave for the 29SDAC. If you peek at the pixels, there certainly is a peak just on axis somewhere between 10 and 15Khz. Then at 60 degrees off axis, there's a dip at 15KHz.

After I built this speaker, somewhat as a homeage to the late Jeff Bagby, I measured mine, and that characteristic certainly is there.
These peak/dip combinations suggest diaphragm break up, IME.
Now whether or not it's objectionable, or favorable to the listener, well I couldn't say subjectively. But objectively I'd prefer it not be there. @hifijim may be able to account for the on-axis response in his crossover, of course, but that won't change the directivity/dispersion characteristic.

Here was the quasi-spinorama of my build of Jeff Bagby's Revolution mini, which uses the MW13P-4, with the SB29SDAC tweeter:


Context is important, as always. IIRC, the original design by Jeff was commissioned by SBA, and originally used another 104mm tweeter (SBA brand new at the time SB29BAC tweeter). I think Jeff wanted a low cost tweeter option builder who may be turned away by the high cost of the beryllium tweeter. And so he found an alternative tweeter the same impedance response and tweeter faceplate size as the original design, for an easy drop in replacement. AND he needed it to crossover at relatively low 1.8KHz acoustic LR4. The B29SDAC fit the bill.

If @hifijim wanted to experiment with different 100mm tweeters, or offer builders other options eg. SB26ADC (aluminium), then I can see why the 100mm SB26STAC may be a better option than the 104mm SB29DAC.
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My reasoning behind selecting the SB29SDAC is that the rising directivity from 2k-10k matches up rather well with both 4" and 5" mid drivers. When simulating with ideal drivers, a tweeter with an Sd of 9.6 cm^2 (i.e. SB29) works better than a tweeter with an Sd of 6.2 cm^2. Keep in mind this is in the range from 1k - 6k. My focus on directivity is through the crossover region. Whatever happens above 6k is inherent to a particular tweeter and can't be simulated in advance.

I also like the high sensitivity and low Fs of the SB29. It simplifies the passive crossover part of the design.

The SB26CDC or SB26ADC are very good tweeters, but I have used those many times, and I want to experiment with a soft dome tweeter.

I'm not sure the concept of diaphragm breakup is really applicable to soft fabric dome tweeters. They basically operate in breakup over their entire range. They never move as a unified homogeneous piston, there is always some modal shape at almost any frequency above Fs. However, they are so well damped that there is no energy storage, and most of them have a very clean CSD waterfall plot. The SB29SDAC is this way. Whatever is going on from 10k-20k, it does not have any energy storage, so it is correctable with EQ.

I will give the tweeter selection more thought.
These peak/dip combinations suggest diaphragm break up, IME.
The high frequency off axis nulling and on axis rise is all part of the dome and surround shape. Fabric domes still have self interference, quite a few SB drivers have a combination of surround and dome shape that make this happen. This could well be more of a problem for the eyes on a graph.
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Do you know if it due to the interface between the dome and the surround? Or if it’s due to the tip (apex) of the dome?

But to be clear, the importance I place on what happens in the top octave, in the grand scheme of things, is pretty low.

I’m just reflecting on what @A4eaudio said in regards to the SB26STAC, and pros and cons vs the SB29DAC…
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Do you know if it due to the interface between the dome and the surround? Or if it’s due to the tip (apex) of the dome?
Not enough to give you a completely definitive answer but wide surrounds tend to make this worse. To get a decent 15K notch you need a 5.75 to 6mm difference in geometary somewhere seen from the angle where it happens.
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Not enough to give you a completely definitive answer but wide surrounds tend to make this worse
The Scanspeak D2905/9900 shows a similar pattern, and that is a well put together soft dome tweeter... about as good as they come.

Here is the CSD waterfall plot for the SB29SDAC, . Like most well-designed soft domes, it is very clean.


Thank you @HiFiCompass

So I am faced with a decision between the SB29SDAC and the SB26STAC. Both would meet my requirements, and both are within the cost constraints.

Normally I don't like buying drivers just to see if I like them. However these tweeters are about $50 each, so perhaps I will get both of them and do my own evaluation. I have been thinking that I may need to make an XPS foam board prototype of the mid + tweeter to confirm my baffle arrangement. I could make the foam board prototype in a way that both tweeters could be tested... unfortunately, one of them has a 104 mm flange and the other has a 100 mm flange... but I think I can work around that.

At this point I am 90% decided on using the SB15NBAC30-4 as the mid driver. For the price, I have not found anything else that equals its combination of smooth frequency response, low distortion, and clean CSD/ step response performance.

Dayton 10" RS270-4 $131
SB15NBAC30-4 $82
SB29SDAC $52
Total $265

So I am within 6% of my original driver budget, even though I changed the requirement from an 8" woofer to a 10" woofer.

Thank you to everyone for all the excellent advice and information on driver selection.

This is the current concept sketch, showing the size/position of the midrange enclosure.

View attachment 1213773
I like this design and also like your considered open minded approach to the design process... Congrats in advance of listening to all your hard work paying off!
I have attached pics of a really great 10 inch sealed box design using the Beyma 10MC 500 ... I loved the results and was blown away by the detailed supple bass and natural midrange... I ran it up to 500 Hz and Eq'd it flat to 40Hz with a BDX pa 2 it really was a joy. I will try and dig out the sim, regardless of the graphs it sounded way better than I had imagined.
Cheers A.


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I found the sim, I was originally going to use them with subs crossed over at 80Hz, but the bass was so good all the way down I ended up not using subs.
My amps (class A/B solid state) only have 250 watts into 8 Ohms so I ran my sims at 250 watts max power even although the drivers handle 500 watts AES.
Also Xmax... I never use drivers anywhere near Xmax in the real world ie Two speakers, plus room gain and listening at around 96dB peaks on loud days... So I rate the drivers at around 4mm or 50% of there rated Xmax.


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Dropping the power to 150 watts but staying at a conservative 4 mm Xmax gives you 98 dB continuous SPL from 1 speaker at 40Hz... So a pair gives 101 dB with massive headroom available for SPL peaks if you have the amplifier power available. Also, room gain will boost the low end even further without incurring any power or Xmax penalties. Not to shabby from a 12 litre sealed box with ideal critical damping Qtc of 0.5....!!
This is the ideal driver simulation for a tweeter with an Sd of 6.2 cm^2 (SB26STAC). No waveguide, tapered bevel cabinet


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Here is the the ideal driver simulation for a tweeter with an Sd of 9.6 cm^2 (SB29SDAC)


I like the bigger tweeter better. However, the differences are well within the limitations of an ideal driver simulation... Real world drivers will deviate from this prediction in terms of directivity. Since these drivers are not expensive, I will get a pair of both, and do some comparisons.



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I can't speak for all AMTs - 'cause some really suck - but the AST2560 was smooth and super well-behaved. Just a tank with what you could throw at it, but like I said, smooth and sounded good at the same time.

It had low distortion and was free from some of the buzzing and IMD that I've experienced with others. The Beyma TPL75 is the only one with measurements that suggest it might be similar but it's also really expensive. So I'm looking for a reasonably priced AMT that can compete with the AST2560 which seems to have again changed their diaphragms. The new Dayton is interesting. They already had one in a similar form factor and price, so I wonder what the difference is with this one. (I think the $99 might only be an introductory price)
Looks much like the SB29SDAC but in a small faceplate if you were interested in testing a very close CTC, I haven't run the numbers to see if it fits below the problem wavelengths.
Thanks... No I had not seen that, I will look into it to see if it might be useful. I will do some simulation to see how close I can move the tweeter to the mid...

Side topic: I am hoping that SBA introduces a small faceplate version of the Satori TW29B and/or TW29TX. I have a future project in mind that would need a high quality small faceplate tweeter, either beryllium or Textreme. Right now, the ScanSpeak D3004/6040 is my best choice, but I would like some additional choices.