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The Anatomy of the Acapella Violon - shocking find

For quite a while now I have been having trouble with the bass on my Acapella High Violon Suboktav 2001. With the help of a friend, we have dismantled this speaker and studied the internal construction and measured the crossover points. I am hoping that my findings will be helpful to all of you Acapella owners.

The bass problem is this: bass can not keep up with the speed of the midrange and top end. On some recordings, the bottom end becomes disconnected - you can hear music from the midrange and the top, followed by the bass response a microsecond later. Furthermore, the bass is poorly controlled and flabby. From my other Audiogon threads, you can see that I have been wondering whether the damping factor of my Cary CAD-211AE amps is sufficient to control the wild bottom end, and whether a solid state amp will cure this problem.

The current iteration of the Violon is Mk. IV. I am not sure what a "Violon 2001" is, I am guessing either Mk. I or Mk. II. Acapella's own website does not reveal any secrets, all it says is that the High version of the Violon has an additional driver inside.

Anyway, this is what we found.

PLASMA TWEETER: 4th order high pass crossover (24dB/oct) at 5000Hz. Measures very flat all the way to the limit of measuring equipment. Incidentally, Acapella marks the recommended tweeter level with a pencil mark on the tweeter volume pot. At the minimum recommended range, the tweeter comes in 12dB ABOVE the reference SPL. I had to wind the tweeter almost all the way down to get a flat response.

MIDRANGE HORN: First order high pass crossover (6dB/oct) at 450Hz, with a very gentle taper between 3dB/oct - 6dB/oct from 5000Hz and up. Goes all the way up to 10,000Hz. The shallowness of the low pass section of the band pass crossover makes me suspect that it is relying driver rolloff.

This is a surprisingly wide band of frequencies (4 1/2 octaves) to ask a horn to handle. As you know, horns are tuned to work over a fairly narrow frequency range and the response drops off at either extreme of this range. Wavelengths which are too long for the horn do not couple with the horn. Wavelengths which are too short will bounce around chaotically. That is only the theory however, because the horn measures very flat between 450Hz - 5kHz.

The integration between the tweeter and midrange horn is very good.

BASS UNIT. As advertised, there are two 10" drivers in the unit, and both appear to be SEAS drivers. The external driver is run through a passive crossover from the binding posts, with a low-pass first order crossover at 450Hz.

And now, the surprise. The internal driver is run directly from the binding post with no crossover in between. In other words, it is run full range, relying on driver rolloff only.

I can think of no advantages for a setup like this, only disadvantages.

Firstly, the drivers are wired in parallel. This will drop the impedance, making it difficult to drive them with valve amps (Acapella supposedly voice the speaker with the Einstein OTL).

Secondly, a configuration like this will result in destructive interference between the two drivers, ESPECIALLY if the crossover introduces phase problems in one of the woofers. Given the other woofer is crossover-less, any difference in phase will definitely cause interference.

Thirdly, running a woofer full range will cause cone breakup at the top, which will muddy the lower midrange (exactly what I have been hearing).

Possible solutions:
- wire both drivers to the crossover, maybe in serial configuration to increase the input impedance (Zin),
- disconnect and remove the internal driver, i.e. convert the speaker from the "High Suboktav" version to normal Violon,
- remove the passive crossover entirely and use a preamp-level crossover (active crossover).

Now, I am no speaker designer. I am just an enthusiast struggling to understand these things. But this just makes no sense to me. I am hoping that someone with more experience will be able to explain why Acapella made these design choices, and what you think of the possible solutions.
amfibius
audiokinesis
2,737 posts
 
Let me see if I understand: The woofer that faces the outside world has a 450 Hz first order crossover, but the inner woofer has no crossover. Is this correct?

Are these woofers identical?

The crossover on the outer woofer - what does it consist of? Just an inductor? Inductor and capacitor? Inductor, capacitor, and resistor?

Are there in effect two internal chambers, one behind the inner woofer and the other between the outer woofer and the inner one?

Are either of these chambers ported?

Are they 8-ohm woofers, or do you know?

Just trying to envision the situation at this point.

Thanks,

Duke
amfibius OP
282 posts
 
Duke, thanks for your response.

Correct - the woofer that faces outside has the 450Hz crossover, and the internal one is full range.

Yes, the woofers are identical.

The woofer crossover has about a dozen parts. 3 inductors. Across each inductor is a capacitor. And there are a few resistors around. I will be getting a circuit schematic so that I can better understand what the designers had in mind, however my friend (who has my speaker at the moment) said that they look like notch filters.

The chambers are not ported, and in effect there are two internal chambers.

I do not "officially" know if they are 8 Ohm woofers or not, but my guess is that they ARE. This is because the "standard" Violon (which only has one woofer) is nominally 8 Ohm. The "high" version which I have is 4 Ohm. Given that I now know that the woofers are wired in parallel, it would make sense that it is an 8 Ohm woofer.

Thanks,

Keith
audiokinesis
2,737 posts
 
Keith,

Thanks for the information. I think the woofer system you have is what would be called a "sealed isobaric".

It has been twentysomething years since I worked on an isobaric system. Basically, the inner woofer dances in sync with the outer woofer in an attempt to keep the pressure constant in that chamber between the two. "Isobaric" means "constant pressure". I did some measurements of an isobaric system years ago and my findings contradicted the conventional wisdom of how to model an isobaric system, but I won't bore you with the details.

Okay, in my opinion the inner woofer of an isobaric system should be rolled or else it mucks up the midrange. I do not know what order the rolloff should be, but either first order or fourth order make the most intuitive sense to me. In my homebrews, I used a first order rolloff on the inner woofer because that was much simpler to do.

There's a good chance that the discontinuity you hear is due to the inner woofer blaring away into the back of the main woofer. In my opinion, it would be a good idea to roll off the inner woofer above 80 Hz or so, and we'll try a first order network because that's probably the safest bet.

If you can by any chance come up with the model number of the woofer, that would be very valuable. Even just a physical description of the woofer would help.

If not, then we can "wing it" and use a 16 millihenry iron core inductor in series with the woofer (like a Madisound Sledgehammer), stabilizing the impedance with a SWAG zobel consisting of a 30 microfarad cap and a high-power 10 ohm resistor.

If you can come up with any identifying information on the woofer, or even just a physical description (cone material, phase plug, surround description, magnet description, frame description) I can hopefully make a more educated guess on the component values for the inner woofer's crossover.

Duke
timrhu
2,779 posts
 
Given that I now know that the woofers are wired in parallel, it would make sense that it is an 8 Ohm woofer.
I'm not a speaker designer but doesn't this mean the internal woofer is operating at the same frequency as the other and not full range?
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