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.

Showing 4 responses by audiokinesis

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 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.

Amfibius, thanks for the link to the photos. I can't tell much more about the woofer from the photos, unfortunately - I was hoping it would be identical to something in the Madisound catalor.

Eyeballing the crossover I see two notch filters and a contour filter, along with some impedance equalization. Looks like some high quality parts, by the way. The absence of an unbypassed inductor makes me think the voice coil inductance is on the high side, so I now propose a zobel consisting of 45 microfarads in series with 10 ohms, along with a 10 millihenry series inductor, to roll off the signal going to the inner woofer.

Isobaric doesn't mean both woofers get the same signal across the spectrum - in fact, in my opinion they shouldn't. If the inner woofer is allowed to play up into the midrange, in effect you have twice as much unwanted midrange energy bouncing around inside the box and re-radiating through the woofer cone. Isobaric loading is only beneficial in the deep bass region.

Keith aka Amfibius, "isobaric" means constant-pressure. It can be accomplished by many different internal geometries, and does not require that the internal woofer be fed the same signal as the outer woofer. I used to build 'em. The very-close-coupled format you describe sounds like what we used to call a "compound woofer" (often configured with the woofers mounted face-to-face), which was only used in the bass region. For operation up into the midrange, a larger inter-driver spacing was used to allow the use of stuffing between the drivers.

Ralph aka Atmasphere, one of the issues with running isobaric woofers in series is that the two woofers will have dissimilar impedance curves because they will have significantly different resonant frequencies. The inner woofer may well have a resonance an octave higher than the outer woofer. Since it's the outer woofer that we're listening to, in most cases it would be detrimental to have in effect a very high resistance in series with it in the octave above its resonant frequency. The impedance peak of the inner woofer would approximate a series notch filter.