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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
amfibius OP
282 posts
 
And Shane ... I have not contacted the manufacturer. Not too sure if they speak English (they make this lautsprecher in Deutschland).

I should also say: I realize that I have probably come across as being pretty negative about this speaker so far. Maybe I have been a little unfair, because in all honesty the bass does not bother me that much with 80% of my recordings.

I do know that the construction is very good. It is fully lead-lined and extremely heavy. The finish is close to flawless. And as Duke mentioned, the parts used in the crossover are of high quality.

The midrange and top end, where most of my music lives, is of supreme quality. Nothing else sings with the same delicacy and speed. That horn does not sound like a horn at all. Some other horns sound a bit quacky but not this one.

However, on some recordings the speaker is brought to its knees. Once I notice the problem, I focus on it and stop listening to music. That's not a good thing.
atmasphere's avatar
atmasphere
11,676 posts
 
Amfibius, I have a simple idea that you might try, since it appears that you have access to people that are speaker savvy. See if you can arrange to put the woofers in series and readjust the crossover to support the same notch frequencies.

Tube amplifiers, unless endowed with a lot of feedback (which makes them sound more solid state) are not be happy with an 8 ohm midrange and tweeter, along with a 4 ohm bass unit. The bass will be loose and flabby sounding, owing to the increased distortion of the amplifier (NOT as popular mythology has it, lessened 'damping factor').

Setting the woofers at 16 ohms will make the speaker much easier to drive with almost any tube amp. The result will be improved detail and a greater sense of authority; this will be a load a tube amp can handle. It will also take care of the rear woofer not being crossed over.

BTW they may voice the speaker with the Einstein, but if they had done that with the woofers in series, the result would have been even better. Sixteen ohms is an easy way to get smoother sound and more detail out of any amplifier.

audiokinesis
2,729 posts
 
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.

Duke
amfibius OP
282 posts
 
Ralph - sounds like a great idea! I'll ask him what he thinks of it. Would the caps and inductors need to be changed as well, or only the resistors on the notch filter? Could be a good temporary solution.

Duke, very interesting and helpful comments, thank you. It appears that my understanding of isobaric enclosures is incomplete and there is a different type of isobaric as you have just described. I will have to do more reading on this - there may be a method in the madness after all.
atmasphere's avatar
atmasphere
11,676 posts
 
Amfibius, Duke's comment sounds like one to be paid attention to. So if you proceed, keep his comments in mind!

The crossover parts would change as the impedance would be different and you would have to set up for the same notch frequencies, and if Duke is right, probably a notch for the unwanted secondary impedance peak.

I'm glad I make amps and not speakers!! :)
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