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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
downunder
1,329 posts
 
Keith

Borrow a pair of CAD 500MB's from your mate at audiophile or better still borrow a boulder amp from Sensis where you bought the speakers.

One thing for sure, I recently tried a Gyrphon Encore, Levinson 436's and 33h's. They certainly have more control than any tube amp. the bass control and added perceived octave the 33h's gave to my speakers were incredible compared to my tube amps. Midrange and treble were another not so clear storey :-)

Considering you can still run tubes on top, you may them have the best of both worlds.
atmasphere's avatar
atmasphere
11,676 posts
 
Rlawry, The speaker you have has only a single woofer and is an 8 ohm load. The way an amplifier handles that is quite different from how it reacts when the woofer load is 4 and the rest of the speaker is 8, especially if the amp is question is a tube amplifier. Your speaker is an easier load for tubes (and transistors for that matter) so your results will be a bit different!
rlawry
477 posts
 
Hi, Ralph, thanks, yes, I would imagine my single woofer version to be an easier load, even for my big SS amp. It will be interesting to hear what Amfibus finds when he tries driving his suboktav version with a SS amp. I must admit the indicated woofer alignment seems a little strange, but I would be hesitant to alter the driver arrangement prior to trying a possible easier fix such as an amp with a much higher damping factor.
amfibius OP
282 posts
 
Hi all, thanks for your suggestion of trying to bi-amp with a SS amp at the bottom. If I am going to go active, I will need two more channels of amplification anyway so a beefy SS amp is on the cards.

NOW ... given that I only need to run these from 30Hz-450Hz, and I have my Cary CAD-211AE on top ... what amplifiers would you guys recommend? I was thinking of the Cary CAD-200 - inexpensive and should be "enough".
rlawry
477 posts
 
I have had BAT SS amps, the VK500 and VK600, and the bass was pretty darned good. The only thing I didn't like was the lack of soundstage size, but for bass, they are great. You might be able to pick up a used VK500 cheap. I would think just about any SS amp with a big power supply would work pretty well for bass and lower midrange, like one of the big Conrad-Johnsons, Krell, Mark Levinson, Classe, etc. The Cary would probably work, although I am unfamiliar with their new SS amps. I have had a couple of their tube jobs and they are great.
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