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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
oshag
11 posts
 
Amfibius,
I thought very seriously about buying this speaker (the high version) but after extensive listening, chose not to proceed.
The Violon is somewhat of a paradox, at once offering such a level of performance in one area that other designs cannot attain, but then falling short on performance in other areas. I've found the upper midrange and HF to be stunning. I've not heard such purity in any loudspeaker before and yet, the lower mids, mid bass and bass, detract from the standard of performance set by the upper-mid and high-freq.
Your findings make sense to me, judging from what I heard for myself. Its certainly not a great idea to run drivers in full range if it can be avoided. The only reason I can think of as to why the designers implemented a crossover-less design for the bass drivers, is to re-inforce the lower midrange and mid-bass regions. strange. If you can bring the performance level of the bottom half of the speakers frequency range on par with the upper half you will have something very special indeed.
rlawry
479 posts
 
I agree with you, Oshag. The midrange and treble on this speaker are stunning. Detail in these areas is the best I have heard, bar none. Soundstaging, dynamics are state of the art. Unfortunately the bass is just ordinary. Extension is OK, speed and pitch definition are good, but the bass doesn't have the weight or dynamics of other speakers, especially the Wilson Watt Pupp 7's and Sophias I had. I spent a really long time getting my JL Audio subwoofer to integrate with the High Violins, and now the bass is liveable. BTW, I just assume my speakers have the single bass driver. On the back there is an emblem stating "High Violin 2001 Bass." Maybe someone knows which bass driver version they are. Thanks.
amfibius OP
282 posts
 
Well i've got in my living room a Cary CAD-200 solid state amplifier which I just borrowed from the dealer. I will unpack it later and see how well it controls the wayward woofer.

Rlawry, mine says "High Violon 2001 Suboktav". Weird - I would think that you and I have the same model being the "High" version. In any case, it is quite easy to dismantle the speaker to have a look - undo all the Allen bolts you can see on the front and top panel, then push the woofer out from the inside.
kotjac
13 posts
 
Amfibius,

The CAD-200 is a great amplifier and definitely one of the best kept secrets in audio. Absolutely an amazing value! It has a beautiful midrange and top end reminiscent of Cary tube amplifiers. Internally it is almost identical to the CAD-500 monoblock and soundwise it is virtually indistinguishable from the monos. Just like the monos it has this really nice signature Cary midrange, but to some extent it also has a signature Cary bass. It should have better control than your CAD 211s but I don't think it will make your problem disappear. What you really need is an amp with a high damping factor that could take an iron grip of these woofers. I would look into something like the Spectron Musician III or Belles 350A Reference. I own the Belles right now and honestly, I have never heard an amplifier that can do bass better that this little beast.
amfibius OP
282 posts
 
Thanks for your input Jacek. I have been listening to the CAD-200 for the past few hours.

Result: it HAS removed most of the muddiness and the bass has better definition - but it still does not go very low, and the woofer is still noticably slower than the mid and the top end.

As a negative - it sounds lifeless. Not unexpected I suppose, given that it is less than 1/4 the price of the CAD-211AE, and that it is solid state not tube.

I would like to try bi-amping, but for some reason it does not like my 10m long speaker cable (!?!?!?!?) and refuses to drive it. Weird.
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