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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
atmasphere's avatar
atmasphere
11,577 posts
 
Amfibus, you have 10 meter speaker cables??! OK- in order to get the most out of the speakers, you will want to shorten those up- over 5 feet and most speaker cables chomp into the impact and definition in a big way! If I had known that earlier, I would have recommended getting that sorted out *before* even taking the speaker apart.

Four ohm speakers tend to be very critical of speaker cables (16 ohms OTOH *almost* don't care). IOW you won't get away with a 10 meter run, regardless of your amplifier.

So I would try an experiment wherein you temporarily use a short run of cable that is only a meter or meter and a half (so you will have to move your preamp and front end to do this), do this with your Cary 211s running full range on the four ohm tap, and see how it sounds then (and report back of course).

I understand that you may need to have your preamp and the rest of the system much further away, but there are preamps that are in fact intended for that (and that are also all tube and zero feedback, so you won't corrupt the sound of the Violins). We'll solve *that* problem *after* you establish the effect of the cable!
amfibius OP
282 posts
 
Ralph, I do not normally use my 10m speaker cables! The 10m speaker cables are left over from when I used to run my rear speakers from the HT and are normally in storage. In normal use - preamp-power amp cable is 5m, and the power amps are right next to the speakers with 1m speaker cable.

I only got out the long speaker cable so that I could experiment with bi-amping ... it was all that I had. I don't want to cut it either, because it is nicely terminated.

Keith
rodman99999
5,984 posts
 
Hello Amfibius: I didn't read any of the responses, so forgive me if any of this is redundant. Your first thought about connecting the woofers in series- You'd be doubling the impedence seen by the crossover and totally changing the filter frequency. If you haven't reconciled your problem: Look into finding a Dahlquist DQ-LP1 passive high-pass/active low-pass filter(usually available used on eBay or on this site). I employed one with wonderful results, bi-amping my system since 1981('til just 3 yrs ago when I sold it for it's original purchase price). You won't need any time-domain correction with your system, so the piece will be perfect for your app. It will allow you to drive your woofers with a SS amp(I use a Hafler TransNova 9505 for my subs)and enjoy the high damping factors and fast transient response attendant, and to bypass any internal crossover to the woofers. The high-pass consists of one cap(I used 2% matched polystyrenes from Michael Percy) and perhaps one resistor(use Vishay 2% bulk foil, if needed) in parallel with the input of your amp to adjust the impedence/crossover frequency. This makes it completely invisible sonically(believe me- I'm picky)! The piece is still supported by it's designers(call themselves, "Regnar" now) and a number of upgrades are available for it. I installed gold WBT RCAs at the ins and outs very easily.
rodman99999
5,984 posts
 
Here's a bit more support for the solution I've recommended. Read this article:(http://www.sonoris.co.kr/review/reviews/tas/exoticagr-part1.html) There's a DQLP-1 at auction on this site right now. None on eBay. A nice thing about the piece: If for any reason you don't want it, they are VERY easy to resell.
atmasphere's avatar
atmasphere
11,577 posts
 
Amfibius, 5 meters is still *really* long. Many 4 ohm speakers will simply not allow lengths like that- Magnaplanar (otherwise a fairly easy load) is a good example- with them its beneficial to turn the amps around backwards and run 6-inch speaker 'jumpers' from the amp to the speaker (the biggest improvement by doing this is in the bass BTW, although the mids and highs get better too. This is one of the arguments for monoblock amplifiers). Seriously, 3 feet or so is about the maximum for many 4 ohm speakers.
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