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 13 responses by amfibius

Duke, thanks for your response.

Correct - the woofer that faces outside has the 450Hz crossover, and the internal one is full range.

Yes, the woofers are identical.

The woofer crossover has about a dozen parts. 3 inductors. Across each inductor is a capacitor. And there are a few resistors around. I will be getting a circuit schematic so that I can better understand what the designers had in mind, however my friend (who has my speaker at the moment) said that they look like notch filters.

The chambers are not ported, and in effect there are two internal chambers.

I do not "officially" know if they are 8 Ohm woofers or not, but my guess is that they ARE. This is because the "standard" Violon (which only has one woofer) is nominally 8 Ohm. The "high" version which I have is 4 Ohm. Given that I now know that the woofers are wired in parallel, it would make sense that it is an 8 Ohm woofer.


Duke, my understanding of an isobaric woofer is that BOTH woofers receive the same signal. In this case, the outer woofer is receiving a signal from the crossover, and the inner woofer is receiving a full range signal from the amplifier. All crossovers introduce phase issues, particularly a crossover as complex as this one. If there were ANY phase differences between the two drivers (in the same enclosure!) they would interact and produce peaks and dips in the frequency response.

I think that Acapella themselves have realized this, which is why there are notch filters built-in in the crossover.

If you want to see pictures of the dissected speaker, take a look here:

One of the pictures shows the back of the woofer (notice the flimsy looking spider?) with what appears to be a model number - SEAS CA25ACA. There is also a picture of the front of the woofer. A google search fails to bring up the driver. A pity, I was hoping to find the T-S parameters.

Brian, thank you for your suggestion however I do not believe a replacement amplifier will solve the problem. A solid state amp will only improve the damping factor, and it looks as if Acapella have "set in wood" the problem with the bass.

Having said that, I can easily borrow some amps from my dealer and see whether 600W of solid state Class A/B would be enough to drive the speaker. I realize that an amp like this will probably ruin the top end, but it should immediately answer the question of whether it can make the woofers move.

In another thread, I asked you if the updates to the Mk.IV version of the Violon can be retrofitted to mine (Violon 2001). The answer you gave was "no". My option at present is to sell the speaker or to modify it.
Duke, the woofers are NOT in isobaric configuration. Isobaric (as I understand it) means: two woofers, fed the same signal and wired in parallel, located very close together. At bass frequencies, the distance between the two woofers is much less than a bass wavelength - meaning no standing waves.

The woofers are about 50cm apart, with the lower woofer firing into an enclosure which is stuffed full of ... umm, stuffing. My friend (who is a speaker engineer) commented that placing them 50cm apart and running up to 450Hz between them would surely cause standing waves. I mean - the wavelength of a 450Hz wave is 76cm. You only need 1/2 a wavelength to set up a standing wave. I calculate that standing waves between 343Hz to the crossover point could potentially be supported.

He also commented that the intention of the stuffing (for want of a better word) may be for acoustic filtering. The High Suboktav is marketed as having an additional woofer allowing for deeper extension. Perhaps this was the intention.

I was unable to find the SEAS CA25ACA by google search, so I can not tell you what the T-S parameters are. However, there are some other SEAS drivers which are almost similar so the T-S parameters are probably very close.

My friend noted that the cone suspension is not very rigid, and it is very easy to make it reach maximum excursion (Xmax).

I am starting to wonder if the non-Suboktav version of the Violon may have cleaner bass. What do you think, Brian? Thank you for your suggestion of trying different amplifiers, but like I said - if the problem has been "set in wood" then it won't go away no matter how many Watts I feed it.

And BTW I am not too worried about resale value. I am a very slow upgrader and I tend to hang on to my goodies until they die. I have NO INTENTION of selling this speaker, because the mids and the top end are sensational. If I could fix the weakness, I would be very happy.
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.
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.
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.
Sorry for not responding for a few days. I went and did some research on this type of woofer so that I can better understand the designers intentions.

However, all my reading indicates that the inner woofer should be rolled off lower than the external one. I can not find any reference that says that running the internal driver full range, and the external driver crossed over, is a good idea.

I think I have hit the limit of my knowledge - either the Acapella designers made a mistake with the design, or there is something about this configuration which I am not aware of.
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".
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.
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.

Thanks Rodman and Ralph. Rodman, I do not think that equalization would help because (1) the rolloff at 60Hz can not be fixed with equalization and (2) the problem is with system Q, which equalization can not fix.

Ralph, I said that the interconnects are 5m long, and the speaker cable is 1m long in normal use :)
Couple of months down the track, and I have had the opportunity to try a number of different tweaks on this speaker. To cut a long story short: none of them have fixed the bass problem. Anyway, this is what I have tried:

- PAUL SPELTZ AUTOFORMER. Impedance multiplying device to improve damping factor. Very slight effect in the bass but detrimental to mids and highs.

- CARY CAD-200 AMPLIFIER. Enough power to poll the woofers and gave much better bass definition - but still muddy and still lacking in low bass. As a minus, substantially inferior to the CAD-211AE in the mid and top.

- JEFF ROWLAND AMPLIFIER. This is the 150W Class A/B model, before they switched to Class D. Like the Cary, this amp had enough guts to poll the woofers. Mid and top end almost comparable to the Cary CAD-211AE - cleaner, sweet instead of warm, but the CAD-211AE had more detail and better soundstaging.

- AUDIOANALYSE AMPLIFIER. This is an ancient 20 year old 75W Class A power amp. Again, much improved bass but dry in the midrange and top.

- AKSA SORAYA / AKSA GLASS HARMONY AMPLIFIERS. Beautiful amps. The Soraya was too SS sounding for my taste. The Glass Harmony was closer to the 211AE in sound but not quite there.

I think I have satisfactorily proven to myself that I can feed this speaker with enough power to poll the woofer AND still suffer from muddy bass. The next step will be to modify the woofer. Planning to remove both woofers, replace the external one with a custom woofer, bypass the crossover, and run the woofer section active.
Hi gents, I maintain the Acapella Violon mailing list. If you would like to join, send me an email. Also - there is an Acapella owners group on Facebook. To find it, search for "Acapella" on FB. See you there.