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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
wireless200
415 posts
 
Atmasphere, what exactly is it that the longer wires cause? The resistance would seem negligible. Is it C or L or something else? I'm thinking about buy a tube amp as a second system and would be interested in hearing more about how speaker wire length affecting performance.

regards, David
atmasphere's avatar
atmasphere
11,577 posts
 
Hi David, When you are dealing with 4 ohm speakers and you are running a long cable, first off the resistance of the cable actually becomes part of the total impedance driving the speaker.

The second thing to understand is that all speaker cables have a property known as 'Characteristic Impedance'. This is not the resistance or impedance of the cable, rather it is the property of the cable such that it will have its best performance when terminated by a specific impedance. For example, a cable with a characteristic impedance of 8 ohms will perform best when terminated by an 8 ohm load. Its actual DC resistance will likely be quite low as will its AC impedance, however.

Even if we were talking about a cable with a 4 ohm characteristic impedance, there are no speakers that are exactly 4 ohms over their entire range, so there would be termination errors. Most cables have a characteristic impedance that is much higher than 4 or even 8 ohms so there is a greater error that results.

The error is in the form of reflected energy which smears the waveform. This increases as the cable length is increased; a simple way to reduce the error is to shorten the cable- hence a good argument for monoblock amplifiers.
rodman99999
5,984 posts
 
Amfibius- I believe you misunderstood the purpose of the DQLP-1. It's not an EQ, it's a high-pass/low-pass filter that will allow you to drive your woofers with a separate amp. This will free your main amp of the burden of reproducing bass freqs, which cleans things up dramatically. It does have an EQ knob that enables the user to enhance the bass below about 40hz, if desired.
amfibius OP
282 posts
 
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.
atmasphere's avatar
atmasphere
11,577 posts
 
Amfibius, based on the results that you posted, it looks like the woofers are in parallel, and so are 4 ohms while the rest of the system is 8 ohms.

If this is so there will not be a tube amplifier made that will play the bass at the right level. Transistors will do OK, but then the highs will be dry as you experienced.

I would consider putting the woofers in series if you want to use tubes. This will get the bass working right, so long as the crossover for the two drivers is the same. If not, it might be a long road ahead for you to get this sorted. Good Luck!
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