The B&C DCM50. Can it replace the K55 driver?

[Al Klappenberger]

To spite the chance if being accused of abusing the form, I am going to share this with the group. Yes, I may be offering this driver and a new 2-inch horn if everything works out as I hope. For now however, it is simply a suggestion.

Ever since I can remember there has been an interest in upgrading the K55 driver in the Khorn. The problem has been finding one that is actually designed to go down to 400 Hz and survive. There are simply none in a 1-inch horn format that will do! This means we need to move to a 2-inch throat horn. Even in 2-inch there are not may drivers that will do. Most are designed to be crossed over at 800 Hz or higher. After finally getting it through my thick skull that I simply have to go 2-inch, I started looking for a suitable driver. I think I have found one. It’s the B&C DCM50. This driver is not the typical driver with a metal or phenolic diaphragm. It’s a “composite”. I think that’s like the “paper” cones you see on direct radiator drivers. I took a picture looking through the bug screen as best as I could. That picture is below. The lines you see radiating out from the cone in the center are actually supports. The diaphragm is farther back and can’t be seen.

I don’t have a 2-inch horn to evaluate the driver properly yet, I will be getting the prototype Trachorn 420 horns from Bill Martinelli in about a week. For now, I can at least do a preliminary introduction.

[Al Klappenberger]

I don’t have a horn yet to do a proper evaluation, but I did do a quick frequency response plot with the driver sitting on the bench. It looks much better than expected with no horn at all. The low end extends down to about 440 Hz even withOUT a horn!

[Al Klappenberger]

Here’s the jury rig I used to make the plot.

[Al Klappenberger]

Here's the pdf technical information on it from B&C.

One thing you should be aware of. B&C is an Italian company. All the dimensions are give in metric units. The mounting hardware is also metric. It requires four 6mm bolts.

Stay tuned. I expect to be doing direct comparisons between this driver and the JBL 2426h 1-inch driver on similar horns. I plan to do intermodulation distortion as well as frequency response and sensitivity plots. I also have a local friend who has Khorns with K55x drivers on the Trachorn 400 who is associated with a local symphony orchestra to do listening tests. This should be a direct comparison of the two drivers. I don't trust my own ears, but I do trust instruments. They are totally unbiased.

Al K.

DCM50.pdf

DCM50.pdf

[ngk]

Al,

The B&C driver looks very promising. Does the extended frequency range to about 10k offer an opportunity to set the tweeter crossover at a higher frequency. From my very limited knowledge of loudspeaker and crossover design, this may be a good thing.

[Al Klappenberger]

ngk,

This driver does go to 10K and is supposed to according to the specifications sheet. That means you do have the option to cross it over that high, but I don't think it's a good idea to do that. After a few forum post exchanges with DJK, who knows a h___ of a lot more about horns than I know, it seems doing so has two major drawbacks.

1 - The distortion in a horn increases the further you go up from its lower limit. This says a smaller horn is better than a big one and to cross it over to a tweeter lower, not higher. Remember that Klipsch has lowered the crossover on the Khorn and LaScala from 6000 Hz. down to 4500 Hz even though the K55x will go to 6000 Hz.

2 - Dispersion tends to collapse in the vertical plane as frequency goes up. This reduces the total acoustic power in the room even though the frequency response on-axis is flat. I think this is intentional horn design to compensate for mass roll-off. That seems to be important in how a speaker sounds.

After careful consideration of rule #3 below, I would hope DJK would jump in here and explain this better.

AL K.

[seti]

Very interesting. When I was chatting with Bill Woods of Acoustic Horns this is one of the drivers he recommended for another project.

[jwc]

I'm eager to see more data.

jc

[Al Klappenberger]

It's good to hear that someone else has suggested this driver too. I just went looking through manufacturers spec sheets and thought this one looks like exactly what the Khorn needs!

I'll be doing more testing as soon as I get the horns.

Al k.

[ngk]

Al,

Will your universal crossover need to be modified to handle the new driver? At present I think you have a 12dB slope at 6kHz on the midrange. With the K55 this is certainly alright as there is a natural drop-off above this frequency. My K55m seems to have little audible output above this frequency. Would a move to 18dB slope be better with the B&C driver to allow bettter integration with the treble (which already has an 18dB slope). Otherwise I can see that it may require your extreme slope crossovers. This is just speculation based upon the specs and frequency response of the new driver.

Nick

[Al Klappenberger]

Nick,

The Universal network could be used with most any driver as-is, but I don't suggest it. This isn't because of the squawker to tweeter crossover at 6 KHz but rather because of the 1st order woofer to squawker crossover. I think B&C suggests a 12 dB / octave filter at 400 Hz to the woofer. It depends too on the size of the squawker horn. A big horn that goes way below 400 Hz would be needed to load the driver for safety because of all the lows that get through such a simple filter. On the other hand, a 1st order filter needs the horns to determine the actual crossover acoustically. That's why it's "universal"! That means the horn should only extend slightly below the electronic crossover. If you have the big squawker horn with the low Fc you will have major overlap and driver interference. Two drivers, squawker and woofer, making the same sound at the same time isn't good. I think the answer is that you need a horn that goes just a bit lower than the crossover and a network that has sharp enough slopes to safeguard the driver from extreme lows. Extreme lows will just cause distortion at high levels if they get to the driver. In short, I just don't like 1st order networks! The network should be sharp enough to reduce everything at or below the horn Fc to a level that is low enough to at least cause no harm to the driver and preferably be a very small part of what is going through the woofer. That's greater then 25 dB on the ES networks.

The plots below whould illustrate the point.

Al K.

Afterthought: I just looked at a 2nd order (12 db / octave) crossover at 400 Hz. It only adds a couple dB at 310 Hz. All a 2nd order will do is protect the driver from extreme lows, like 20-30 Hz. The squawker transformer also adds another element down that low as well to increase the safety margin.

[jwc]

Al.

I'd like to ask you a question and to let you know there isn't any attempt of criticism. I just want to understand.

Both the BMS and BC drivers are 2" drivers and what I would consider companies with a "good reputation".

I read your comments on a previous thread explaining the benefits of the BC over the BMS. But I'm interested in hearing them again and/or maybe with some additional info.

When looking at the two different horn/driver combos that are being compared......my thoughts seem to think that the biggest "weight" of difference will come from the horn. I would wager that both the drivers are "good" and a plus for sure over a K55. I would also wager that lining up forum members to compare one driver over the other on the same horn would give mixed results......yea....maybe there could be a winner...but I would bet the results would not be a landslide.

I would like to hear more on one horn over the other. The offer of both setups seem to be a range of the usual 400-6000Hz...... Yea we could talk about using other frequencies like using your combo out further or gregs lower...but the common use will be to the above mentioned/usual frequency range.

Greg's horn will have a lower Fc and there doesn't seem to be much problem with off axis response when you get to 5000Hz....although does fall off more than say a horn that is pure tractrix with an Fc of ~360Hz. Or, I guess yours may have the same/similar Fc but be truncated?

Your horn is gonna have a much smaller mouth and shorter length with the same size throat. What is the Fc of the Trachorn 420? Is it about 300-310Hz?

I'm carrying this out too far........

Martinelli is a well respected horn maker.....no doubt. But what is aspect of the 420 is gonna set itself apart? Maybe it is something in the throat...or baffle depth...or construction? Curious here.

Anyway.....I must say that I think this this is a healthy challenge. This whole upgrade concept is not well favored on the forum but I'm in the camp of thinking that they are "an improvement". I love Klipsch, the forum...and have respect for PWK. The K400/401 combo isn't to my liking compared to DIY combo mids I've heard.

These threads may get Nixt....but..thought I would post anyway thinking they may stay alive....

jc

[Al Klappenberger]

JC,

The differences in these various horn and driver combination's is not major. The BMS driver is a fine driver, it's just expensive and larger than necessary. It's intended for other applications where you need response down to or below 300 Hz. The same thing applies to a larger horn. All you accomplish by using stuff that is capable of working well below the 400 Hz top end of the Khorn woofer is to run the cost up. It's all overkill. Even a 2-inch horn could be considered overkill for a home stereo. My thinking is that the objective is to address the weakest link left, the K55 driver. That's my rationalization for going to a 2-inch horn. When I measured the BMS driver, it took 110 dB SPL to see any I.M. distortion increase by the 1-inch driver. Martinelli says that he believes a 2-inch horn actually sounds better than a 1-inch horn. I can't say from my own experience one way or the other.

Another point is the suggestions to move the crossover from 400 Hz down to 300 Hz. There is virtually no difference unless you do it with an extreme-slope crossover. Even a 2-nd order network has little influence over the the actual attenuation near the crossover. The B&C driver is less expensive than the BMS and will do just as well crossed at 400.

The size of the mouth seems to determine the Fc of the Edgar type horns. The size of the throat doesn't seem to effect the calculated Fc. I will see if that's true when I get to the measurements. It's 310 Hz on both.

I don't see any reason this thread would get the axe unless it moves to the dark side like the one where I first mentioned the B&C driver.

Al K.

[Al Klappenberger]

JC,

Here’s another factor to consider, sensitivity. The plot below shows the BMS 4592 2-inch driver versus the JBL 2426h 1-inch driver I have been using in my Belles. The BMS is rated at 113 dB on a certain horn. The JBL is rated 107-110 dB on JBL horns. The B&C is rated at 108 dB on their horn. I won’t know what its sensitivity actually will be on my 420 horn until I measure it. One thing I never liked about the JBL was that it required a transform tap setting of 0-1, or about 12 dB attenuation, to match the woofer. This limits the number of possible tap combination you have available to adjust it to your situation. The plot shows that the BMS 4592 will have the same problem. It has nearly the same sensitivity and will also require nearly the same settings as the JBL. All three if these drivers are 8 Ohms, by the way. A 16 Ohm driver with need a 3 bB higher setting for the same level. I am hoping that the B&C will have lower sensitivity, more in line with the Klipsch woofers, allowing a higher setting on the transformer and more flexibility.

Al K.

[DrWho]

The distortion in a horn increases the further you go up from its lower limit. This says a smaller horn is better than a big one and to cross it over to a tweeter lower, not higher.

That
isn't true, but by all means, carry on...just wanted to clarify for the
masses reading along. You can have two horns with the same Fc that are
very different in size where the HF distortion is the same between
both, and the only difference is the polar control at the lower
frequencies.

It looks like the DCM50 starts beaming above 6kHz,
which looks to correspond pretty well with the inductance corner of the
driver that also sets in at 6kHz. I need to double check my data, but
I'm pretty sure the BMS 4592 isn't beaming the response to boost the
on-axis response....which makes sense considering it was designed to be
used with the coaxial tweeter, which happens to have very wide polars.

All
that to say, on-axis frequency response measurements are only one
snapshot of a multi-dimensional system. To compare two drivers on-axis
without polar response data can be very misleading if you're not
careful. This is why plane wave tubes are so handy (it takes the horn
and polars out of the equation).

Btw, I just wanted to make a comment that intentionally throwing
away efficiency kinda defeats the whole purpose of using horns in the
first place....driver nonlinearities dominate the innaccuracy of a
sound system by a rather large margin, and those nonlinearities only
increase as efficiency is removed. I don't think I would encourage
striving for a lower efficiency system just because it makes tap
settings easier...

[Al Klappenberger]

No science here. I just got some pictures of the new 420 horn from Martinelli. Here's one. I will have them in a couple days.

Al K.

[Deang]

Man, that looks stout. Nice.

[Al Klappenberger]

Dean,

Thanks.

That's why I have Martinelli make them. He's definitely not your average carpenter!

Al K.

[Rudy81]

Nice work Al. I may have to take a look at this to replace my JBL 2470 and Trachorn combination. I had been trying to attack the low crossover problem by looking at using a Jubilee bass bin and crossing it higher. But this other option might work just as well, depending on cost structure and the results of your tests.

The lower sensitivity shouldn't be an issue. My current active filter setup has the mid amp sensitivity turned almost all the way down, two clicks from the very bottom to match the bass bin on the Khorn.

Keep it coming Al. []

[Al Klappenberger]

Rudy,

Yes. Don't jump to an conclusions just yet. The driver problem for the Khorn is a long-standing one. The B&C may be one practical and affordable answer.

I just fitted a K55 to a 1-inch Trachorn 400 so I can compare three drivers on virtually identical horns for the instrument tests. The K55, JBL 2426h and the B&C DCM50.

Al K.