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Old 28th July 2006, 07:57 AM   #1
DAMIC is offline DAMIC  Serbia
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Location: Beograd
Default Apogee C/M Mod – what I ended with


I acquired Apogee Centaur Minors almost two years ago, hybrid speakers, tweeter is 26 inch ribbon, and middle-bass 6,5 inch cone driver positioned on the front baffle near the middle of the ribbon, in sealed enclosure, front panel is 100 cm high and 35cm wide and back attached enclosure is app. 20 x 20 x 75 cm.

From the early beganing, due to lack of bass notes, coupled them with active Mission stereo sub. Overall, I was satisfied with sound. Last year, I made stands, look alike original I saw on few pictures, only the height is 27 cm instead of 20 cm. Actually, I used one of my old speaker – mini monitor enclosure, heavy and very rigidly made of 22mm thick MDF (wide 20 cm, height 27cm and 32cm deep) with internal net volume of 9,3 l. and only attached on the front side 4cm thick MDF dim. 35cm x 27cm. From the profile, the combination look like letter L, but gave more stability and height to Minors, as well as better sound. As the stands were pretty heavy by themselves, I left them empty.

Meantime one of my mid-woofer broke and I found excellent substitute for original Vifa driver with 6ohm Audax AP170Z0 HDA (high definition aerogel). Driver is with bigger voice coil diameter 30mm, and T/S parameters similar to original, except 1,5dB greater sensitivity.

But, sometimes when I was lazy to switch the sub on, the sound really bothered me a bit. Just to mention, my two previous pair of DIY speakers utilized old but for me still nice Kef bass driver B-139 (one design was reflex and second horn type) so it is understandable why I missed a bass. I even thought of making new bigger enclosure, similar to Centaurs, middle model, with new 8 inch 6ohm driver.

But an idea come on my mind. Why don’t use the volume of stand to get a new bigger reflex enclosure and install the duct port on the back of the lower box? The project should be easier to realize than to make totally new enclosure and C/M design wouldn’t be visibly degraded.

I did some measurements and found that net volume of the C/M box is 15,7 l, what with volume of stand 9.3 l makes total of 25 l. The new volume should be almost ideal for existing Audax driver (Vas 24,65 l) I ended with duct internal diameter 4,6 cm, 7,7 cm long ( plastic tube I already have at home) and theoretically tuned the bass reflex enclosure to app.43 Hz. I must note that my primary intention was to tune the enclosure to 48 Hz (drivers resonance is 48,5 Hz) but due to same error with calculator, enclosure is tuned to 43 Hz. I discovered that later and checked several times with different calculators.

I removed the driver from C/M and pull out the stuffing.
In order to reduce the vibration of the front and back panels, I made cilindrical shape brace from the hard wood, diameter 3,5 cm long 16,2 cm which just slipped through driver opening about 5 cm below, applying dense glue.
Than, with crown saw, I made circular opening on the middle of the bottom side of C/Minor box, diameter 7 cm. On the appropriate place of the top side of stand/lower box, I made the same diameter hole. The back side of the stand/lower box was removable, so I connected Minor's cabinet with strong 4 screws and applied a thin rubber sealant in between. For stuffing, I used about 1/3 of the original quontity and put only one layer, along back side in the C/Minors and on the bottom in lower box. I hope, you can imagine the picture.

The final result is astonishing. A couple of my friends came on audition, all knowing previous C/M sound and they also did not beleived that bass notes comes only from 6,5 inch driver. All asked whether the sub is powered on. Bass was so deep and sound natural, no boomy, no one note bass but smooth almost like from TL. Such results you could expect from good 8 inch driver in good tuned box but not from 6,5 Audax. Very, strange? Several good recorded disc was played, all sounded excelent. Later, we put test CD, freq. range from 10 to 300 Hz with 1 Hz increments, and we did not noticed any audible dip or high abnornalities. Unfortunately I do not have noise meter to make exact measurement.

I was intriguing, how come that bass now sounds so good, and after almost two months, and some research on this forum, It seems that without any intention, I actually made double chamber bass reflex box instead of classic bass reflex. Basicly the concept for making DCR is similar to ordinary bass reflex, but second internal port should be tuned to double value than the main port. In fact, it seems that cilindrical empty space where two box were connected (two layer of MDF with rubber sealant in between) formed the second port, dia.7 cm and 4,5 cm long, which I later found is tuned to 90 Hz, almost ideal, one octave difference.

Now, I do not use my sub any more. The difference with sub switched on is still noticeable only at higher volume, but somehow, I prefer to listen without it. Integration with ribbon remain good as earlier.

After 2 months listening, I can say that combination sound fantastic and don’t believe that could be better. Well, I think that double chamber reflex enclosure is made as I sow on Claudio Negro site http://www.claudionegro.com/ (look under projects-speaker-DCR) the third case is applied, so only one outside port on the lower box.

But there are some incongruity with the theory: I lightly stuffed the box as a bass reflex one, while I found later that in double chamber reflex design there is no stuffing applied.
Secondly, the ratio of volume between primary and secondary box is not as I found in theory. Primary box with driver has 15.7 l and lower 9.3 l, while in theory first one should be 2 times smaller.
Only positive data I found, was that internal port (if it really is) is accidentaly tuned to dezired one octave more i.e. 90 Hz.

But after all, whether I ended with reflex or double chamber reflex design I am not dare to touch/further modify anything. Speakers works so good on low and moderate levels, with ease, the feature which I prefer the most.

I am wondering can anyone from this form clarify me whether I have now reflex design or maybe double chamber reflex. If it is DCR, maybe I could remove the stuffing and further improve the sound? Or whether I should reduce the length of outside duct a bit to tune at exact 45 Hz? Any ideas?

Thanks in advance
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Old 28th July 2006, 08:56 AM   #2
claudio is offline claudio  Italy
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Default Re: Apogee C/M Mod – what I ended with

Originally posted by DAMIC
Well, I think that double chamber reflex enclosure is made as I sow on Claudio Negro site http://www.claudionegro.com/ (look under projects-speaker-DCR) the third case is applied, so only one outside port on the lower box.
Hello Damic,
the third case showed in my HP is a DBR, alias Double Bass Reflex, patented by Elipson (french society) and studied by Hiraga, Kondo and Ohara.

The DCR (Weems-Augspurger) studied in my HP is different from the DBR, that is 3 ports are used instead of the 2 used on the DBR.
My Home Page: www.claudionegro.com
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Old 28th July 2006, 10:01 AM   #3
sreten is offline sreten  United Kingdom
Join Date: Nov 2003
Location: Brighton UK

IMO the DCR has no advantage whatsover over normal reflex loading
except in the case of low Qts drivers in boxes that are essentially
very large compared to the Vas of the driver.
In your case if you have a DCR IMO you don't want one.
(the only way to make sure is measure the impedance)

Did a bit of modelling of the Audax.
Reflexes well in 15L tuned to 45Hz. 25L at 45Hz gives stronger
output at 45Hz but a stronger "knee" to the bass roll-off.
Not much point going much over 25L as Vas = 25L.

I estimate your internal port is tuned to 140Hz as the effective
volume of both sides of the port is 5.75L (one side of the port).

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Old 28th July 2006, 10:01 AM   #4
DAMIC is offline DAMIC  Serbia
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Hello Claudio,

You are right, I ment on the situation like DBR, but just thought to direct the people who will search your site to folders Projects, than Speaker and DCR, where appears 5 examples of bass reflex and its variations.

Best regards,
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Old 28th July 2006, 12:51 PM   #5
DAMIC is offline DAMIC  Serbia
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Join Date: Jan 2006
Location: Beograd
Hello Sreten,

Thank's for reply. Well, you think that only reliable way to check whether reflex or double chamber reflex design is in question, is possible by measuring the impendance, i.e. are there 2 or 3 peaks in imp. plot? Unfortunately I do not have equipment required.

BTW when discovered that 25 l enclosure accidently was wrongly tuned to 43 Hz, I wonted to reduce external port length to 5.3 cm that should give freq. to almost optimal 48 Hz, but the sound is so good that I fear to ruin it. Particulary after the moment when discovered that other type enclosure maybe was achived. So, stays my dilema whether to tune total 25 l volume to 48Hz as reflex type.

Best regards
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Old 28th July 2006, 01:41 PM   #6
sreten is offline sreten  United Kingdom
Join Date: Nov 2003
Location: Brighton UK

My modelling indicates the bass gets peaky if you go above 45Hz.
If anything err on the lower side of 45Hz, less bass but a more
rounded knee.

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Old 2nd August 2006, 07:45 AM   #7
DAMIC is offline DAMIC  Serbia
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Sreten, thank's

For now, I decide not to change anything from present configuration.

best regards
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