B&W CM4 alteration advice needed - diyAudio
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Old 27th November 2011, 12:55 AM   #1
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Default B&W CM4 alteration advice needed

I have a pair of B&W CM4 speakers for a number of years. I never like the treble of the speakers which appears to be too harsh for my taste. A few years ago, my son pressed on the treble. I could never restore the diaphragm back to the original. Although it never seriously affect the speakers sonically, I am thinking about replacing it (or them). I come across a pair of Chinese speakers call Aurum Cantus, and I like the sound of the speakers. I have no idea how much work is involved in replacing the CM4 treble with a pair of ribbon tweeter.

CM4 is a 2-1/2 way speaker. It is bi-wirable. The high pass filter is a 3rd order filter, and have a cross-over frequency of 4000Hz. I don't know the impedance of the treble unit. Using some of the crossover frequency calculator on the web, it appears the impedance is about 8ohm. The AC ribbon tweeter has a 6ohm impedance and a recommended crossover frequency of 2500Hz.

I have attached the links to the service manual of CM4 and the spec for the CM4 and the ribbon tweeter at the bottom.

1) I dont know if I am doing it correctly, if I dont re-work the crossover, the crossover frequency will be around 5000Hz? Between the 4000-5000Hz, the sound will drop off?

2) Assuming, I put in new capacitors and inductor to alter the crossover frequency to be 4000. Since the AC ribbon tweeter has a recommended xo frequency of only 2500Hz, will be affect the performance of the AC ribbon tweeter? If I need to switch the xo to 2500 HZ, I'll need to change the xo of the lower pass filter? What would happen to the sound quality of the mid range and the woofer?

3) There is a resistor (R1 OR22) in series with the 3rd order high pass filter in the CM4. What is the function of that? I cannot find the spec (ohm?) for the resistor?

4) Am I crazy to do this alteration? What other options do I have?

Thanks

B&W CM4 Service Manual (pg 12 schematics/pg 9 values of components)
B&W CM4 Servie Manual.pdf - File Shared from Box - Free Online File Storage

B&W CM4 spec
B&W CM4 spec.pdf - File Shared from Box - Free Online File Storage

Aurum Cantus Tweeter Spec
Aurum cantus
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Old 27th November 2011, 01:46 AM   #2
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Quote:
Originally Posted by rsdofny View Post
...If I need to switch the xo to 2500 HZ, I'll need to change the xo of the lower pass filter? What would happen to the sound quality of the mid range and the woofer?

3) There is a resistor (R1 OR22) in series with the 3rd order high pass filter in the CM4. What is the function of that? I cannot find the spec (ohm?) for the resistor?
You don't need to change the crossover frequency from 4K to 2K5Hz.
The resistor is just a small R of small value, 0,22 Ohm or 0R22, to adjust a little phase and attenuation of the tweeter with mid.

Quote:
Originally Posted by rsdofny View Post
4) Am I crazy to do this alteration? What other options do I have?

Thanks
Is just fine and you can look for a lot of people have done it here in diyAudio. Try a search (on the top menu).
Other options are repairing the tweeters and adjusting the crossover for the level desired.
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