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Old 7th April 2012, 03:05 PM   #1
Ron AKA is offline Ron AKA  Canada
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Default Tweeter - How much series resistance do I need?

One tweeter has failed on my B&W DM7 MK2 speakers. B&W no longer seem to make an exact replacement. However it looks like I can get a similar one from a newer Matrix 801 S2 speaker. Wondering what I would need to do to the series resistance in the crossover to keep the output sensitivity the same?

Original DM7 MK2 - TS26 fabric dome tweeter
Overall sensitivity of system - 86 dB
Series resistance - 3 ohms

Matrix 801 S2 - metal dome tweeter
Overall sensitivity of system - 87 dB
Series resistance - 0.47 ohms

So if I want to keep sensitivity at 86 dB but with the Matrix tweeter installed, what value of resistor do I need?
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Old 7th April 2012, 03:17 PM   #2
AndrewT is offline AndrewT  Scotland
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The resistor may change the shape/slope of the response curve.
Simply getting the right sensitivity (at one frequency) may leave a response anomaly that means that at other frequencies the sensitivity sounds wrong.

It may take quite a bit of trial and error or measurement and software.
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Old 7th April 2012, 04:28 PM   #3
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It's not just a question of matching sensitivity. The new tweeter will most likely require different crossover component values to get the response right.
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Old 7th April 2012, 05:55 PM   #4
Ron AKA is offline Ron AKA  Canada
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I'm thinking it should not be all that complicated. If you just consider 15K Hz for example that should be well past the crossover frequency influence of the inductors and capacitors, and only the resistor should be influencing sensitivity. And, to put things in perspective my sensitivity right now at 15k Hz is 0 dB. I'm thinking of putting a 1 ohm in parallel with the existing 3 ohm, to give me an equivalent resistance of 0.75 ohms, or just a touch more than the Matrix resistance of 0.47 ohms.

If you want to dig a little you can look at the crossover circuit drawing of each speaker. I don't know how to link direct to the file, and the PDF is too large to upload to this site. But, if you go to the link below, you can find the "service manual" for each of these two speakers in the appropriate section; DM for the DM7 Mk2, and Matrix for the Matrix 801 S2. The sections are arranged in alphabetical order. And for the matrix the circuit drawing is on page 16, with the component values on page 15.

B&W Service Manuals

However, this said, I think the only factor influencing the basic sensitivity of the tweeter is the resistor. On some models such as the DM6 B&W gives you a 3 position selector switch to change resistance on the tweeter from 0 or 1, or 3 ohms.
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Old 9th April 2012, 03:39 PM   #5
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You can also test the "variable L-pad project" very common in this forum. That is, using one (or a stereo pair) to adjust for the right volume before substituting with fixed value resistors.
L-PADS
Solen Electronique Inc.

A tweeter can also be repaired, if you look for.

Last edited by Inductor; 9th April 2012 at 03:45 PM.
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Old 9th April 2012, 11:29 PM   #6
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L pad calculator - attenuation dB damping impedance decibel loudspeaker speaker voltage divider - sengpielaudio Sengpiel Berlin
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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency
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Old 10th April 2012, 09:14 AM   #7
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Quote:
I'm thinking it should not be all that complicated.
You are thinking wrong.
What let you think that the 801S2 tweeter is similar to your DM7/II one? Appearance? Frequency response? Impedance vs frequency? If they both have the same FR and impedance plot they can be swapped otherwise they are just two different tweeter. Without measuring you are essentially shooting in the dark.
If you insist in using the 801S2 tweeter and padding it down, keep in mind that the amount of series resistance needed is dependent on that of the tweeter, which we don't know. And moreover series resistor before the crossover is not an L-pad, and varying the value will change the behavior of the x/o.
Ralf
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Old 10th April 2012, 11:00 AM   #8
system7 is offline system7  United Kingdom
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What the other forum members are telling you, Ron, is that it is much more promising to replace the tweeters as a pair. If the new one is more sensitive but has a similar power handling and frequency response and Fs (fundamental resonance), you have room to manouvre with Zobels and L-Pads.

On a cursory glance, both speakers employ 4th order Linkwitz riley highpass filters, with an input resistor as you say of 3R on your speaker and 0R47 on the Matrix, at roughly similar frequencies, and they are not too different except the Matrix employs a Zobel (2.2UF and 15R) on the tweeter too.

Since a protection circuit is built in too, there are other considerations too. This is not a totally simple problem. I've seen enough to suggest this is doable, but if it was my speaker, I'd do the work on the two manuals first. Particularly check sensitivity and crossover frequency and tweeter impedance if you can. But yes, it looks promising.
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Last edited by system7; 10th April 2012 at 11:22 AM.
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Old 10th April 2012, 02:41 PM   #9
Ron AKA is offline Ron AKA  Canada
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Quote:
Originally Posted by system7 View Post
What the other forum members are telling you, Ron, is that it is much more promising to replace the tweeters as a pair. If the new one is more sensitive but has a similar power handling and frequency response and Fs (fundamental resonance), you have room to manouvre with Zobels and L-Pads.

On a cursory glance, both speakers employ 4th order Linkwitz riley highpass filters, with an input resistor as you say of 3R on your speaker and 0R47 on the Matrix, at roughly similar frequencies, and they are not too different except the Matrix employs a Zobel (2.2UF and 15R) on the tweeter too.

Since a protection circuit is built in too, there are other considerations too. This is not a totally simple problem. I've seen enough to suggest this is doable, but if it was my speaker, I'd do the work on the two manuals first. Particularly check sensitivity and crossover frequency and tweeter impedance if you can. But yes, it looks promising.
System 7, first I much appreciate your taking the time to review the circuit drawings.

As an update, I ordered two ZZ09709 tweeters (original Matrix 801 S2 component) from the B&W Group. I decided that I'm going to keep these speakers for some time, and I don't know how much longer parts are going to be available. The plan is to install both and keep the one good original DM7 tweeter as a spare.

On the simplicity side, I just discovered yesterday that I had missed that parallel resistor/capacitor on the 801 S2 crossover circuit. I assume this is what you mean by the Zobel. Yes, that does complicate things, and gave me some reason for panic when I discovered it. The other piece of this B&W evolution of tweeters is that they updated the ZZ09709 to the ZZ04372, which was used in the 801 S3. The main difference seems to be the addition of ferrofluid to the 4372. B&W now sell the later tweeter as equivalent to the older 9709 design, and recommend it for the 801 S2 as well.

If (big if) one assumes the performance characteristics of the two tweeters are the same, and they are interchangeable, then it makes sense to consider the crossover circuit used in the 801 S3 as well. It is very simple with no Zobel compensation. The overall S3 speaker claims 87 dB sensitivity and uses one single 2.2 resistor in series.

So now I'm thinking if a single 2.2 gives 87 dB and I want 86 dB with a single series resistor, then I must be real close with my current 3 ohm, and my first cut attempt at this would be to just put in the new ZZ09709 with the crossover untouched.

I will play around with the L Pad calculators a bit to see if I can get a similar level of comfort working back from the S2 circuit with the Zobel compensation. Thanks PeteMcK and Inductor for those.

But, I still would appreciate opinions on whether or not the existing 3 ohm is in the ballpark.

Last edited by Ron AKA; 10th April 2012 at 02:49 PM.
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Old 10th April 2012, 03:28 PM   #10
system7 is offline system7  United Kingdom
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Those replacement tweeters aren't cheap, eh?
B&W Parts Retail Price List

I think you'll be able to slot them in with no change. 1dB is nothing really.

You can figure out the crossover circuits to compare from this:
2-Way Crossover Designer / Calculator

Generally, increasing the input series resistance to a filter raises the Q of the filter and produces a peak in output. A resistor in series with a tweeter will slope top end response up. A Zobel rolls off high frequency response while flattening impedance. Ferrofluid flattens impedance and increases power handling. Confusing, isn't it. I think you'll be alright.
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Well, there it is! Best regards from Steve in Portsmouth, UK.
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