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Old 10th April 2012, 05:25 PM   #11
Ron AKA is offline Ron AKA  Canada
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Quote:
Originally Posted by system7 View Post
Those replacement tweeters aren't cheap, eh?
B&W Parts Retail Price List

A resistor in series with a tweeter will slope top end response up. A Zobel rolls off high frequency response while flattening impedance.
Yes, they are not so cheap, but these were quite expensive speakers in their day. In one of the technical manuals I think they gave their cost for the original fabric dome tweeter in the 801 as $79 around 1980. I couldn't justify the $360 they want now for the ferrofluid version, plus they only had one in stock. I gather the main reason for the ferrofluid was to allow them to drop the self powered protection circuit. I will be leaving the circuit in place.

Yes, I wondered also about the high end frequency response effect of the Zobel. One review I found on the 802 S2 showed a marked roll off effect, 5 dB at 20 kHz. See image below. Makes you wonder if they were not trying to control the peak beyond 20 kHz. At my age, I'm not thinking I will hear anything up there unless it creates sub harmonics in the audible range. With CD's it would seem there is a pretty hard cutoff above 20 kHz in any case, and I doubt my MC cartridge is producing much at that frequency, although it may be more probable than the CD source.

Click the image to open in full size.

Thanks again for the input. Now thinking I will just put in one tweeter and see how it sounds compared to the undamaged original speaker. Assuming it is OK, then I will replace the good tweeter too.
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Old 10th April 2012, 05:29 PM   #12
AndrewT is offline AndrewT  Scotland
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yes, adding a Zobel in parallel to the driver decreases the impedance seen by the amplifier and/or the crossovcer components before the Zobel.
That reduced impedance has the effect of introducing a low pass filter to that part of the passband. It also increases the demands on the amplifier and reduces the sensitivity of the speaker.
Only add a Zobel when you know that is what is actually needed.
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Old 23rd April 2012, 01:02 AM   #13
Ron AKA is offline Ron AKA  Canada
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I have now received and installed the new ZZ09709 tweeter and have done an initial quick test with the original crossover untouched (3.0 ohm resistor in series). The new tweeter has a DC resistance of 5.8 ohms. The purpose of the test was to compare the speaker with the new tweeter to the other untouched speaker with the functioning original TS26 tweeter. My tools are quite crude (Bink test tones, iPod touch dB Volume app). The environment is far from a test chamber but the readings (except for one frequency) were quite steady and seem repeatable. My purpose was not to determine absolute magnitude of output but to compare one speaker to the other to determine relative difference. Amp volume was unchanged, and used the same channel, and speaker positions were identical relative to the iPod Touch mic (3' straight in front of the tweeter). The crossover is at 3000 hz Here are the results:

Freq - dB New - dB Old - Diff
800 - 85 - 85 - 0
1250 - 85 - 85 - 0
1600 - 84 - 84 - 0
2000 - 79 - 72?? - 7?? Reading not steady
2500 - 85 - 85 - 0
3150 - 85 - 85 - 0
4000 - 83 - 84 - -1
5000 - 83 - 85 - -2
6300 - 80 - 84 - -4
8000 - 76 - 81 - -5
10000 - 83 - 85 - -2
12500 - 77 - 80 - -3
16000 - 76 - 76 - 0
20000 - 65 - 67 - -2 (low but the rolloff may be the iPod Touch mic)

So my first conclusion is that the new tweeter is down about 2-3 dB compared to the old. Your thoughts?

I'm now thinking of shorting out the 3.0 ohm series resistor and repeating the test to see what I get. Again your thoughts? It would seem I will have two data points on performance and should be able to fair closely determine how much resistance I need if 0 ohms is a little hot.
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Old 23rd April 2012, 09:40 AM   #14
AndrewT is offline AndrewT  Scotland
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Try adding a temporary resistor in parallel to the 3r0. Start with 10r.

Keep adding more of the 10r, until you think the sensitivity is about right.
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Old 23rd April 2012, 01:08 PM   #15
system7 is offline system7  United Kingdom
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Ron, this is your interesting problem, not mine, so you're going to have to figure it out yourself. Start by getting the issues clear in your mind.

Your old tweeter was a fabric dome with 3R input resistance to the DM7 MkII filter, your new one is a (shudders...LOL) metal dome with 0.7R input resistance on a slightly different filter. B&W see fit to add a 15R + 2.2uF Zobel to the metal dome in the Matrix 801 S2, perhaps to tame it at the top end.

The filter in the DM7 MkII looks like Linkwitz-Riley 4th order ca. 4000 Hz. You should find out how the Matrix 801 S2 filter works too. Write the values down!
2-Way Crossover Designer / Calculator

I would then model it with a similar metal dome tweeter and mid/bass woofer from the visaton range, but that's just me:
Visaton - Lautsprecher und Zubehör, Loudspeakers and Accessories

Broadly speaking, reducing that 3 ohm input resistance raises the level of the tweeter. Adding a Zobel rolls it off at the top end. You might be able to add a small resistance to the output of the filter which will slope its response upwards too. This depends a bit on the resistance Re of the two tweeters too, which you should measure and ideally match. Try modelling the effects in that Boxsim program so you get the hang of it. Beware that the woofer input will affect things, particularly near crossover.

IMO, you're probably going to have to adjust this to taste. I would think the Zobel is worth fitting, and then hopefully you can just adjust that input resistor to get the level about right overall. You'll then just have to put up with the tonal change due to the different tweeter.
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Well, there it is! Best regards from Steve in Portsmouth, UK.

Last edited by system7; 23rd April 2012 at 01:35 PM.
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Old 23rd April 2012, 03:54 PM   #16
Ron AKA is offline Ron AKA  Canada
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Thanks for the comments. I should have mentioned it in my last post, but my objective is to match the new tweeter output to the old as step one. I bought two new identical tweeters, so step 2 is to replace the functional old tweeter by using the same compensation for both.
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Old 24th April 2012, 05:51 AM   #17
AllenB is offline AllenB  Australia
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Considering the replacement tweeter is a similar type to the original, matching a single measurement of the new to one of the old tweeter should get you close. (The quality of the mic you use won't be too critical as you don't need to know how they actually measure, just whether they are the same).
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Old 24th April 2012, 03:42 PM   #18
Ron AKA is offline Ron AKA  Canada
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Checked the crossover and found the existing series resistance was 3.3 ohms (not the 3.0 on the drawing). I did some trial and error input to one of the calculators for series resistance and found that if I wanted to gain 2.5 dB I needed to drop the resistance to 1 ohm. That was using the 5.8 ohm DC resistance for the tweeter. If I use something higher like 9 ohms impedance then I need to drop resistance to near zero.

So the easy resistance to try was zero using a jumper on the board. I compared the two speakers with zero resistance in the one with the new tweeter, with the following results:

Freq - dB New - dB Old - Diff
800 - 85 - 85 - 0
1250 - 85 - 85 - 0
1600 - 82 - 81 - -1
2000 - 85 - 85 - 0
2500 - 85 - 85 - 0
3150 - 84 - 83 - -1
4000 - 81 - 84 - +3
5000 - 85 - 85 - 0
6300 - 85 - 85 - 0
8000 - 84 - 84 - 0
10000 - 82 - 82 - 0
12500 - 77 - 77 - 0
16000 - 73 - 68 - -5
20000 - 63 - 62 - -1

Still some odd readings, probably caused by reflections in the room, but I'm thinking this is probably as good as it gets and I'm done! I can't hear the 16k or 20k tone in any case, as I am 62. I also suspect they are probably higher than they measure and the iPod Touch mic is just not that sensitive up at that frequency. I'm guessing the mic starts to roll off at 10k.

Agreed? Next step is to install the other new tweeter to replace the original and solder in jumpers on both boards.
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Old 24th April 2012, 10:02 PM   #19
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by Ron AKA View Post
Still some odd readings, probably caused by reflections in the room,
Using 1/3 octave tones it will be hard to get around extraneous things. I would try taking the measurements at 1', and pull it out from the walls and lift it a few feet off the ground if possible.

But it does look like you're close already.
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Old 25th April 2012, 04:02 AM   #20
Ron AKA is offline Ron AKA  Canada
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Thanks all for the suggestions. I soldered the jumper in both speaker crossovers, and now have the new tweeters in both speakers. Did one final check and with the usual measurement glitches they pretty much match with 0 dB difference at most frequencies, and most of the rest within 1 dB. Listened to a couple of CD's and it sounds good! Still a bit surprised there was that much of a sensitivity difference between the older fabric dome tweeters and the new metal dome. But all is good now.
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