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Old 23rd December 2012, 03:24 PM   #1
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Default Questions regarding electronic crossovers, bi-amping and impedance equalization

Hi, I have some questions regarding electronic crossovers, bi-amping and speaker impedance/impedance equalization.
I'm planning on building a pair of speakers over the holidays, a 2 way TQWT design. The low/mid frequencies are handled by a Tang Band W6-623C paper cone woofer, and the remaining mid/high frequencies are handled by the 28-847SE silk dome tweeter. I'd like to ditch the passive crossover and go active.

The speaker design I'll be using is called "Pipe SIX", a 40 liter TQWT enclosure tuned at ~25Hz (see impedance peak in attached impedance/phase plot), originally with a 3rd order (18dB/octave) Butterworth-type crossover at ~2500Hz and an impedance equalization network for the woofer. Since the drivers have a very similar sensitivity, there are no series resistors necessary for attenuating the tweeter, and the crossover is about as simple as it gets.
Thus, an ideal candidate for bi-amping, methinks. I have two chipamps, an LM4780 single chip stereo and an LM4780 bridged dual mono, a cute little mini Aleph, and also a pair of tube amps, so I got the amp section covered.

I would run the LP of an electronic crossover into a chipamp for low/mid, and directly feed the woofer from said amp. The HP section I would like to run into the mini Aleph, and feed the tweeters through a large series connected MP motor run cap (for mandatory DC protection), which I also happen to have on hands. How big a cap do I need? With a crossover frequency of 2.5kHz, one octave down would be at 1.25kHz, one and a half octaves down would be at ~940Hz, and two octaves down would be 625Hz. The 28-847SE tweeter has a nominal impedance of 8R, so with a 23uF cap I would get a high pass with -3dB at f=1/(2*pi*Z*C)=865Hz, which should be fine if I aim for >1.5 octaves. So far, so good.

Now to my questions:

Is this electronic 18db/octave Linkwitz-Riley crossover a good choice? Project 123 (Figure 1)
I would have all the necessary parts on hands. A matched sextet of 13.6nF S&H styroflex caps for the high pass filters, and a bunch of 10nF Wima FKP1 5% caps for the low pass, some 1% MF resistors are easy enough to get, and I also have some LME49720 opamps and LM317/337 pos/neg voltage regulators for the supply.
Do I need input buffers? I guess output buffers with some level setting ability would be nice too, perhaps even with a DC coupling cap? Would some unity gain configured opamps (I don't want excessive gain in my system) and some 20k pots (trimpot even) suffice?

Do I need impedance equalization networks?
The original design incorporates one for the woofer (6.8uF and 10R), and none for the tweeter.
If I calculate the values myself, I get the following values:
Some sources say R_z=R_dc, others say R_z=1.25*R_dc. There seems to be agreement that C_z=L_e/(R_z)^2.
The W6-623C has a nominal impedance of 8R, a DC impedance of 6R (measured by K+T 6.11R), and L_e=0.61mH.
The 28-847SE has a nominal impedance of 8R, a DC resistance of 6R, and L_e=0.016mH.
R_z would thus be 6R or 7.5R for the woofer, and the same fot the tweeter. The resulting C_z/R_z values would thus be 16.9uF/6R or 10.8uF/7.5R for the woofer, and 0.44uF/6R or 0.28uF/7.5R for the tweeter. Do I need impedance equalizing networks for both? How critical are the values?

Well, that'd be it for now. Am I missing something?

Attached you'll find the original crossover schematic, an impedance/phase plot, the drivers' datasheets, and construction drawings (everything in millimeters).
Attached Images
File Type: png impedance.png (67.9 KB, 282 views)
File Type: png xover.png (15.5 KB, 280 views)
Attached Files
File Type: pdf W6-623C.pdf (452.6 KB, 11 views)
File Type: pdf 28-847se.pdf (48.7 KB, 8 views)
File Type: pdf PipeSiXe_Bauzeichnung.pdf (148.3 KB, 17 views)
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Old 23rd December 2012, 03:41 PM   #2
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OK, I'll take a stab at answering a few of your questions.

Quote:
Originally Posted by Rodeodave View Post
Is this electronic 18db/octave Linkwitz-Riley crossover a good choice? Project 123
Yes, it's a good choice. But it's not a Linkwitz-Riley filter, it's 3rd order. I like 3rd order electrical filters.

Quote:
Do I need input buffers?
Maybe, but I doubt it. You can start without.
Quote:
Do I need impedance equalization networks?
Probably not. The chip amps should have a low enough impedance that the impedance peaks won't effect the current flow. It won't hurt to leave them in, but I doubt it you'll hear any difference with them out (unless that Zobel cap is really nasty).


Quote:
Well, that'd be it for now. Am I missing something?
Just that it will not sound the same. The passive crossover's filter function is determined by the impedance of what it's driving. The speaker is not a flat line impedance, even if the Zobel wants to make it that way. So the stock, passive filter isn't a textbook 3rd order, tho it might be close. Your active filter will be close to textbook, and that's gonna sound different.

If I have the time, I might run this thru the Passive Crossover Designer to see what you've really got. But it's probably close enough to nominal that it won't matter. You may like the change that the active filters bring.
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Old 23rd December 2012, 03:55 PM   #3
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On second glance, I'm not sure about your passive crossover values being 2.5 KHz, 3rd order. Need to do the math.

EDIT: The passive values you show for the woofer don't make sense. Is there a mistake there?
For the tweeter it looks like a perfect 3rd order Butterworth at 3500Hz.
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Old 23rd December 2012, 04:39 PM   #4
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Thank you for your input, Pano!

I just realized I made a mistake, the cap in the woofer section is supposed to be 6.8uF (not 0.68). Revised schematic is attached.
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Old 23rd December 2012, 04:54 PM   #5
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Thanks for the update, Dave. Those woofer values are still kinda odd, but they may be there to correct FR bumps in the woofer and baffle.

To me it looks like a spread frequency crossover. Woofer ~2K, Tweeter @ 3K. You can see that in the combined impedance plot you posted.
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Old 23rd December 2012, 06:47 PM   #6
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Hi,

Its all wrong, except for the bass loading perhaps, and going
active won't help at all with the basic flaws of the design.
Which is wrong, for equal sensitivity drivers the tweeter
always needs attenuating for any half decent design.

An active copy of a crap passive is still crap.

rgds, sreten.
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Old 23rd December 2012, 10:19 PM   #7
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Hm, I never really dealt with crossovers until two days ago. Never heard of a spread frequency crossover before, but it makes sense. Setting frequencies is easy enough with the crossover design in question.

Now having found the formulae for 2-way 3rd order Butterworth filters, the suggested crossover values do look odd. The high pass seems to be set for -3dB at around 3400 Hz, and the low pass at around 2500Hz, maybe?

And to address the critique by sreten...is the design really that bad? Can you elaborate what leads you to say that?

Also, here are the TSP for both drivers:

28-847SE
Fs = 850 Hz
Qms = 2,37
P-Dia = 28 mm
Qes = 1,87
Re = 6 ohms
Le = 0,016 mH
Z = 8 ohms
Pe = 8 watts
Qts = 1,05
1-W SPL = 90 dB



W6-623C
Fs = 46 Hz
Qms = 3,18
Vas = 23,88 liters
Cms = 0,858 mm/N
Mms = 13,8 g
Xmax = 4 mm
Sd = 140 sq.cm
Qes = 0,44
Re = 6,11 ohms
Le = 0,68 mH
Z = 8 ohms
BL = 7,4 Tm
Pe = 30 watts
Qts = 0,39
1-W SPL = 89 dB
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Old 23rd December 2012, 10:29 PM   #8
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The crossover isn't "all wrong", tho the low pass is certainly odd. I do agree that copying a bad passive is no way to go. I just don't know how bad this is. I still suspect that either the low pass values are wrong, or the designer was doing something strange electrically because of the acoustical response.

Do you have Microsoft Excel? If you do, I would suggest a copy of Jeff Bagby's most excellent Passive Crossover Designer. It also allows active crossovers or a combination of the two.
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Old 23rd December 2012, 11:14 PM   #10
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Quote:
Originally Posted by sreten View Post
No. The design is clueless and until you really know
your stuff don't even think about an active version.
This tells us almost nothing. What is "clueless" about it? What do you object to? Can you give details about your objections?

The high pass filter is pretty much textbook 3rd order Butterworth @ 3500Hz. (given the published tweeter impedance curve) and that's not a bad place to be for these drivers. There may be better crossover point, but 3500Hz isn't a terrible choice. The woofer values are strange, and probably a mistake. It's worth looking at that further. We do NOT know the acoustic response of the woofer in this box, and that is important to the crossover design.
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