Which Impedance for Crossover Calculation? The Nominal, the One at Crossover or? - diyAudio
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Old 4th October 2013, 11:18 PM   #1
T101 is offline T101  Bulgaria
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Default Which Impedance for Crossover Calculation? The Nominal, the One at Crossover or?

Which Impedance for Crossover Calculation? The Nominal, the One at Crossover or?

The topic is weakly addressed in this thread: calculate crossover with Re or impedance ?
Generally the Internet is lacking serious read on the matter or at least Google fails to present in the first pages with results. The thread from the link above is one of the few sensible results at all.

There are some self propelling myths on the matter though.
One myth is that the impedance of the driver at the crossover frequency should be used for the calculations.
Another myth is that Zobel networks can be avoided.

According to my humble results, in order not to use a Zobel on a conventional driver and not have anomalies in the summing at crossover frequency, one should use some very heavy R&D department in order to have the exact high Q crossover alignmet requied to fill the holes.

Actually it is either flat impedance or nothing. It occurs that it is that simple and this is yet another constraint to add to the already commonly known ones. In order to avoid impedance equalization crossovers should be placed at fequencies where drivers show relatively flat impedance.

Now my simple experiment.
I ran simplified simulations of a symetric Butterworth crossovers at 2000 Hz as follows:

- 1st order with values for 8 ohm loads loaded with 14 ohm load in the LP section;
- 2nd order with values for 8 ohm loaded with 14 ohm in the LP;
- the same as above, but instead the resistor, the LP is loaded with a 0.45 mH inductance with dcr for 6.2 ohm;
- 3rd order with values for 8 ohm, LP section loaded with 0.45 mH 6.2 ohm dcr;
- and in the end, the same 3rd order as above but with values of the LP section for 14 ohm load.

On all simulations are shown the curves of the response of ideal 8 ohm loads and the close to real load for comparison.

(a typical "8 bass driver with not very long 1.5 in (37-38mm) two layer voice coil would have about 0.45 mH Le and 6.2 ohm Re)

As it becomes obvious, you can't avoid Zobel networks and if you take the impedance at the crossover frequency it becomes worse.

Originally the thread is posted in Bulgarian language here: Bulgarian language thread on impedance at crossover frequency

Best regards!
Attached Images
File Type: jpg 1stOrder8vs14ohm.jpg (95.0 KB, 206 views)
File Type: jpg 2ndOrder8vs14ohm.jpg (100.2 KB, 202 views)
File Type: jpg 2ndOrder8vs14ohmLeDCR.jpg (101.8 KB, 196 views)
File Type: jpg 3rdOrder8vs14ohmLeDCR.jpg (100.4 KB, 195 views)
File Type: jpg 3rdOrderNOzobelLPfor14ohm.jpg (100.8 KB, 194 views)

Last edited by T101; 4th October 2013 at 11:43 PM.
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Old 5th October 2013, 02:55 AM   #2
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the one at crossover.
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Old 5th October 2013, 03:11 AM   #3
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As the passive components are reactive (caps and coils) with the inductance of the woofers drivers having some undesired effects (peaks) when making crossovers, Wayne Parham gives a good rule of thumb...

Quote:
...So a good “rule of thumb” is that woofer compensation RC damper circuits may be omitted if voice coil inductance is less than crossover inductance.
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Old 5th October 2013, 03:26 PM   #4
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If you use some software tools like the Passive Crossover Designer (or other) where you can virtually try a crossover topology on a real impedance curve, you'll understand quickly what's going on. It makes a difference!

I often don't Zobel my woofers, just design with the rising impedance in mind. It can often help achieve the roll-off I want.
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Old 5th October 2013, 03:37 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by T101 View Post
Another myth is that Zobel networks can be avoided.
Hi,

Your wrong, the myth is that they are needed, which they are not.
When something similar is needed, values are not Zobel values.

Analysis of electrical x/o functions is a waste of time as they are
all wrong. Right is acoustic functions taking into account driver
and box effects diffraction effects, everything else is wrong.

e.g. 4th order L/R is acoustic, not electrical. The electrical
functions are the difference between 4th order L/R and
the acoustic responses of the drivers in the box design.

rgds, sreten.
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Last edited by sreten; 5th October 2013 at 03:42 PM.
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Old 5th October 2013, 04:00 PM   #6
Pano is online now Pano  United States
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Quote:
Originally Posted by sreten View Post
Right is acoustic functions taking into account driver
and box effects diffraction effects, everything else is wrong.
Yes. This is the very important fact that most people miss. It is the acoustical result that matters. You use whatever electrical filter function you need to achieve the acoustical results. That's why passive crossover values that have been optimized often look odd.
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Old 5th October 2013, 05:26 PM   #7
T101 is offline T101  Bulgaria
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Please bear in mind that the "optimization" and "experimentation" capabilities of the ordinary hobbyist are very limited especially when inductors are concerned.

I thought of two examples when impedance equalization is not required, this is when the case is LP crossover of bass driver between 100 and 600 Hz and the second is a HP crossover for ribbon driver.

I have Visaton's free software Boxsim. I took the first 8 inch bass driver which happened to be W200S, put it in 30L sealed enclosure and applied a textbook second order Butterworth crossover for 8 ohm load.
See what happened... I apply SPL curve and another simulation showing the exact influence of the crossover. The scale of the simulation is the same as the ones from LT Spice.

Optimisation is nonsens yet for another reason, it means that the crossover quality factor would be altered. Then the syne wave response might be satisfactory and the curves and phases might seem alright as well, BUT the impulse response and the crossover induced distortion would be unacceptable.

Now let us see what will happen if we design a Zobel and apply it without any other optimization or whatsoever. I designed it with this calculator: Impedance Equalization (L-Pad) Circuit Designer / Calculator using the published data: W 200 S - 8 Ohm

Wow! The ugly peak disappeared!

Values: LP Butterworth crossover 7.03uf and 0.9 mH, Zobel: 7.5 Ohm and 39.1 uf.

Guys, you will have to elaborate harder on this.

Why would I put the labor to "optimize" when I can design a Zobel with inconsiderable cost within seconds while in the same time I will have the benefit of keeping the crossover Q factor with the desired value. I might be a fan of LR crossovers for their great impulse and transient response, "optimization" would deprive me from the opportunity of using them.

I am not going to bite the LR-4 bait. But... with such steep slopes, summing is not a problem even if the acoustical slopes are deviated for some reason... Phases can be aligned in a matter of putting spacers beneath one driver flange... and the words "transient perfect" are pure majic
Sreten, please open another thread if you are so very interested in the topic.

Best Regards!
Attached Images
File Type: jpg Butterworth2orderW200S.jpg (107.2 KB, 33 views)
File Type: jpg Butterworth2orderW200S2.jpg (41.3 KB, 30 views)
File Type: jpg Butterworth2orderW200S2ZOBEL.jpg (40.9 KB, 30 views)
File Type: jpg Butterworth2orderW200SZOBEL.jpg (106.8 KB, 25 views)

Last edited by T101; 5th October 2013 at 05:28 PM.
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Old 5th October 2013, 05:33 PM   #8
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The one at and above crossover frequency. Zobel can be avoided if inductance of the voice coil is very low, which is not the case with the bulk of cheap drivers in the €10-100 range. If not using Zobel your filter function will change with frequency and reduce its steepness above the selected crossover point, which may not be desireable if breakup nodes are present. Textbook values are useless 99% of the time, unless a Zobel is used and DC resistance equals the reactive one.
I would also go for a LR2 function or even a sub Bessel instead of the Butterworth.

I see you`re from Bulgaria, I can lend you ( free ) my measurement rig if you promise to be good with it - calibrated mic, Behringer mixer for phantom power/mic amplification and a Behringer external sound card. Can also give you an impedance measurement device ( pretty simple to construct on your own ) and an RLC meter. You`ll need to download HOLM for acoustic and Limp for impedance. I`m pretty busy in the last weeks but if I have more time I could help you with the crossover, should be a weekend, otherwise impossible for me. Beer comes from you

Last edited by Mario Pankov; 5th October 2013 at 05:49 PM.
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Old 5th October 2013, 07:10 PM   #9
SY is offline SY  United States
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Quote:
Originally Posted by T101 View Post
Please bear in mind that the "optimization" and "experimentation" capabilities of the ordinary hobbyist are very limited especially when inductors are concerned.
What you're not listening to (but should) is that what matters is target acoustical transfer functions. The crossovers might look a bit odd and will generally NOT resemble 1950s textbook topologies. That's one of the reasons that modern hobbyist speakers are so much better. If you want anything even approaching acceptable results, you need to:

1. Measure the SPL response of the drivers in the baffle/box.
2. Measure the complex impedance of the drivers as a function of frequency.
3. Determine target acoustic transfer functions.
4. Using a decent CAD program (e.g., Soundeasy), load in the SPL and impedance data, then simulate different crossover topologies and run optimizations on them to try to get close to the targets.
5. Then build and measure the complete system to see how close you are to the target (generally, you'll be very close).
6. Tweak component values to hit the target more exactly or to adjust the sound to something closer to your ideal.

Now, all of this ignores other critical issues like polar pattern (which is a function of driver choices, baffle shape, and driver placement), but good CAD software will even let you simulate that as well- and these days, the sims are pretty accurate.

Yes, you will need tools. If you want to design and build automobile motors, you need milling machines and lathes; you can't get good results with chisels and sandpaper! The tools you need to do good speaker design aren't cheap, but they're no more expensive in total than a pair of good woofers. You'll also want to read and understand a few basic texts like "Loudspeaker Design Cookbook" (Dickason) and "Testing Loudspeakers" (d'Appolito) so that you get meaningful results from your measurements and simulations. A good digital crossover (e.g., the emulations in Soundeasy or a Behringer DCX2496) can be extremely helpful as well.

This is not an easy thing, but some diligence will pay great rewards- doing things right with cheap drivers will get you hugely better results than doing things poorly with expensive drivers.
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Old 5th October 2013, 07:20 PM   #10
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by T101 View Post
Guys, you will have to elaborate harder on this.
Hi,

No. Having completely the wrong end of the stick is your
problem. Analyse the designs here : Zaph|Audio

rgds, sreten.
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