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February 28, 2017 / consort3

Minimalist Duelund crossover

mcc
A remarkably cost effective 3 way active circuit is due to Egerton in WW Circuit ideas, Jan 1988. A subsequent analysis by Brian Pollard in WW Feedback July 1988 showed a slight error in Egerton’s analysis but despite that the circuit is adequate. As given the crossover frequencies are 363 Hz and 1.456 kHz. Egerton found he needed a 200pF capacitor across R5 to suppress high frequency oscillation. The midrange filter is a maximally flat type. I haven’t tried this circuit myself as I have only just found it. Such an elegant circuit demands a decent implementation. However the mid-range driver needs to be high quality and ATC domes do not come cheap!  A 2 way design could be made by eliminating the woofer components.

The resistor values given are exact presumably so you can see the relationships. Putting the nearest E24 series values slightly changes the results. I scaled the resistor values to the highest I would be comfortable with. That also gives the lowest capacitor value.

Try R1,2,3,4 &6 as27k, R5 &13 as 16k, R12 10k, R8 68k, R10 39k all Cs 8.2n. If the Cs are made 4.7n the crossover frequencies are 655 and 2.675kHz, which is suitable for the Faital pro 3FE22 I want to try.

The circuit appears to be a minimalist form of the State variable filter.  The tweeter circuit unusually uses differentiators and an op amp is saved by combining the summing functions. Rod Elliots adjustable version  of the filter (an analogue DCX2496!) is explained here:

https://sound-au.com/project148.htm

The circuit seems pretty close to a Duelund crossover. There is also a good analysis of the Duelund by Linkwitz on his site here:

https://www.linkwitzlab.com/crossovers.htm

John Krekovsky did an analysis of the Duelund here

http://musicanddesign.speakerdesign.net/Duelund_and_Beyond.html

The other John K used a Dayton RS52 in one of his designs, which might be an appropriate mid-range unit. However it would need a notch filter to get rid of the breakup spike. Talking of the RS52 this project uses it:

http://ampslab.com/falcon.htm

Although the designer does not mention it, the crossover looks “Duelundish” to me!

Using Ltspice, I summed the outputs in simulation by using a simple mixer circuit. That showed that the polarity of the mid had to be inverted as Pollard had noticed, also that the mid out was 3db more than normal which could be compensated by the mid amplifier of course.

An easier way to sum the outputs is to use Plot settings/Add trace then in the “expressions to add” window, add the woofer and tweeter outputs (+sign) and subtract the middle output. This shows an underdamped response. Reference to Don Lancasters Active Filter cookbook which has a good section on State variable filters showed that R5 could be tweaked to vary the damping . With R5 at 7.1k the mid response is a plateau but 4.4dB above nominal.

mcc1

With R1,2,3,4 & 6 as 13k, R5 9.1k R12 3.6k R13 6.2k R8 47k and R10 27k also all Cs 10n you get crossover frequencies at 447 and 3.57k suitable for the venerated ATC mid dome. If R5 is made 12k you get the plateau response with a 4.4dB lift.

mcc2

There is a good discussion on the Duelund here particularly post 26 by Jon Marsh

http://www.htguide.com/forum/showthread.php?33931-Beyond-the-Modula-NeoD-CC-(split-from-original-thread)

Jons piece plus my sims showed that the mid output should be below the woofer and tweeter outputs. To get a level sum on the original circuit, I found it easier to raise the woofer and tweeter levels by raising R2 & R6 to 18k and making R5 9k. Mid in and out of phase plot with this mod:

mcc3

I am sufficiently intrigued by this circuit that I am going to try it with a HiVi TM1a mid/tweeter combination plus my long suffering CP168. Crossover points would be 1.4K and 5.5K

A 4 way Duelund!

http://audioselvbyg.com/articles.php?article_id=17

The above site has a helpful application to work out values for a passive Duelund circuit. To emulate the Egerton circuit, I used an A factor of 2.3 and got this:

torben1d

With this outcome:

torben1c

Another useful Htguide thread on a derivative of John K’s RS52 design:
http://www.htguide.com/forum/showthread.php?33852-ZDT3-5-(sort-of)-Duelund-crossover-design-(I-hope)

The electronic filter slopes are also the target acoustic slopes, but the drive units will not perfectly convert their electric inputs to acoustic outputs. It is the amount of tweaking needed, the less the better, which will determine the success of this filter topology.

Pdf of original article:

egerton

Thanks to the insights provided by Messrs Marsh and Krekovsky I have tweaked the Egerton circuit into a likeness of the Duelund crossover.

I noticed that Parts Express were offering the RS52 at a sale price of $40 so I came up with an active notch filter to add to the Active Duelund circuit . It uses a bridged T configuration with positive feedback to increase the Q. It needs setting up by having a signal generator that is set up to run at the break-up spike frequency by monitoring the break-up spike with a microphone and a scope. Then insert the filter and tweak the pot for minimum signal.

The usual provisos about supplies and decoupling for the op-amp apply In addition the input needs to be at 0V. If it is AC coupled it needs a 150k resistor to ground to provide bias for the op-amp

rs52notch

rs52notcha

I have had second thoughts about the RS52 notch filter. Rather than have the hassle of setting a pot I have widened and deepened the notch so that no setting is necessary. R1 is now 3.3k and R3 is 220 ohm The pot is replaced by a short circuit.

rs52notch2

Another interesting choice for the mid/tweeter combination would be the Tannoy coaxial which meets Duelunds timing criteria. Lower crossover frequency would be 160-250 Hz and would allow a smaller cabinet. Other candidates would be the KEF 105 and the B&W Matrix 801. These have the neccesary driver offsets. Meantime I will build my own design inspired by the 105. The question is how greedy to be with the mid bandwidth?
Update 3/9/18 I just noticed that Parts Express are selling a fabric dome RS52 which does not need the notch filter. Incidentally it is worth exploring Michael Chua’s Ampslab website referenced above as he has done a lot of work on amps and speakers.

My technique and indeed the Synkron technique (component values rather than acoustic slopes) do not allow the tweaking of the design if the real world drivers do not conform to their specs. This article shows how much a standard 3 way design has to be tweaked.

https://www.physical-lab.com/images/pdf/FilterShopApp_05.pdf

I would argue that with wide bandwidth drivers and a limited bandwidth ratio the techniques will work OK.

Why bother with Duelund? It is a development of the Filler driver concept introduced by Baekgaard of B&O in 1977 which gives linear phase. Good explanation here:

https://www.acculution.com/single-post/2017/08/23/009-Time-Alignment-in-Loudspeaker

It is worth reading Duelunds original article!

https://duelundaudio.com/wp-content/uploads/sites/3621/2013/12/duelund-filter.pdf

There is not much on the internet about the Duelund crossover. However I encountered this from some French enthusisasts:

http://forums.melaudia.net/showthread.php?tid=5956&page=1

And this (use  Google translate)

http://forums.melaudia.net/showthread.php?tid=8739

dbdb

More on this type of design:

http://www.gammaelectronics.xyz/ax_2007-01_unique-crossover.html

A tone control based on similar state variable filtering techniques

http://waynestegall.com/audio/svtone.htm

Update 18/1/21

The activity on the equal capacitor 3rd order crossover gave a clue that some judicious tweaking would get a flatter response and I used standard E24 values as well. R5 is now 9.1k, R2, 6 are 18k R12 is 9.1k R13 is 13k R8 is 51k and R10 is 36k. Cs are 10n Crossover frequencies are 373 and 1.463KHz  and the graph follows:

For a wider band midrange i.e 443 to 3.406kHz R2 &6 are 11k R5 is 8.2k R12 is 3.9k, R13 is 5.6k R8 is 43k R10 is 30k and the Cs are 10n. The flatness is a function of how close the R10/R12 ratio is to the R8/R13 ratio. Another good set is R8 62k, R10 43k, R12 11k and R13 16k. The phase of the mid and tweeter tracks but is not coincident  with the woofer. Perhaps the best way of getting the drive unit z axis positions right is to set the mid to positive phase and measure the scary ghost like response. The mid position is tweaked for the  best first null and the tweeter position is tweaked for the best 2nd null.

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