A rarely discussed crossover is the Series Crossover (in contrast to the universally popular Parallel Crossover). It offers considerable advantages especially with a subwoofer and a bass midrange. I used a series passive crossover in the
Bitches Brew Live Edge Dipoles (which are featured in
the new October 2023 issue of AudioXpress). Here is the schematic:
ABOVE: In a series crossover, you wire the woofer and midrange in series. You short the woofer with a capacitor. You short the midrange with an inductor.
The circuit above is what I used in the
Bitches Brew design. The Bitches Brew has (2) SB Acoustics 15OB350 subwoofers and (1) B&C 15CXN88 coax used as a bass midrange. Note the reverse polarity of the midrange; usually but not always required.
This is in contrast to the usual, BELOW, where the woofer is in series with an inductor and the midrange in series with a capacitor (and both sections in parallel).
ABOVE: Look closely because this is super interesting.
I'm showing the woofer half of the coax. I'm using actual measured impedance of the drivers in my model, and showing the filter voltage at the driver terminals. The parallel circuit above is not "optimized" for anything except to illustrate the problems with standard parallel crossovers. It just happens to have the same values I used in the series circuit.
In the subwoofer section, the inductor interacts with the impedance of the 15OB350s to create a peak around 50Hz. Then it rolls off above 100Hz.
In the midrange section, the capacitor interacts with the impedance of the 15CXN88s to create a dip at 100Hz,
followed by a 6dB peak at 35Hz. Very bad!!!
This +5.5dB peak at 35Hz is HIGHLY undesirable and happens ALL THE TIME in subwoofer-mid passive xovers. The reactance of the capacitor cancels the inductance of the bass-mid just below the bass-mid resonance and creates a peak exactly where you least want it.
(The average speaker designer is likely not aware that a passive crossover can BOOST, rather than cut the voltage below the crossover frequency! I discuss this phenomenon in detail in the thread
Open Baffle Bass Boost: +4 to +7dB w/ Passive Xover, No DSP where I show some cool ways to use this to your
advantage. Especially in dipoles.)
Usually, nobody aware it is even going on. You build a 100Hz crossover for your subwoofer and satellite and you don't even know the satellites are getting a few dB EXTRA bass at, say, 60Hz. Right where it's most harmful.
There is no easy fix for this in a conventional parallel crossover.
However in a series crossover all of the impedance peaks work in your favor instead of against you.
#1: The impedance peak of the subwoofer cuts the signal received by the bass-mid by over 20dB at 28Hz. Which is exactly where we most want to protect it.
Don't forget, this is 'just a 6dB per octave crossover' yet the electrical slope between 60Hz and 30Hz is closer to 24dB per octave (!) which is fantastic. It's effectively a steep 18dB shelf filter.
#2: The impedance peak of the bass-midrange (which is about 60Hz) interacts with the crossover to boost response 3dB. While this is not desirable per se, the roll off above that point is immediate. In my opinion, the mild 6dB/octave slope above 100Hz helps the subs blend better with the mids.
I corrected the peak at 60Hz with DSP. In most bass-mid crossovers, the midrange would have a higher resonant frequency than 60Hz and would be even easier to work with.
Above: Impedance of the circuit. In this design the 60Hz 2ohm minimum is OK for me. It's possible to choose different crossover values to achieve a higher Zminimum. Actual measurement is below (I've added a Zobel to flatten the impedance.)
In an earlier
discussion with
@Lynn Olson we discussed the difficulty of getting subwoofers and bass-mids to seamlessly integrate. This is a very good way to do it especially because the cone sizes are all the same. In this design, both subs and bass-mid are 15" and both in the same dipole array.
The
Bitches Brews are bi-amped with DSP, with the active crossover between the coax midrange and tweeter. I could have triamped them, but I felt that a series crossover as shown here was much more simple and elegant. Fewer amps, fewer DSP channels, fewer signal cables and speaker cables.
I feel air core inductors are worth the extra money for an "ultimate" no compromise design like this; I didn't want the hysteresis of iron core inductors. So I used Madisound air core copper foil inductors with DCR below 0.6 ohm (low DCR is very important, the higher the DCR the less attenuation of low frequencies that you get in the midrange signal).
That's especially important in this design, because the DSP boosts low bass 12-15dB around 30Hz and the deep notch in the midrange filter is very desirable there. I can shove hundreds of watts at 30Hz into these, and the midrange cone only moves +/- 1mm or 2mm.
The Bitches Brew has one of the most seamless crossovers I've ever heard. The acoustic crossover between subs and mids is at about 90Hz, all the cones are 13" in diameter, and the coax tweeter integrates perfectly with the midrange. The radiation pattern is extremely well behaved and there are no lobing problems at any frequency.
Above: Measured polar response, +/-90 degrees, 1/3 octave. I don't have a good way to measure these at low frequencies, but VituixCad calculations are below.
Above: VituixCad vertical directivity calculations for 2x 15OB350 and 1x 15CXN88. Variation is only about +/-2dB.
Expanded details on the Bitches Brew crossover are
here.
@Yourmando
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