Analog Crossover Network
Nelson Pass's LXmini Analog Crossover, otherwise known as the Analog Crossover Network (ACN). Please read Nelson's LXmini Crossover article to learn more about it, then head over to diyAudio to discuss it in the LXmini crossover thread.
This crossover is highly configurable and can be used to create an active crossover built using only discrete analog components. Nelson has now posted a MicroCap sim file you can use to create crossover simulations for your own speakers. If you have any questions, please post them in the LX Mini Crossover discussion thread.
Chassis / Hardware
What is an active crossover and why do I want one?
In a loudspeaker with more than one driver element, a crossover network divides the audio power into frequency bands that are appropriate for the drivers. For example, a two-way loudspeaker may have a woofer and a tweeter. A three-way speaker might have woofer, mid-range and tweeter drivers.
In a passive crossover filter, this function is provided by coils, capacitor and resistors so that the woofer sees the low frequencies, the mid-range gets the mid frequencies and the tweeter sees only the high frequencies. This gives very much better sound but also protects the mid-range and tweeter from lower frequencies that would cause them to distort and possibly break.
An active crossover replaces that passive network with an active circuit that divides up the frequency bands and feeds them to separate amplifiers which then drive the loudspeaker elements directly.
There are advantages to this approach which often make it worth the extra effort, and they all relate to performance quality:
Lower distortion – loudspeaker drivers fed directly by amplifiers generally have lower distortion and better transient response due to the high damping factor of the amplifier. Passive crossover networks placed between the amplifier and driver compromise this effect – for example, there will often be frequencies where the damping factor seen by the driver could go from values of 100 or more down to as low as 1 or so, allowing considerably ringing or overhang in the transient response. An active crossover allows the loudspeaker driver to see the consistently high damping factor of the amplifier at all frequencies.
More and better power – two amplifiers splitting the audio band will typically allow power performance approaching four times that of a single amplifier of the same power, and these amplifiers will also show much lower inter-modulation distortion.
Adjustable and Flexible – perfect for the DIYer, active crossovers can be made very adjustable and are not necessarily fixed to the needs of a specific loudspeaker, so they can often be used with different or modified designs. Even in crossovers with fixed characteristics it is still easy to adjust the levels of the different drivers, allowing different choices of amplifier types between drivers. For example, you could use a Class D amplifier on the bass, Class A amplifier on the mid-range, and tubes on the high frequencies.
What's so special about this one?
This particular active crossover was designed around the Lxmini at the specific request of the designer, Siegfried Linkwitz as an alternative to DSP based crossovers. There are people (particularly vinyl users) who prefer traditional analog electronics for their sonic character and also to avoid artifacts from the conversion of analog signals to digital for DSP and then back to analog.
As analog filters go, this one is aimed at high-end audiophiles with Class A discrete JFET circuitry, mostly buffers operated without feedback. The JFETs have been individually matched and mated to resistor values in each filter to achieve less than 0.01% distortion.
Does it support speakers other than the LXmini?
Yes. You can stuff the board with alternate RC values for different frequencies and slope character. There is also a (cheaper) version available with just the pc board and JFETs so that you can use your own capacitors and resistors and create your own filters. There will be design templates posted which can be used to model in LTSpice, FilterCad, MicroCap and other simulation software.
As it includes equalization for bass boost and high frequency notch, this crossover is particularly useful for two-way designs with full range drivers and woofers in open baffles as well as more conventional enclosures.
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