4th Order Linkwitz Riley Crossover Filter


Having worked with Vifa's PL18 woofer and PL27 tweeter for many years, I can safely say that 1,500Hz with a 4th Order Linkwitz Riley Filter is the best crossover point as far as sonic quality is concerned.

A 24dB/octave network is ideally suited for this speaker. With the tweeter right at the edge of it's specified bandwidth, such a steep rate of rolloff is a necessity. At the same time, the woofer nasties beyond 2kHz would be sufficiently suppressed too.

A Linkwitz Riley Response has some very appealing features for crossovers. Acoustically, the two drivers sum at unity at the crossover point. The amplitude response throughout the passband is flat, no ripples whatsoever. At the crossover point, it has a zero phase difference between drivers.

For more information about this crossover, Linkwitz Riley Crossovers by Rane Corporation is worth reading. And for the man himself, visit Linkwitz Lab. You can see some of his amazing speakers there.

Using an excellent software freely available on the internet, component values calculated by Passive Crossover Designer are displayed at Figure 22.

Let's take a closer look at this 24dB/octave filter.


(Figure 22)


(Figure 23) 4th Order Linkwitz Riley Passive Filter

4TH ORDER LINKWITZ RILEY FILTER NETWORK

Figure 23 is a schematic of a 4th Order Network with components for Linkwitz Riley response. Let's do a computer simulation using B2Spice to verify the nature of the response.

Sweep is at 1,000 points per Decade resolution. Tweeter is at 6 ohms and woofer at 8 ohms.

 

SPICE MODELING

Figure 24 is the result of Spice's AC Sweep of the filter networks.

High Pass and Loss Pass look symmetrical. Let's examine the data from Spice's tables to see how many dB down at one octave up and one octave down from 1,500Hz.


(Figure 24) Bode Plot of 4th Order Linkwitz Riley Passive Filter

(Figure 25) Crossover Point at 1,500Hz

(Figure 26) Tweeter is -24dB at 750Hz

(Figure 27) Woofer is -24dB at 3,000Hz

According to Spice's tables, the crossover point is -6dB down at 1,500Hz (Figure 25). This conforms to Linkwitz Riley Filter. In Figure 26, the tweeter (Green Curve) is -24dB at 750Hz. This is one octave away from the crossover point. In Figure 27, woofer (Red Curve) is -24dB at 3,000Hz, one octave up from 1,500Hz.

Spice confirms that the component values for the 4th Order Linkwitz Riley Filters are correct. So we have a 24dB/oct rolloff, or do we.

 


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