Passive Line-Level Crossover
This page started from a BASS List post by Peter Rettweiler (and has evolved since then)

A spreadsheet to assist in calculating values: PLLXO_Calculator 21-dec-10

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A passive line-level crossover (PLLXO) is a very good solution if you don't need circuits that are too complex circuits and can live with the insertion loss. You'll be rewarded with all advantages of biamping and you won't need opamps in
your signal path.

I'm currently using a 12 dB PLLXO for my satellites and I'm very satisfied.

6 dB filters would look like this, you can cascade them for higher orders, although beyond 2nd order the drawbacks become overwhelming.

First Order Lowpass:

Select a value for R1 (around 5-10 k W should work well with most amps), then calculate the C's like this:

f = crossover frequency 

First Order Highpass:  

Ramp = the input impedance of the HP amp

If the input impedance is close to R1 you need to consider it for the LP as well and raise the value of C1. 

Note: If R1 is left out, Ramp becomes the load R for the filer,

So for example, if we want a 60 Hz crossover point, and the total R is 10K, a 265.26 nF capacitor would be required.

You may also want to add a potentiometer for level matching.

The 12dB version looks much closer to a Linkwitz slope if it looks like this:

Second Order Lowpass:

Select a value for R1, then:

f = crossover frequency

We must consider the R of the amplifier so:

Solving for R2':
Second Order Highpass:

So for example, if f=1kHz: R1=5 k W, R2=50 k W, C1=31.8nF, C2=3.18nF

Note that the smaller resistor will more or less determine the impedance which your preamp will see, while the larger one is the main factor for the insertion loss, so things may get tricky if your amp has a low input impedance.

The amp's input impedance lies in parallel to C2 and R2.
 

In the hi-pass the input impedance of the amp is parallel to the capacitor C2. If the input impedance gets lower, this has two consequences: the insertion loss gets higher and the xo frequency is shifted upwards. To compensate, calculate C2 in the hi-pass as follows:

Note that the input impedance of your power amplifier needs to be fairly high to make a 2nd order hi-pass feasible. You can allways use the input impedance of your amp as the 2nd shunt R when calculating C2, and just leave R2 out.



Art Ludwig discusses PLLXOs

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