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July 26, 2020 / consort3

A 4th order LR state variable Xover

There is not much info out there on designing state variable filters. Having said that there is a good article on the basics here:

https://www.electronics-tutorials.ws/filter/state-variable-filter.html

I happened across this excellent paper on how to do a 4th order Linkwitz-Riley type. I used E24 values to implement the design.

https://jahonen.kapsi.fi/Audio/Papers/Statevariable.pdf

You could replace R13, 15, 16 & 18 with a 47k 4 ganged linear pot each in series with a 3.9k resistor. This would tune the  filter from 310 to 4kHz. Such a pot is availiable here:

https://www.omeg.co.uk/wp-content/uploads/2019/12/PC4G16ECO.pdf

4statvarB
4statvarA

As you may know, I have a liking for 3rd order crossovers so I thought i would try a state variable one, by eliminating one integrator. You have to swap the connections, then you find it does not sum flat.  I offset the frequency of one of the integrators and have ended up with a compromise which has low phase change and should give a good pulse response, as the drivers are in phase. Methinks it is a 3rd order version of the Harsch crossover as it has the Bessel type tweeter response and the low phase change characteristic of the Harsch type. When I modelled it with LT1057 op-amps, I got a sudden phase transition at 102kHz on the woofer output which might be a sign of oscillation. A 1k in series with C5 fixed that.

3statvarA

3statvarB

Change the components (E12 this time) to get the crossover frequency indicated

R15, R18 R16 Frequency
6.8k 1.8k 3.65k
8.2k 2.2k 3k
10k 2.7k 2.46k
12k 3.3k 2.04k
15k 3.9k 1.66k

Interesting discussion on a passive third order crossover here (post 83) :

http://techtalk.parts-express.com/forum/tech-talk-forum/50359-third-order-transient-perfect-passive-crossover?p=737401#post737401

I like this active 3rd order crossover. Time to stop designing and start building!

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