[Garbz]

This isn't strictly headphone related but I've seen a few people here playing with bass boost / treble cuts so I may get a decent response.

I've just built a subwoofer that I'm having difficulty integrating into my system. The sub is a standard sealed box with a linkwitz transform filter to boost the bass down to 17hz as the -3db point. (this will be taken up to 25 shortly as my room shakes too much)

The problem i'm having is integrating it with my speakers in the top end. My speakers being ported had to have their ports sealed for phase linearity. This puts the -6db point at 75hz, and thus I've tuned my sub to roughly the same frequency. Currently i'm using an RC filter to cut the treble before the sub amp, and this is causing 2 problems. Firstly the frequency response is all over the shop between about 60-90hz, and secondly becuase of the slow rolloff of the sub's treble it's not as invisible as I'd like as it's not cut out enough at 150hz. So currently I've the choice of hearing the sub in the system, or having a frequency hole, so I need a better filter.

Does anyone know of any filters preferably bessel type filters which are at 2 pole minimum, and the key problem is adjustability. I was looking at VCVS filters http://www.engin.brown.edu/courses/e...res/FILT03.htm but since these require 3 identical part values I can't adjust the resistance with a dual pot. Another option was a state variable active filter like the UAE48 from burrbrown, but this isn't exactly designed for audio applications.

I'll try the UAE anyway but I was wondering if anyone knows of any other steep adjustable filter types?

 

[ATAT]

Going off the top of my head, wasn't chebyshev adjustable?

 

[Garbz]

They all are adjustable the point between the different filter configs that is bessel, chebyshev, butterworth, and the other one, is the the different responses and phase in the pass band and stop band. The resulting filter depends on the ratios of the resistor and capcitor values in the VCVS design.

I just noticed something very interesting. I modelled a 2 pole Bessel filter using the VCVS design. Althought there are 4 resistor, 3 set to the same value and one at a ratio of .286*R, only 2 of them actually determine the rolloff of the filter. The resistors at the +ve input of the opamp determine the corner frequency, but the resistors at the -ve feedback loop and ground don't need to match the input ones to be a bessel filter, they only need to be at a ratio of 0.268:1

That means I can use a stereo pot for 2nd order lowpass filter, but I'd still be interested if anyone knows of any higher order filters which are adjustable.

 

[JahJahBinks]

Usually adjustable higher order filters are done in digital. I am interested in seeing an analog one too.

 

[Garbz]

Nope VCVS are the only types of active filters left to choose from. The state variable and biquad filters I've looked at allow you only to select a centre frequency and a bandwidth, so if you move the high pass section you move the low pass. Not my idea of adjustable

.

I'll try and find a really cheap local 4 gang pot and concatenate 2 VCVS lowpass butterworth filters for a nice 24db/octave drop.