I realize this is probably possible, but I lack the electrical know-how to implement it. What I'd like to do is have the "new" Fc controllable by a pot or something, so that one could adjust the equalization to match room gain. Is this possible?
Before my subwoofer amp, line level.
Essentially a standard Linkwitz Transform circuit for an Adire Tempest (Fs = 18.8Hz, Qts = 0.38, Vas = 317L) in a 4.5 cu ft enclosure for a Qtc of 0.71, except with a variable "new" Fc, so that I could adjust the amount of gain of the circuit. Then I could equalize for any room's gain for maximally flat response.
With that driver in a closed box room gain might be desireable. It will start it's roll off around the point where you room will start to gain (< 35 Hz).
Placed in a corner you will be very close to what would be the flattest bass reponse possible with that driver.
Qts is a little low for a closed box which will give a small peak in the bass around 30-40 Hz which can be adjusted with a band rejection filter, but that is hard to do over internet.
In theory it's easy, but needs trial and error development in reality.
It would be interesting to see the variable Linkwitz transform circuit. So far I haven’t figured out how to implement this.
I also searched for variable frequency and variable Q (Bass boost) circuit with constant gain but with no great luck. So I decided to gather together the little info I had about active filters and draw such a circuit for simulation. Simulation has run good so far. There is nothing new - just standard variable hi pass filters and variable gain circuits linked together. I believe I have seen something similar in the web somewhere also. Again it’s not Linkwitz transform but lets you raise or lower the Q and/or frequencies you want to boost if this is what you are after.