Vari-Q Bandpass Filter With Constant Gain

Well here's the progress so far. I still need to get the power supply board connected up and then do some testing, assuming it actually works. Then make a box of course.

The two pushbutton switches are for bypass and swap. It can swap the filters between the L/R channels to show the person in the passenger seat what they're missing out on.

Well, tis actually working pretty well, with one exception - the frequency pot. It's just too sensitive at the top end of the travel, and definitely not linear. I used a log taper pot as it seemed that the relationship between freq and resistance would require it, but I think it compounded the situation. I'll try some test sims tomorrow to try to get a good plot of the relationship.

mojozoom said:

Well, tis actually working pretty well, with one exception - the frequency pot. It's just too sensitive at the top end of the travel, and definitely not linear. I used a log taper pot as it seemed that the relationship between freq and resistance would require it, but I think it compounded the situation. I'll try some test sims tomorrow to try to get a good plot of the relationship.

Click to expand...

You need RevLog pots! Or a different arrangement that allows law-steering of Lin pots.

Wow, yes that's definitely the problem. It would have been better of with linear pots than with the log pots I have. I ordered some but that are non stock so it might be a few weeks before I can do any more testing. That should give me time to build a box though.

The good new is that the rest of the circuit does exactly what it should. It was my first foray in Eagle CAD, and my first full scale SMD project as well. Thanks to everyone for their help!

> it might be a few weeks before I can do any more testing.

Switch the leads on the pots. (Perhaps just move the wiper-jumper to the other end?) Number the knob backward.

CW=higher or CCW=higher is totally arbitrary. Just like the BBC wired slide-faders so if the engineer fell asleep and pushed all the sliders "away" it would go silent, not LOUD like most other consoles. I do not know how universal this is, but the gas-stoves of my youth all had "more" tuning CCW. You just have to re-train your muscle memory. (Or suffer until convention-pleasing pots arrive.)