Second order shelving filter


2011-10-13 8:52 pm
Hi there,

After building a few mics, I wanted to be able to switch in a 'bass boost' in a preamp for a cardioid mic. I realize there is the proximity effect but the WM-55A capsule has roll-off at around 300Hz, down 10dB at 100Hz when used at anything over a few cm away. So as an experiment to 'flatten it up' I thought I'd knock up a simple shelving filter to counter-act this so I can test it out, comparing it to an omni, WM-61A (but with less ambient noise).

Alas, I cannot find much info on active second order shelving filters, mostly first order. Also the linkwitz lab website (which got the most hots for such a filter) seems to be down.

The only 'trick' I've done so far is to design a second order stellan low pass filter, with a Q of around 5 - this gives a resonant peak of +10dB at 100Hz, rolling off too 0dB at around 400Hz. Gives a nice sharp roll off after 80hz too, but it is rather steep.

So if there are any examples of shelving filters with steeper roll-off, I would appreciate it :) Again, this is less of a 'project' and more just for testing purposes, as nothing beats knocking up circuits, recording and tweaking to test mics!

Cheers again,