Seeking Help with Passive Filter Design

[ryansupak]

Hi, I bought a circa-1959 passive filter set. The faceplate writing, I've determined, is all in Hungarian.

I think that it used to be part of a vinyl mastering setup, since it basically consists of an LPF and an HPF in series (the idea being, presumably, that you'd want to cut out the rumble and hi-frequency energy to make the deepest vinyl cut possible). The net effect is that of a Band Pass Filter with adjustable width.

Anyway, the thing is super-cool and sounds great (loaded with giant inductors and transformers) but what I'd really find much, much more useful is an adjustable Band Stop Filter -- in other words, a thing more like a "smiley face EQ" curve.

So, I started doing research since based on my limited knowledge of filters, I knew that many simple versions of filters were the same few components with their orders rearranged. Sure enough, a Band Stop filter turns out to be a Band Pass filter, except in Parallel instead of Series.

I decided I wasn't going to do any mods until I fully understood what was going on. So, I created a SPICE model of the unit, in a given knob state, and in the Parallel configuration. Here's my first draft:

smiley.cir - inductive bandstop filter
*voltage source
v1 0 1 ac 5

*lpf
l1 1 2 0.5

*hpf
c1 1 2 1000p 

*output load
rload 2 0 1000

*test
.ac lin 20 30 20000

.end

The problem is that this creates a very steep "smiley" and I'm looking for something much more gentle. Any suggestions how I would change my simple circuit to achieve that goal?

Attached is a pic of the current "too-steep" response curve, and since it's Gearslutz, a picture of the faceplate

rs

[trumq]

I'm not sure how well this would work, but could you split the signal that you want to filter, invert one side, feed that side into the bandpass filter, then mix the inverted, bandpassed signal back into the non-processed signal? Then the level of the processed signal would control the depth of the bandstop. If that works, then you wouldn't need to modify the unit at all, and could still use it for its original purpose if the need arose.

[ryansupak]

Thanks for that suggestion. I plan on trying that, at least in the short-term.

rs

[ryansupak]

Half the battle is knowing exactly what term to search for -- and "shelving" should have been obvious:

Active Filters