Notch Filter Control

[amz-fx]

A new post in the Lab Notebook about a notch tone control using a bridged-t filter:

http://www.muzique.com/lab/main.htm

Should work well with fuzz pedals that need the highs toned down.

Enjoy!

regards, Jack

[Mark Hammer]

Another nice little sub-circuit that will no doubt be referred to a lot....that is unless distortion pedals somehow fall out of favour.

Thanks, Jack

regards,
Mark

[GibsonGM]

Nice study on that, Jack, thanks!  

[Kipper4]

Bookmarked thanks Jack

[glops]

Thanks, Jack! I have referenced your site very often over the past 4 years. Nice to see a new addition...

[midwayfair]

This is very timely ... I just spent the better part of Saturday afternoon messing with Duncan's Tone Stack Calculator on the BMP tab with the mids maxed trying to work out a notch filter! Now I have some more things to try out.

Some more ideas with tone controls for notch filters:

-I think you can move the notch with a control by flipping the shredmaster control sort of upside down, like this:

    |-------cap-----|
In > pot1         pot2
              pot 3
                |
              cap
               |
               G

-Mark Hammer's mod for the Companion Fuzz (a common use of the notch filter) puts a pot in series with the cap to ground, to reduce the midrange scoop.

-I think something slightly different happens if you flip jack's new control so that the input goes to the pot instead of the output going to the pot, but I'm having trouble visualizing it at the moment. I think it should move the notch to a higher frequency but also trim the highs, instead of Jack's which moves the notch lower but trims the highs.

-A pot + cap in parallel with the high-pass capacitor that bypasses the whole thing (the 500pF in the first example) will obliterate the notch for the most part. Using a fairly large value and an all-pass capacitor could actually make an interesting GAIN control. You'd get a fairly large signal boost with the right values, and more midrange at the same time, bumping up the gain a lot and going from a really nice clear clean sound with very present highs to a more saturated midrange-y lead sound (where the highs aren't cut but are simply not as prominent in the signal).

[Mark Hammer]

I knew I had seen it somewhere before, when I recommended something like this to a forum member recently.  Jack mentions it was in the Marshall Shredmaster.  The only change I would make is to add a fixed resistor in parallel with the leg of the pot going to the 47nf cap.  It actually doesn't take much resistance to effectively eliminate the notch, so reducing the effective resistance of that leg would avoid squishing the usable part of the pot rotation down to the last 5 degrees or so.

Exactly how much parallel resistance, I couldn't say, but one can always start with 47k.  This will make the combined parallel resistance of the pot leg and resistor about 24k at the midpoint, between the 47nf cap and ground.

[J0K3RX]

These work really well in conjunction with a Baxandall... adds great tonal shaping and control over the mids. Work good right before a Marshall tone stack as well!  Wampler makes good work of notch filters and so does Amptweaker...

[rm77]

Not sure how helpful this is since I'm pretty new to this stuff, but here is something I found with the Duncan Tone Stack Calculator, following Jack's lead on adjusting the values of the BMP tone stack in the DTSC to make it a useable test circuit for this new notch control:

-Set R3 to 100k and R1 to 10k, and you get a nearly identical scoop as far as frequency, and similar hi cut response as described; but a shallower scoop
-Set R3 to 10k and  R1 to 100k, and you get nearly identical results as well, just a deeper scoop
-Set R3 to 25k and R1 to 47k, and you get basically the same response as the 33k/33k example, just a couple db deeper and a slightly lower center frequency of the scoop

These values are just me trying to recreate the results from parts I know I have and are common...maybe useful in trying to apply the circuit to a build?

Kind of like how Jack's body pot for the BMP control adjusts how the mids behave when the main tone pot is adjusted, I wonder, is there a way to adjust the depth of the mid scoop via a second pot while keeping the mid scoop frequency basically the same, and while the main tone pot still alternates between a mid scoop and a low pass?

[acehobojoe]

Very nice!

[tca]

Works nicely! Thanks.