One band parametric/notch filter? Any suggestions

[ChrisPtacek]

Have any of you guys bumped into a product like this?  Looking for a one band parametric eq to use as a notch filter for a bassist.  He likes to sweep a room and find out where that "boom" happens so he can eliminate it and have his bass tone remain clear and non-muddy.  Ideally, he wants to put it in a stomp with a big MXR style knob to be able to adjust it with the foot.  I haven't seen anything quite like this in stompbox form.  There are some DIY 4 band parametrics, but I don't know about a "Notch in the Box."

[MikeH]

I believe there is a craig anderton project like this.  A simple parametric EQ for guitar or bass.  And I think there is also an article on how to build one a geofex.com

[Mark Hammer]

Simplest thing is to build a 2-stage phaser and use a dual-ganged pot instead of FETs.    Since you want to be able to manually tune where the notch is, you don't need the FETs, their biasing circuit or the LFO to sweep them.  If you co-opted the Phase 45 circuit, you'd really only need 3 op-amps.

The location of the notch is given by the usual 1/(2*pi*R*C) formula.  So, if the cap between the output of the previous op-amp and the noninverting (+) input was .047uf, and the dual-ganged pot was 10k in series with a 12k fixed resistor to ground, the tuning range for that notch would be between 154hz and 282hz. 

If the cap was .047uf and you used a dual-ganged 25k pot in series with a 10k fixed resistor to ground as the varied resistance replacing the FET, you'd have a sweep range from 97hz-338hz.  If you wanted to, you could use a pair of .047uf caps in series and use a toggle to shunt one of the caps in series as a way of switching the range.

Normally, the "intensity" of the phasing effect depends on the relative amplitude of the straight vs phase-shifted signal at the mixing stage.  In the Phase 45 that mixing stage is a simple pair of 10k resistors.  As the 10k resistance from the phase-shifted path is increased, the amount of cancellation is decreased, and the notch depth is reduced.  So, a 10k resistor in series with a 50k-100k variable resistance, would set the notch depth from maximum to barely audible.

[R.G.]

The single-notch phaser is one way that will work.

There's another at GEO - see http://geofex.com/Article_Folders/EQs/paramet.htm for the twin-T solution. Another, possibly better one is with a state varable filter used as a notch filter. The state variable filter does the notch directly, with full control of notch frequency and depth. Both Twin T and SV use one dual pot for a frequency control.

I think your friend will be more happy with two knobs - a frequency and a depth - than with one.

It was interesting to me to note that of these three approaches, the difference in complexity is just about nil. All require dual pots and all use either one or two dual opamps.

[ChrisPtacek]

Great stuff, guys.  Thanks!

[rocket]

the boss sp1 spectrum is such a thing as well. (also now copied from behringer)

[rocket]

but I don't know how deep a notch the sp1 can make.

[Mark Hammer]

Not particularly.  There is a difference between notches which have a simple tonal impact, and ultra-deep notches which are created for rejection of specific problematic frequencies or for other very specific purposes.

You should note that both RG's suggested circuit, and the one I outlined, are really co-opting of tonal change circuits for the purpose of "taming" problematic frequencies.  Neither would qualify for "lab quality" notch filters.  You CAN find, however, plenty of designs for much steeper, narrower notch filters in many of the various filter cookbooks or application circuit collections around.  There are also those assorted switched-cap filter chips, like the MF4/6/10, etc., that let you construct very complex notch filters.  These have the advantage of being controlled by a master clock with a single pot, where normally "re-tuning" a multi-pole filter would require simultaneous adjustment of so many resistors as to require the sorts of pots you'd have to custom order, and even if you could get them they would be so large you could never stick them in something like a 1590BB.

[Steben]

Regardless the circuit, it is always nice to put the freq control in a footpedal like a wah. With a Q control on the state variable filter (notch or boost) you can go from faux-phaser to wah, all controlled in one pedal.

[coxter]

I thought that was the Nathan East, NE-1 Para-EQ for Bassist?

[Paul Perry (Frostwave)]

I wonder what is the shape of the frequency response of a problem room?  I don't imagine it would be REALLY sharp.

[Mark Hammer]

The room isn't, but you put the room, the P.A., the sound system positioning, and the players' locations together, and out of all that morass you get a couple of "conspiracy resonances" that you have to tame.

[R.G.]

Which makes me think. Peavey used to put twin-T notches in their PA gear for suppressing howling. I think other makers did too. These were pretty carefully aligned twin-Ts but not boosted for super sharpness. I think that bass muddiness might respond best to notches that were not particularly sharp. It would certainly be easier to find them. Super sharp filters are so selective that it's hard to find the frequency setting that works.

One interesting thing that might be good for this situation is that for varying the frequency of notch filters you need at least a dual pot, and a quad pot is what the math really wants. That's kind of hard to get these days.

You could use two dual 100K digipots (US$1.42 each, Mouser), a single frequency control pot, and a US$1.00 PIC to read the control pot, then set the digipots to the same reading. You don't get the issues of zipper noise because this is a set and forget unit. The programming is so trivial that it's a good beginner's programming exercise. For that trouble, you get four pots that both match and track within tight tolerances, which is hard to get in the analog world. I suspect that the total cost for the PIC and two digipots is cheaper than finding a quad pot, and a lot more available if you can find a quad pot.

[Mark Hammer]

Steve Giles will probably want to blurt it out, but I'll blurt first: CMOS switched resistors.  MXR cleverly used 4016/4066 quad CMOS switches, controlled by a HF clock whose pulse-width was varied, to do a number of filter-like tasks.  The Envelope Filter uses this approach, as does the Analog Delay pedal, and one of their later-issue phasers.  The beauty of the approach is that one can use essentially conventional filter topographies, and stick in a CMOS switch section in series with a fixed resistor to vary the overall effective resistance, and frwequency of the filter section.  In theory, one can construct fairly complex, and tightly matched, filters in this manner.  The only downside, I suppose, is that, much like through-zero flangers, if you have several notch sections controlled by individually variable HF clocks, you run the risk of clock noise and heterodyning.  But that's something you can always tame with board layout, proper shielding, short leads, etc.

[puretube]

Maybe Mr.Nyquist can give some food for thought on "unusual multiple pots-implementation"...
(fig.6)

[db]

You can make a parametric EQ with a desired amount of Q-gain dependency.  This is useful, because at small amounts of boost or cut, a fairly wide/low Q can be obtained for a more subtle effect whereas when you increase the boost/cut, a narrow/high Q results.  This can also mean that the Q control can be fixed rather than variable so less knobs are required.  Also, you can make a parametric which has high Q on cut and broader Q on boost so it can be used to increase mids or null out problematic signals - this has more use on a mixing desk I would have thought.

[Paul Perry (Frostwave)]

You can make a parametric EQ with a desired amount of Q-gain dependency. 

I guess someone - not me! - could make a parametric with a F -Q dependency too.
Which would be very useful for making a wah 

[mills]

Regardless the circuit, it is always nice to put the freq control in a footpedal like a wah. With a Q control on the state variable filter (notch or boost) you can go from faux-phaser to wah, all controlled in one pedal.

This is something I've really been wanting to do lately, but with decently wide frequency range (or a few that are switchable).  Have you actually done this, and is there a specific state variable filter that you used or thought would work well (or just any generic one would work)?  Right now, my only thought was the geofex parametric schematic with a switch for a few ranges... any thoughts or experiences would be hugely appreciated.

[Paul Perry (Frostwave)]

I don't think an anti-feedback unit (as used to stop howling in a PA) would help, if the problem is "muddiness".
Because, the "muddiness" probably comes from actual room resonances (which can - you never know - be quite sharp and fairly long-lived.)