A Simple One Opamp Parametric

[George Giblet]

This was spawned off another thread,

http://s476.photobucket.com/albums/rr130/ggeffects/?action=view&current=gg_simple_parametric.png

It's quite a well behaved parametric and uses the bare minimum of parts.

You can cascade a few bands together without any problems.

[Processaurus]

Neat, does the input need to be a low impedance to soak up the opamp's output that comes back through RV1?

Sweepable mids are really where it's at.  I'm working on a PCB board that has the Metal Zone parametric EQ section on it with the same goal, figuring a parametric eq on any distortion pedal would make it versatile.  I believe small bear has a 2x 100KC pot (for univibe's originally).   Not in 16mm, but beggars can't be choosers.  Or beggars could choose a higher value linear pot and taper it with resistors.

[jacobyjd]

You can cascade a few bands together without any problems.

Cascade, meaning place them in series?

My first thought would be to run a couple of these in different bands in parallel--maybe I'm misunderstanding though.

Either way, it looks like a neat little snippet--I'll give it a whirl on the breadboard

[Processaurus]

2 in parallel and you'd lose 1/2 of what you accomplished in the other stage.  The iron hand is better than democracy when it comes to filtering audio.

[oliphaunt]

George this looks very interesting, thanks for posting.  Do you have a plot of the curve?  Do you know how to change the width of the Q?  Sorry for the silly question, but what does each pot control?

[jacobyjd]

2 in parallel and you'd lose 1/2 of what you accomplished in the other stage.  The iron hand is better than democracy when it comes to filtering audio.

Ah, that makes sense. Thanks for clarifying.

[ianmgull]

2 in parallel and you'd lose 1/2 of what you accomplished in the other stage.  The iron hand is better than democracy when it comes to filtering audio.

Best analogy ever. From this day forward:

Series=Iron Hand
Parallel=Democracy

[fuzzo]

Sweepable mids are really where it's at.  I'm working on a PCB board that has the Metal Zone parametric EQ section on it with the same goal, figuring a parametric eq on any distortion pedal would make it versatile.  I believe small bear has a 2x 100KC pot (for univibe's originally).   Not in 16mm, but beggars can't be choosers.  Or beggars could choose a higher value linear pot and taper it with resistors.

Yeah, I'm thinking about the same idea, take the MT2 tonestack and putting in a stand alone box. That additional bass/treble controls are really interessting to don't just work on mids.

[George Giblet]

Do you have a plot of the curve?  Do you know how to change the width of the Q?  Sorry for the silly question, but what does each pot control?

I will put up a plot (I don't bother with it because it just looks like the usual humps).

The Q and amount of boost are linked, that's the limitation of the simple circuit.  Also, I'm wouldn't recommend trying to get more than 15dB boost/cut because you can get some weird effects (that's the bug in the MT2).     As is the only thing you can play with is the ratio of the capacitors.   IIRC a ratio of C1/C2 = 1 gives 9.5dB boost with a Q of around 1.   The Q produced by the circuit is good for general use.

It's possible to mod the circuit but by that time you are better off choosing a two-opamp parametric.

For the pots,
  RV1 = boost cut
  RV2a, RV2b = frequency

[fuzzo]

Someone has tried it ?