Parametric EQ (RG keen)

[fuzzo]

Hi guys,

I made an one band parametric coupled with an overdrive. I just made the parametric section :




the overdrive works good, but the parametric section doesn't work. I did it on stripboard with this layout:



it works like a buffer, any gain boost and the freq. pot. doesn't work too.

Any suggestions ? mistakes ?

[Silent Fly]

I may be wrong but I think one of the problems might be that the Q and freq. pots are actually in series.

[earthtonesaudio]

The schematic looks good to me (not sure what Silent Fly is referring to...?) except for the jack with the sleeve connected to where Vref should be.  That's a problem.  If the sleeve of the jack is grounded, or if you plug in anything with a grounded sleeve, then the op-amp will be referenced to ground instead of Vref.

[fuzzo]

Hi guys,

Actually in test phase I didn't wire the jack connected to the freq. pot. like I didn't put the "reso" pot (that I won't put in the final circuit).

If the sleeve of the jack is grounded, or if you plug in anything with a grounded sleeve, then the op-amp will be referenced to ground instead of Vref.

Yeah I thought about that, I stuck some scotch tape on the box to avoid the jack touches the box (and goes to the ground)

[fuzzo]

Hi,

I bump my thread I decided to make with that a two band EQ (like the original in RG page) . the input buffer stage , and last stage work perfectly but I made a mistake that I don't manage to see in the gyrator part. I can hear the boost/flat/cut action of the pot but the freq. pot doesn't do what it should.

So, I think I made a mistake in my gyrator but I don't see where, do you ?

[fuzzo]

a little bump for my project, I try to find out where's the mistake but I don't. Someone ?

[earthtonesaudio]

Your stripboard layout doesn't show any track breaks underneath the op-amps.

[fuzzo]

yeah I know I foget all the time to show the cut in the stripes but there're in my stripboard of course.

[fuzzo]

little bump

I retraced the schematic with my stripboard to see I made mistake and  all seems good. I don't understand if someones can check that and see where I did something wrong.

Thanks.

[fuzzo]

I've sound, I can clearly hear the frequency movement, sometime that sounds like a %^&*ed wah but the  boosted frequency seem to be a little bit low , not really boosted compared to the bypass sound. Normal or not ? I thought that will give something similar to a boss GE7 with a only one band (-12B/0db/+12dB).

[earthtonesaudio]

Have you tried this out on a breadboard?

[fuzzo]

Nope I haven't a breadboard. I did it directely on a stripboard.

[fuzzo]

Bump

to have a bigger boost maybe I could increase the 2K7 resistor in the feedback loop of the final AOP.

[fuzzo]

  Still no one ?

[George Giblet]

There doesn't appear to be any circuit to generate Vr, either on the schematic or on the layout.

You need a voltage divider constructed from say 2x10k in series, connected to the power rails.  Then you connect a cap say 47u to 100uF from the join of the 10k's to ground.  The Vr voltage is where the 2x10k's and the cap meet.

[fuzzo]

Of course I installed a bias system in my layout. To be honnest, I gave up that circuit (but not really the Parametric EQ idea).

That doesn't work at all, any boost any cut .

I'm looking for another project (I'd prefer with PCB) with the same ideas (the MT2 tonestack can be real great too) 

[George Giblet]

> Of course I installed a bias system in my layout.

OK, no worries.

> I gave up that circuit (but not really the Parametric EQ idea).

The main problem with that circuit is the resonance control.  If the 10k pot is set too far away from the shorted end there is virtually no boost or cut.    If you replace the resonance pot with a short that type of EQ will work, it's used in many amplifiers, particularly bass amplifiers.   It's not the best parametric EQ, technically it has few problems in that only one of the skirts moves when the frequency is varied, but it's one of the simplest.

> (the MT2 tonestack can be real great too)  

Yes, you should stick with that one.  The parametric EQ in the MT2 isn't a bad circuit although the actual circuit in the MT2 has some weird behaviour/bugs.

Here's a simple Parametric without the quirks of the MT2, it similar to the MT2 but it's simpler and works perfectly well,

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

[fuzzo]

Hi george !

Actually I did not install the "resonnance pot", the  wipper of the boost/cut was directely connected to the gyrator. I could hear the boost/cut but not really present like it supposed to do, just a tinny boost which didn't boost a specific frequency.

Anyway, I'll take a look into your schematic.

The MT2 tonestack looks cool, espcially beause it has a bass/treble controls with a full parametric mids, that must be more interessting than just two parametric band . I don't know I'm not an expert about that kind of circuitry.

Otherwise, just coping  an graphic EQ and transform it in parametric in keeping one band with moveable fequency.

EDIT: your schemo looks good and fun, very simple.