Geofex parametric - nice boost. No notch.

[blackieNYC]

I have a single band parametric like the geofex  http://www.geofex.com/article_folders/eqs/paramet.htm With the 10k instead of 2.7k resistors from the sabrotone schematic for some reason. http://www.sabrotone.com/?attachment_id=571
The Q works great, the freq range is a nice 500-2000 hz.  The boost increases the center freq by 6dB, which I thought would be higher but it sounds just fine. At 12:00 it's at unity and flat, but below 12:00 all the way CCW there is no notching attenuation. It remains flat. 9.6v supply, 4.7v on all input and output pins.  The cut/boost pot, checked with an ohmeter is connected to the (-) input to the 3pin of the pot, and the + input of the TL072 is attached to the 1pin of the pot.  The pot wiper sweeps from the (-) to the + as you turn the pot.  Neither end of the cut/boost pot is shorted to ground, or anywhere near it.  Can't find anything visibly or measureably wrong, yet, and I can't understand how this can be happening.
I was so excited with my sharp-Q fixed-wah sound I didn't notice the lack of notchness.  Anyone have a guess?

[R.G.]

(1) post the actual schematic so we don't have to guess so much
(2) is your pot linear taper?

[blackieNYC]

Hi RG
I'll post a schematic -it'll have to wait until tonight, but the schematic is yours at geofex, with the (2) 2.7k resistors relaxed with 10k.  Single band. Linear pot, 10k.  Befuddling.

[blackieNYC]

This is the schematic I used, but I used a single 9v supply, and only the "mid" circuit above (a one-band eq)
The ohmmeter measurements I mentioned were from the pot legs to the op amp legs, so my connections appear to all be there.  Again, everything is working but there is no cut.  And to repeat RG's article at Geofex, correctly I hope, when the pot wiper is up and connected to the (-) input, the feedback loop is grounded thru the gyrator, providing no feedback (high resistance) and high gain at that center frequency - a boost.  If the pot wiper is swung over to the + input, the input is grounded at Fc and flat at other frees - a notch.  I have no notch.
If any leg of the pot, or either input of the 2nd op amp were disconnected, or faulty, I think it wouldn't work at all.  In what situation would I get boost but no cut with that pot?

[R.G.]

Something bothers me about that schematic, but I can't make it come to mind right now. I'll have to think about it a bit.

I checked my circuit and some other similar ones, but I can't quite put my finger on it.

Why did you raise the gain setting resistors from 2.7K to 10K?

Something is bothering me with the highish value of the resistances in series with the resonators and the other parts of the circuit versus the change to 10K's.

It'll come to me. Worst come to worst, I'll run it in the simulator.

[blackieNYC]

I can tack on a pair of 3.3k Rs parallel to the 10ks and see what happens. 

[amz-fx]

In what situation would I get boost but no cut with that pot?

If pin 1 of the pot is not connected or faulty.  It would boost but not cut.

regards, Jack

[R.G.]

Yeah, that would have been my choice, but he said he'd checked those connections out. Still, could be intermittent, maybe.

[blackieNYC]

It is the 10k resistors!  Dropping them down to 2.5k or so (tacking a 3.9k to each) brings back the cut.  And the boost and cut are where they should be: +/- 15 dB.  
So, why?   The only remaining difference on the sabrotone schematic is the +/- 9v supply.  Geofex is +9v and ground.  Could this contribute?  Or, what is it you suspect about that 10k value?
I used the sabrotone schematic back when I was breadboarding the geofex and could not get it to work. Had 3 circuits stuffed into the breadboard at once.  Trashed it again and again, and started over looking at a new schematic.  Which actually did help me correct my initial mistake.  And snuck in a new one.  
But before I go off slagging the sabrotone version, I'm wondering if the bipolar supply works with the 10k values.  i could do a little testing out of curiousity before I close up. I don't think I'll go so far as making a negative 9volt rail.
Anyway, delighted now.  Thanks for he help!

PS - I will wiggle the pot wires around a bit to see if all I've done is disturb a bad connection. Seems like a solid fix, but then I did create an intermittent pot very recently.  Snipped the unwanted right angle legs off an alpha pot.  Too violent - hairline break In the contact. 

[R.G.]

Try a different pot value, or tinker the "Q"  resistors in series with the resonance.

[blackieNYC]

The change from (2) 10ks back to your (2) 2.7ks did the trick.   
If you're low on puzzles at the moment - why?

[PRR]

Ground the pot wiper.

This MUST cause total gain to vary from zero through unity to infinite boost.

If it does one but not the other, you have a wiring fault.

[blackieNYC]

I got it, I tells ya!   The 2.7k resistors, replacing the 10ks.  Head scratcher.