How to vary output on geofex parametric EQ?

[Morocotopo]

Hi everybody, I´m planning to make the parametric EQ featured at Geofex.com. What I want to know is, is it possible to vary the output from 0 (mute) to above unity gain (say, 9dB)in the circuit? I want to add a level control pot, because I think, if you attenuate a big part of the signal, you might want to compensate by boosting the output, or if you boost a large part of the signal spectrum, you might want to reduce the level to avoid distortion or other problems. I guess maybe use the last opamp for that, maybe it´s as simple as replacing a resistor with a pot (I hope)? Or add a single transistor/opamp booster before the output? Any help welcome.
By the way, I never found any reference about somebody building this circuit. No one ever did? No one ever made a PCB layout for it? Am I going to be the first one to build it?

Thanks

P.D: I hope it´s okay to post the image, I tried to link to it but couldn´t get the original article web adress, It´s on geofex.com, strange page formatting... If it´s wrong, tell me and I will remove it.

[bioroids]

Hola!

Creo que reemplazando la resistencia de 2K7 del ultimo operacional por un pote de 25K te alcanza para tener control de ganancia (tal vez seria mejor poner una resistencia de 1k en serie con el pote por las dudas). No se si esto afectara el comportamiento del filtro. Tambien cambiaria la resistencia de entrada (100K) por una de 1M.

Saludos

Miguel

In english:
I think replacing the 2K7 resistor of the last opamp with a 25K pot is enough to have a gain control (maybe with a 1K resistor in series with it). I don't know if this can affect the filter.
I would change also the input resistor (100k) with a 1M resistor.

Luck

Miguel

[R.G.]

I think replacing the 2K7 resistor of the last opamp with a 25K pot is enough to have a gain control (maybe with a 1K resistor in series with it). I don't know if this can affect the filter.

It affects the filter. That resistor helps set the max boost in the bands. Both 2.7K's really need to stay as they are. It's simplest to add gain either before or after the EQ. Not that changing it won't give you more gain, it just changes the gain amount of boost and makes the boost/cut asymmetrical.

I would change also the input resistor (100k) with a 1M resistor.

I'd go along with that as long as the opamps are low-input current devices. TL072's will work fine there with a 1M.

What I want to know is, is it possible to vary the output from 0 (mute) to above unity gain (say, 9dB)in the circuit?

Maybe. You could convert the first opamp into a gain-of-9db stage, then make that 100K into a 1M pot. Take the audio signal off the wiper to the noninverting input. It's not the best possible for noise considerations, but it's easy.

[John Lyons]

I built one on perfboard.  I made mine a high band and a mid band. It works well.

[Morocotopo]

Thanks for the answers, so I think the best solution is a booster after the circuit...
Options:
- Mosfet booster with gain set at above unity gain with a trimpot, and with an external level control before the output, so I can go from boost to unity to mute with a single knob
[edit: I think this is the best one because it´s the cleanest, most linear one judging from what I read]
- Fetzer valve
- MXR microamp
Wich one do you think is best? Or will they work at all?

Thanks

[R.G.]

- Mosfet booster with gain set at above unity gain with a trimpot, and with an external level control before the output, so I can go from boost to unity to mute with a single knob
[edit: I think this is the best one because it´s the cleanest, most linear one judging from what I read]

No, it's not. A single device booster is only "linear" when run as a follower, at a gain of one. The linearity isn't all that bad, but it's probably 2-6% distortion.

If you want a clean (i.e. nondistorting) boost, you use an opamp. The output stage of the Adjusticator at GEO would do this job just fine.

The others are quite complicated answers to a simple problem.

[Morocotopo]

Oh, that´s great R.G.! Thanks. Didn´t know that circuit existed. Your website is such a treasure for people learning, like me! But, so deep that it´s easy to miss things. I think that´s exactly what I need...
If I use just one section of it at the circuit out, the output would be inverted, right? So, it would be OK to use both, just to get a non inverted output, or its not that important/worth the extra parts? I´d prefer to have the EQ be non inverting, but that´s just me being extra obsessive

[R.G.]

You can get non-inverting with only one section. You'll notice that the sections are identical.

Simply:
(a) ground the existing input
(b) use a new 0.1uF capacitor to the + input
(c) use a new 2.2M resistor from the outboard side of that new 0.1uF cap to ground. Your input now goes across the new 2.2M resistor.

The gain is now 1+ (whatever it would have been for the inverting case), or a minimum of 1.1, which is fairly close to unity.

[kag]

Would it have any difference in performance between inverting and non-inverting??

I've built a couple of adjusticators on diffrent boxes, and them both do at some time have some weird oscillations at some volume pot placement, it goes away if I turn down very low the volume or very high...
I'd like to solve that problem, I've tried different op-amps but didn't helped.

Thank you,

[kag]

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