Question about the geofex parametric eq

[MikeH]

The fourth diagram down on this page-

http://www.geofex.com/Article_Folders/EQs/paramet.htm

Are the control for frequency and resonance dual ganged pots?  I'm looking to make a basic parametric eq.  I have one built in on a yamaha amp that I have, it has level, Q, and frequency controls, and I'd like to create something similar to use on my other amps.

[aron]

It looks to me like it's that way.

[MikeH]

It looks to me like it's that way.

And the 10k pots too?

[Ben N]

If you look carefully at the text, each of the sections is independent, so no, no dual-ganged pots that I can see.

[MikeH]

Hmmm... I think I'm looking to make something a bit more basic than this.  I'm looking for something with 2 to 3 controls, not 6.

[R.G.]

The fourth diagram down on this page-
Are the control for frequency and resonance dual ganged pots? 

No. There is a single pot per EQ band. That is a schemo for two bands. They're independent.

And the 10k pots too?

Nope. Those are independent too.

Hmmm... I think I'm looking to make something a bit more basic than this.  I'm looking for something with 2 to 3 controls, not 6.

Actually, if you're after parametric, that's about as simple as you can get. "The parameters" in 'parametric' are Frequency, Resonance (or Q), and gain/loss.  Three controls is about as simple as it gets PER EQ BAND. Higher performance parametrics are usually state-variable based and will need at least one dual-gang pot, if not more.

[Ben N]

... but a state variable filter can give you 3 bands with adjustable frequency on four controls--a worthy compromise if a true full multi-band parametric is too much.

[MikeH]

Dang.  I wish I could find a schematic for this amp.  It's a yamaha G100 212 II.  It has a really nice parametric eq on it; it's probably single band.  I really want to re-create it.

[Ben N]

IMHO, a single parametric band makes a very nice super-flexible mid control together with a fixed-frequency treble-bass tone stack, like a baxandahl (or whatever you have on your amps). IIRC, this is somewhat like the approach Ampeg used to take in its bigger amps, with its inductor-based mid controls with mid-shift and resonance switches. (By itself, on the other hand, I don't think a single parametric channel would be flexible enough for most needs.)

You can easily make a single channel version of RG's parametric, just by leaving out a section (cutting not only the part within the dotted line on his schematic, but also the two 10k pots and the cap between them). Then you would need 3 opamps, which you could do with a dual and a single, or two duals and leave one opamp section unused (or use it for something else).

[Mark Hammer]

When you're trying to simply revoice something, boost and cut is sufficient if you know what band/s you want/need to alter.  If you don't know or don't have that many bands, a second parameter of centre-frequency is useful since it can locate the region of maximum boost or cut exactly where you need it without having to use a 31-band device. 

The 3rd parameter - bandwidth - is really most useful when using the EQ to tailor the frequency response for the room.  You first identify which frequencies are providing the shriek or boxiness or feedback in the P.A., then you gradually narrow the selectivity of any cut required so that you're only affecting that part, and none of the other frequency content.

For guitar, a simple 2-band "quasi-parametric" EQ (i.e., 2 out of the 3 parameters) is generally sufficient to do 90% of the re-voicing that one might wish to do with a guitar.  Set the bandwidth to medium, and tweak away.  If your amp lacks any sort of useful EQ-ing itself, then maybe a 3rd band would be helpful, but 2 will do a surprising amount of work.

[MikeH]

Thanks everyone!

I found this at colin's site:
http://experimentalistsanonymous.com/diy/Schematics/OOP%20Japanese%20Electronics%20Book/parametric-eq.gif

Looks like this is 3 controls? (I know I'm a little hung up on the "3-knob job" aspect but I already have the perfect pre-drilled enclosure and I want it to behave as much as possible like the pm eq on my yamaha amp).  So in this frequency is controlled with a dual gang pot?

[Ben N]

Any true parametric will have three controls per band, AFAIK. That is as true of the Japanese design as RG's.

[earthtonesaudio]

I'd use R.G.'s design, and just leave out one set of controls.  You could do it all with a TL074. 

...Or use dual concentric pots and get the full functionality of two independent bands, but without having to drill any more holes.

[Mark Hammer]

Here's one: http://damien.douxchamps.net/elec/param_eq/
Here's another: http://www.waltzingbear.com/Schematics/Urei/545.htm
And another: http://hem.passagen.se/robin2/im/giteq.gif
And another: http://www.rane.com/pdf/old/pe17sch2.pdf
And another: http://www.retrosynth.com/docs/ra204/ra204_45.gif

And just for good luck, another: http://files.muziq.be/schematics/moog_pareq.jpg

[MikeH]

Thanks Mark!

...Or use dual concentric pots and get the full functionality of two independent bands, but without having to drill any more holes.

Good idea!

[Ben N]

Not exactly a true parametric, but worth seeing:
http://www.all-electric.com/b&c.html

[zencafe]

Hey, I was just wondering about this EQ yesterday.
Looks great being able to have 2 bands with just a double Opamp, and 3 single ganged pots.
In the article, RG says that the range is 4.5:1 (that's why it can go from 100Hz to 450Hz) but what If I'd like to get m aybe 10:1?

Thanks in advance!

Best Regards

Douglas

[Mark Hammer]

Sometimes more range is not what you want, because it makes the control "twitchy", and makes it difficult to reliably dial in what you want.  And when you consider it, two sections that go from 200hz-900hz and 600hz-2700hz (following the 4.5:1 ratio) would cover pretty much 99% of a guitar player's revoicing needs.

[zencafe]

Thanks for the answer Mark... I agree with you! Was asking because the parametric EQ in my Bass Mesa head has 10:1 ratio... but it has no Q control and it uses dual gang pots for the frequency control.

Thanks a lot!

D.

[R.G.]

In the article, RG says that the range is 4.5:1 (that's why it can go from 100Hz to 450Hz) but what If I'd like to get m aybe 10:1?

This is one reason to pay attention in math classes.   

The mathematics of the filter are such that the filter center frequency varies as the square root of the controlling resistance. That means to get a 4.5:1 range, you have to have a 4.5*4.5 = 20:1 range on the pot, which is possible, but the circuit quits acting linearly when the pot gets too small. So for only one pot controlling one parameter, you're limited in range. If you use a dual pot, changing two circuit frequency determining parts at the same time, you get wider range. Even better yet, use semiconductors to replace those elements and get really wide range.

A state variable filter, set up as a parametric EQ stage, can easily have a 10:1 or 20:1 range with pots; if you use OTAs to replace the pots, you can get 1000:1.