GEOFEX Simple Easy Graphic Equaliser PCB Layout

[PRR]

Look at the middle picture. R.G. has labeled the series resistor. But: the 10K pot is also a series resistor. With wiper in the center, we don't care. With wiper at either end, this resistor goes to zero. But for all intermediate positions the added pot resistance reduces the Q. This is probably good musically. It is certainly good for the manufacturer because other topologies cost more. In any case it limits your choices of R and Q for "reasonable" response at intermediate settings.


That's for simple case of C and L.

These days, we don't use actual coils so much. The 3.7H coil in the example is a BIG hunk of iron.

A gizmo called a Gyrator turns a cap into a coil. Within limits. But since transistors and opamps got cheaper than large coils, that's how we do it.

See bottom picture.

The Gyrator needs two resistors. Each has an opposite effect on Q. Both factor into Frequency. (Note that in this version, changing Frequency also changes Q.) Q is also (still) affected by boost/cut pot resistance. And if you wish, you may reduce Q with added resistance.

> mathematically how series resistance affects Q

When working with a boost/cut pot and a Gyrator (not to mention a Freq knob), it gets very messy.

IMHO: don't re-invent the wheel, and don't invent new sizes of wheels. 13" 15" 17" wheels serve most cars and trucks. Likewise many equalizers have been designed by guys hoping to get-rich (or break-even), and some of these are published. I think RANE had most of their not-new EQ schemetics posted. The Dynaco graphic EQ manual must be floating around.

[PRR]

Oh, you didn't ask but you will.....

In this topology, the maximum boost (cut) is the 2.7K divided by Rseries.

At resonance, reactance cancels, the L-C-R circuit is just the R. Put that simplification into the opamp network, wiper at one end, and you will see that gain (loss) is ratio of these two resistors.

[Derringer]

thanks PRR!

[uhbom]

Over a year later...

sorry.

[Guga Brandao]

Fellows, do anyone knows how calculate the Q using different resistor values? I need to achieve other frequencies and Qs... I was using this gyrator calculator, but the Q isn't resulting like the 4QD... Looking to the circuit, there are some differences and the math must be different at all...

This is the calculator: http://www.muzique.com/lab/gyrator.htm

Really need this, fellows... Could anyone help me? OO

[Guga Brandao]

Guys, after some research I was able to program this calculator, using the 4QD equations to find the frequencies and de Q's with the R1 + R2 + C1 + C2 values...

Here's the link:
http://www.contraplonge.com.br/eq/eq_calc.html

So, I'm about to build this EQ, so is there any news about the hiss noise, or the actual changes?? I'm redesigning this using 5 bands simulating the Mesa Boogie Mark IV Graphic EQ (trying to achieve the frequencies and Q's). I'm gonna try to put this on a Hammond 1590A with trimpot instead os slide pots and put a lever 3pdt instead of a switch.

So, I've found the original schematics from Mesa Mark IV and I've used a RLC calculator to find the right frequencies and q values...

I hope the calculator help us to develop a definitive and verified EQ for the forum. I've put the standard values to achive my choices, but you can change them to calculate yours.

So, there are still some questions: What is actually the R27 value? Do we have to change any caps? Does it have a Diodo or not? OO

I can remake the design and share, but we need to be sure, right?

Any news, @Brymus??

Let's do this, guys!  

thank you all!