Parametric EQ Sanity Check

[bartimaeus]

Hi all, I'd really appreciate if someone could give this a quick look over before I map out an smd board for it. I'd be fine fixing my mistakes if it was a through-hole board, but smd makes it a lot trickier...

It's a single parametric EQ band combining elements of r.g. keen's parametric and saprotone's (which is based on keen's). Dual supply from a TC1044 to NE5532s. My goal is to control between 4khz-8khz.

Thank you in advance for any advice!



Also, does anyone have thoughts on picking relative cap sizes. It seems like one cap is usually ten times the value of the other, but I'm not sure this matters? Also, some circuits put the larger cap in C1 and some put it in C2, so sequence doesn't seem to matter.

[knutolai]

Hey

U3B is wired incorrectly. Swap pins 5 and 6. Can't see anything else jump out.

Generally I'd advice that you annotate the whole schematic before posting in order to make it easier to point out possible mistakes. Linking your sources also wont hurt

[bartimaeus]

Thanks you for catching that!

These are the two sources I used:
http://www.geofex.com/Article_Folders/EQs/paramet.htm
https://www.sabrotone.com/?p=578

And here's a version with better annotation. I also used the unused half of U3 to re-invert the output.

[knutolai]

The inverter you added on the output won't work. You're wiring it like a non-inverting buffer. Head over to amz and read the "buffers" article (on mobile atm, hard to type)

[bartimaeus]

Looks fine to me?



From the tl072 datasheet: http://www.ti.com/lit/ds/symlink/tl072.pdf


Although now I'm curious why not use an inverting configuration for the input buffer?

[mth5044]

Looks fine to me?



From the tl072 datasheet: http://www.ti.com/lit/ds/symlink/tl072.pdf


Although now I'm curious why not use an inverting configuration for the input buffer?

You aren't using either of those set ups. You have a combination of figure 21 and 22.

[bartimaeus]

I used the second setup but left out the resistors since I want unity gain. Since for unity R1=R2, I figured I could just set both to zero (aka not use any). Is that not the case?

[mth5044]

I used the second setup but left out the resistors since I want unity gain. Since for unity R1=R2, I figured I could just set both to zero (aka not use any). Is that not the case?

There is probably a technical explanation, but I don't know it. The input signal needs to be isolated from the signal that's being fed back through the feedback loop, which is why you have the input resistor. There's likely a lot more to it.

Also think about the equation, if you have zero resistance, your equation becomes Vout = 0/0 x Vin and ain't nobody want to divide by 0.

[bartimaeus]

Wow, I really should've thought about plugging it in to the formula....

Thank you both for the help!

[PRR]

What they are telling you is correct.

> for unity R1=R2, I figured I could just set both to zero (aka not use any).

But look at the other consequences. Input impedance of an inverter is R1. If R1 is zero, input is a dead short. Actually the impedance at this node is R2/opampgain. Since opamp-gain is very high, we have zero divided by a large number, so very-very teeny impedance. (For the parts used here, much less than 1 Ohm.)