Can someone explain these cut/boost circuits?

[MrBinns]

[the first is a pot between the inverting and non inverting inputs of an inverting amplifier](http://geofex.com/article_folders/eqs/paramet.htm)

[the boss pq-4](http://www.synthdiy.com/files/2007/boss_pq-4.png) i have no idea what is going on with the 100K w taper pots. and don't know what the 4th(bottom) leg is for.

[last one is a 10K pot in the feedback loop of a inverting amp](http://sound.westhost.com/articles/state-variable.htm) idk why the non-inverting input of ua1 is going to where it is. I'm assuming to connect multiple filters you would add them in parallel to the cut/boost pot.


This article  also states there are other ways of connecting multiple filters. I'm wondering if anyone can expand on this idea. Can I use a buffer and have multiple filter sections in parallel, going to their own boost/cut section and then all into a summing amplifier?

Last question. Can someone explain the level knob on the pq-4? i mean it's like a non inverting amp but with a resistor between inputs. and what does C33 do? There was another level design that a saw recently, but  i can't find it.

Sorry if this post isn't clear enough. It's very late at night early morning here, 6 a.m.

Thanks in advance

Goodnight,

-Binns

[MrBinns]

I found out what C33 does. It needs to be a low leakage cap equal in impedance to the resistor above it. It is used to give a low impedance at low frequencies.

It's a bypass cap

[Mr. Lime]

Sorry for being a little offtopic but would the parametric EQ/SVF be useful as guitar onboard preamp?

Maybe the values should be altered for guitar, I once saw a SVF schematic for guitar, that looked pretty much the same expect of the boost/cut stage but had other values.

[PRR]

Don't look at the pots, look for the filters.

> the first is a pot between the inverting and non inverting inputs of an inverting amplifier
http://geofex.com/article_folders/eqs/paramet.htm

R.G. gives a very direct explanation, re-read it:

"...the wiper of each pot is grounded through the low impedance of the LC filter at that frequency. The pot wiper may be dialed to the input side, in which case it shunts the input to ground, producing a notch in the frequency response at its center frequency. It may also be dialed to the inverting input, which shunts the feedback to ground at its resonant frequency, producing a peak when directly in the center, there is no effect from the LC filter at all - frequency response is flat. In this position, the LC filter's effect is equal on both + and - inputs, so there is no overall effect."

> explain the level knob on the pq-4?

Turn the pot wiper toward pin 1. What is the gain? Roughly 1.5K/11.5K. Turn the pot the other way. Gain is roughly 11.5K/1.5K.

You might also note a similarity to the R.Q. EQ, without the EQ.

> what does C33 do?

Audio swings both ways. The battery is wired one-way. So we have to bias-up opamps at half of supply so they can swing both ways. Here we need a point that is +4.5V for DC but "zero" for audio.

> cap equal in impedance to the resistor

The impedance of 10uFd varies from 1,000 at 17Hz to 100 at 170hz to 10 at 1.7KHz and maybe 2 ohms at the top of the guitar band. Where is it "equal"?

It's a big buffet. Don't bite-off too much all at once. Being GOOD at basic resistor ratios, and opamp biasing, is the foundation for the fancier stuff.

[MrBinns]

> Where is it "equal"?

"The value of this capacitor must be chosen to give a low impedance at low frequencies. It is typically chosen to be equal in impedance to R1 at the lowest frequency required - this will give a -3dB fall at this frequency." -from http://www.radio-electronics.com/info/circuits/opamp_non_inverting/op_amp_non-inverting.php

reading more on bypass caps in Electronic Principles by Malvino, he talks about how it reduces the output voltage offset to a minimum.

thanks for the reply. I guess i skipped that part on geofex. that's embarrassing.