parametric EQ's, transformers, etc.

[dave h.]

so im looking at this:

http://www.harpamps.com/schematics/4bandeq.pdf

notice a few things...
A. its got the whole power cord, transformer bit going on.
B. its using dual pots for the freq. knobs.
C. its got an LED in a weird place that im assuming is just a "power" indicator.

so here are my questions as a novice:
A. why does this circuit require the higher voltage and transformer? if i use a different IC can i run it off 9v? could i somehow get rid of the whole bottom portion of that schematic including the transformer and etc.? any tips or ideas?

B. can the dual pot business be worked around? not a big issue.

C. how would i modify this so that it would be a 1-band parametric EQ that would cover the entire spectrum from 30 or so hz to 15-20 khz? this would be a separate project altogether. but in general, i think it would be good to know how to modify and completely control the EQ's frequencies, because eventually id like to build an amplifier, and that would come in handy.

if you have any other creative ideas regarding parametric EQ (or any EQ) or related schematics, articles, etc. please point me in the right direction.

i have found that on this board, there is a number of extremely knowledgable people who are very willing to help. i applaud this board on its politeness between novices and professionals.

[Mike Burgundy]

Higher voltage ( a whopping 3V higher) will yield more headroom. This is a good thing when working with studio (line)levels, but at guitar levels you're ok with 9V.
The 5532 will work at 9V. It's a low-noise opamp, which can directly be replaced (same pinout) with a TL072, 4558, etc. The 072 is nice for inputs- it has high input impedance because of its JFet input. The 5532 has lower input ompedance, but is a lot more quiet.
The dual pots are part of the gyrator design. If you want a gyrator with single pots, have a look here:
http://burgundy.users.balpol.tudelft.nl/bin/schem/parametricbass1a.gif
You'll need some other values though.
Required reading:
http://www.geofex.com/Article_Folders/EQs/paramet.htm
If you read carefully, you'll find out that it would be very hard indeed to build one spanning the entire spectrum. Increasing the frequency pot value increases range, but also does weird things to its behaviour. Try it out...
I now use 1M pots.

[Paul Perry (Frostwave)]

For that circuit, the pots have to be duals. But what isn't really obvious, is how they are wired!