[sdiy] 3 band EQ

[Derek Holzer]

Hey all,

I'd be very grateful for a simple recipe for a 3 band EQ, such as those 
found in my favorite el cheapo mixer of all time, the Behringer 1604. 
Extra points for a sweepable mid, but that's not the point here. I just 
need something for a basic tone control, with a minimum number of pots 
(input gain, hi, mid, low) and parts (maybe a few op-amps), which goes 
from near complete attenuation up to maybe about +10 dB gain. My 
favorite part about the Behringer EQ is that it self-oscillates very 
well when put in feedback situations, so that would be nice too. And, 
like the Behringer, this EQ I want to build can be a bit dirty ;-)

I understand the basics of making low and high pass filters, however the 
midrange has me a bit stumped, since I can't seem to come up with 
something that doesn't have a too-steep resonant cutoff. I know that 
some notches will always occur, but it would be nice if the sound 
closely resembled the original when all three tone controls are set to 0dB.

Keep in mind, I'm not an electronics engineer. However, I'm quite open 
to simple theoretical explanations as well as links to existing 
projects. In particular, the YuSynth Fixed Filter Bank has a simple 
explanation of how to do the low and high pass, even if the band passes 
there are much too steep for my purposes:

http://yusynth.net/Modular/EN/BANK/index.html

I also checked a guitar effect 3-band here:

http://www.zen22142.zen.co.uk/Circuits/Audio/3bandton.gif

which was very unsatisfactory...

Thanks much for any suggestions!
best,
Derek

-- 
derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 52:
"Do nothing for as long as possible"