Need some help with a custom EQ/Reverb box

[Heemis]

So I've done my share of fuzzes and ODs, phasers, trems, and chip amps... but now I've gotten a request for something a bit more subtle.  I've been asked to build a box with Reverb, a variable low pass filter, variable high pass filter, and two parametric EQs.

I'm fairly certain I'll be using the GGG reverb kit for the verb... and I'm partial to this schem for the 2 parametrics (Thanks RG!):  http://www.geofex.com/Article_Folders/EQs/parmet.gif

But I'm having trouble finding good active variable low pass and high pass filters.  I did find this one for a variable high pass: http://www.circuit-projects.com/miscellaneous/20hz-to-200hz-variable-high-pass-filter.html 

It looks good, but appears to work on a bipolar power supply.  Anyone know if it would be ok to run this circuit on regular 12v power?  Also, I'm assuming there would be a way to convert this to a low pass... usually by reworking the placement and value of the caps/resistors... can anyone shed any light?

Last question:  Would it be better to put the verb before or after the EQ?  I'm thinking after, but my other thought was to make it switchable.

[Heemis]

Just a quick bump to see if anyone has any info.

[sean k]

Okay, I've got my R.A.Penfold book in front of me and a bunch of what you want but my scanner is outside so I'll bring it in tomorrow morning and scan the stuff you want. Opamp and transistor, which has gotta be easier, high and low pass filters on single supplies, 12, 18 and 24dB per octave versions. All set at 1kHz but you can change that if you want.

[Heemis]

Sean, thanks so much!

[sean k]

Sorry about not getting onto it, it was raining yesterday and I didn't get to make the trek outside.

Anyways, my thoughts revolve around using the basic paralooper.
http://www8.plala.or.jp/KandR/cir_bassthru.html
http://moosapotamus.net/THINGS/paraloop/paralooperSCH.gif

Which means that my way of helping naturally involves what I'd do in your situation. It has a LP filter already on the lower leg and using a dual pot and having two filters in series would double the cutoff. What I'm thinking is that something like this would allow you to take the bass frequencies one way, and keep them clean, and put the same, but reversed, filter on the top leg, either before or after the send/return. Most probably after as you can then use the opamp already there. It's even got the first .047 cap (on the bass thru jap schem.) I'd follow that with a 2.2k R over one section of a 10k dual pot to ground then do it again, .047 with a 2.2k and the other half of the 10k pot, for two sections of HP then put a .01 cap in to isolate the bias voltage on the + input and away you go.

.047 and 2k equals 1.6kHz and .047 and 12k equals about 340Hz. f = 1.59/ R x C. I use uf x Mohms. .047 x .0122 (12.2k)

Then you could just follow on into a parametric as a stand alone or in the same box. I'd tend towards parametrics being in parallel as opposed to series... which RG's is. Heres another very similar from this page http://sound.westhost.com/project30a.htm
 

I think the reason that I'm dallying with the Penfold schematics is that they are fixed and designed to be fixed and, as he says in the text, changing them too drastically might make them unstable while they aren't really open to being adjustable with pots. But I will get around to getting that scanner as I've drawn the schematics up on a single page.

[sean k]

There you go.

[Heemis]

Sean, thanks a ton for all this stuff... I'll need to mull it over for a while before I have any other questions.  Thanks again!