Porter Net EQ questions ~ center tapped pots?

[James HE]

I gave up on the other eq design I was working on, and started looking at the Porter EQ. I just found the PDF readme file that goes along with the schematic. Reading it I figured out that the amplitude pots need to be center tapped to ground, otherwise the sections interfere with eath other badly. (I had though maybe the casing was grounded or somthing and couldn't figure out why!) I had noticed this in spice simulations, which was unacceptable of course, so I had figured out sort of a work around.

I doubled the inverting amps that the pots feed and seperated the hi / low and mid bands. (I'm adapting this to be hi / lo shelving only with 2 maybe 3 mid bells) This seems to keep the mid bands from affecting the shelving bands, but they still affect each other when their frequencies are near each other. It was just an experiment I performed in the simulation, and seems to work ok. I am afraid of using the two extra opamps however, I don't want to introduce more noise/distortion/bad vibes than I have too. I would post some graphs, but I have to redo all my simulations being that I deleted all of those files accidentally!

So center tapped pots should allow me to keep the 2 amp topology as opposed to 4. Great! Where do I find them!!!!! lol. I'm assuming center tapped is not the same as center detented. Is there anyway to possibly mod a pot to have a center tap?

 

[James HE]

More stuff. I redid some work in the similator.

I also wanted the shelving slope to be a little steeper. So I tried simply making those filters 2nd order instead of first order like this (shows both hi and low sections):

Here is a graph showing the difference of the slope (hi shelf, moded version on bottom):

Now it looks like the shelving slopes on the Waves ren eq. lol.

So now here's an example of what I was talking about above. This is with the hi shelf and the hi mid bands simulated. In both graphs the hi-shelf is in full boost and the hi-mid is set to flat.

The top shows the filters seperated into to sets of inverting amps, the bottom with them going into one. Even with the hi-mid set to flat it causes a dip in the slope of the shelf where it's frequency is. This is why Mr Porter specd center detented pots apparently. I'm trying to figure out how to simulate a center detended pot to check the difference, and to figure out why it works, but I'm not sure how. Any ideas?

I'm using Linear Teachnologies SWCad III. (It's free)My first time using any simulation software. I'm sure I'm doing something wrong or not taking something into account.