[sdiy] String Filter PCB project

[JH.]

Hi,

I plan to resurrect my old String Filter project, and offer PCBs for a new 
version of this.

Old version: see http://home.debitel.net/user/jhaible/jh_string_filter.html

New pictures of this (old version) string filter:
http://electro-music.com/forum/phpbb-files/dscn4350_718.jpg (small)
http://electro-music.com/forum/phpbb-files/dscn4351_121.jpg (detail 1)
http://electro-music.com/forum/phpbb-files/dscn4352_312.jpg (detail 2)
http://electro-music.com/forum/phpbb-files/dscn4354_114.jpg (inside 1)
http://electro-music.com/forum/phpbb-files/dscn4355_194.jpg (inside 2)
http://electro-music.com/forum/phpbb-files/dscn4357_177.jpg (inside 3)
http://electro-music.com/forum/phpbb-files/dscn4356_203.jpg (inside 4)

The two boards that are stacked on the left side are the 40 BPF filters.

That'll be the main thing on the new boards, too. Plus, I add some of the 
overall circuitry, like feedback control, stereo spread, etc.

The plan is to have a self-contained 40-band filter with just these two 
boards. This is enough to get the effect demonstrated in this sample:
http://jhaible.heim.at/string_filter/jh_string_filter_resonance_ob8.mp3

For (more or less) realistic emulations of string instruments (like in th 
esecond part of this sample: 
http://jhaible.heim.at/string_filter/jh_string_filter_benadecello_cs50.mp3 ), 
you need the 40-band filter plus a few broader resonances that I have 
created with a 3-band parametric EQ for each of the thre "presets", violin, 
viola and cello. (Thats the big veroboards with loads of trimpots).

Now, I expect much bigger interest in the 40-band filter than in the preset 
section, so I think I'll offer the 40-band filter PCBs separately. 
(Actually, these will be two equal boards for 20 bands each.)

I'm still looking for the best solution of the Para EQ stuff.
Maybe I'll make it part of the "Famous Fixed Filters" project.
Maybe I'll recommend using the Moog Parametric EQ board for this.
Maybe most buyers of the 40-band Filterbank have enough means for parametric 
EQ-ing already, and don't need a dedicated board for this at all.
I'll try to find out. :)

Meanwhile, I concentrate on the heart of it, the 40-band Filter Bank.
Back then, I used a design I thought clever - only to find out that it only 
was stable with NE5532 chips of a specific date code. :( That's why I didn't 
publish the old PCB layouts in the first place.
Now, I go for a different circuit topology, that also has a lot of benefits 
over the typical single-opamp solutions.
These will be 2-opamp biquads, which utilize a negative resistance to create 
a non-inverting integrator. (Also known as Deboo-integrator).
The main benefit of this - apart from it's low noise - is the low resistor 
spread, and zero capacitor spread.

If you are interested in this - a set of two boards for the 40 filters - 
please let me know.
Pricing will be in a similar range as my other boards so far. And I'm *not* 
taking orders right now. Just testing how many of you would be interested.

JH.