4-Pole Lowpass Filter?

[kinski]

Hi, anyone know of a good 4-pole low pass filter that would work in a guitar pedal format. I'd like to make one that uses an expression pedal to control the sweep for some synth-like tones. Or maybe cram the whole thing into a wah enclosure.

The only thing I can find are meant for euro rack type setups.

Thanks!

[ElectricDruid]

The only difference with something for Eurorack use would be that Eurorack uses a +/-12V power supply.

You've got two solutions to that:

1) Use a '1044 type circuit to create a negative supply to give you +/-9V. That's close enough for many circuits. Anything based on the CEM3320/V3320/AS3320 would work like that, for example.

2) Use a circuit that can cope down to 9V. The LM13700 is the obvious choice here, and there are plenty of 4-pole filter designs that use it (you need two for a 4-pole filter). The SSI2144 would be another possibility - its minimum supply is +/-4V.

[Mark Hammer]

I'm pretty sure I saw a 4-pole diode ladder schematic somewhere that runs off +9V.  Sifting through my hard drive and haven't found it yet.  Sadly, these things never seem to have filenames that meet one's search criteria. 

[Mark Hammer]

OK, found it.  It's a 25 year-old circuit from Tom Gamble.  Never built it, so I can't vouch for it.  Here's the schematic and a layout.

[ElectricDruid]

Another possibility would be the Mutable Instruments Shruthi filter schematics (there are several - the synth was designed to be able to use different filters):

https://pichenettes.github.io/mutable-instruments-diy-archive/shruthi/build/

The original supply for this was +/-5V, derived from a 9V input, if I remember correctly. It uses an LT1054 for the negative supply. Here's the schematic for the basic "SMR Mk2" Roland-style filter:

https://pichenettes.github.io/mutable-instruments-diy-archive/static/schematics/Shruthi-Analog-SMR4-mkII-v02.pdf

If that's too polite for you, there's a Polivoks filter too! (though that's not 4-pole).

[PRR]

It's a 25 year-old circuit from Tom

It is essentially Moog's ladder filter. Patented October 28, 1969.
https://www.google.com/search?q=Moog+ladder+filter

[ElectricDruid]

It is essentially Moog's ladder filter. Patented October 28, 1969.
https://www.google.com/search?q=Moog+ladder+filter

True. Wasn't the diode ladder version first designed as a way around the patent by EMS for the Synthi or VCS? That filter was most famously used on the track "On the run" from Dark Side of the Moon. Diode ladder filters in general are probably most famous for the somewhat-odd implementation in the Roland TB-303.

[Mark Hammer]

It's a 25 year-old circuit from Tom

It is essentially Moog's ladder filter. Patented October 28, 1969.
https://www.google.com/search?q=Moog+ladder+filter

True.  My poor phrasing implied it was Tom's original idea.  It was his stab at a Moog circuit that would run off a +9V supply.  I've built his +/-12V version twice, and still can't get either of them working.

Before Behringer started resurrected all the great analog synths of the '70s and '80s, the synth-DIY crowd were hard at work providing layouts for modules.  I managed to score most (maybe even all) of Tom's old EFM site before it vanished.  It included several of his single-board all-in-one designs, although they were generally single-VCO units.

[PRR]

It is essentially Moog's

SORRY: the 3,475,623 patent is not the classic Moog ladder, but a complementary version. Up to a point they are similar but the overload will be different.