A "full-range" resonant lowpass filter?

[WhenBoredomPeaks]

I've built the Escobedo Sallen-Key filter ages ago, it is working well (imo, never heard an other one) expect even when it' frequency pot is wide open it still cuts noticeable amount of highs, even from a guitar which have much less high harmonics than a synth. Then i put some synths through it and it sounded even more bad of course because it filtered out even more content from the signal.

Is there an other design i could build which doesn't really cuts my highs when it is wide open?

Or the escobedo sallen-key should be good but i made some building mistakes?

Btw i use them from 9V supplies.

[Taylor]

Yes, lower the 100k resistors from the freq pot to pins 1 and 16. I think I made them 10 or 20k, which raised the max cutoff quite a bit and made it more usable.

[WhenBoredomPeaks]

Yes, lower the 100k resistors from the freq pot to pins 1 and 16. I think I made them 10 or 20k, which raised the max cutoff quite a bit and made it more usable.

Whoa, thanks, it looks like that 1-2 years after i built it, i will maybe have an usable circuit.

BTW if it will still be bad, (i mean the freq. range shifts in a bad way or something) should i look into the synth world for other filters or there are some "proven" resonant filters on this forum?

[Taylor]

I think this circuit is plenty good for resonant filter sweeps on 9v with low parts count. Of course, there are synth filters running on bipolar power which are cleaner, offer more poles, more even resonance, less/smoother distortion, etc. A 4-pole filter gets more in the direction of what we're thinking of when we talk about "that synth filter sound", this one's only 2 poles.

I also want to give credit forum member "mrslunk" who gave me the idea to mess with the resistors going into the Iabc pins.

[WhenBoredomPeaks]

The more analogue glitches, the better, nowadays as i use more and more software in my "setup", when i use hardware i want them to add analogue "randomness", glitches and weird stuff which are hard to model in software, i want them to give back some analog feel to my digital sounding stuff.

Interestingly the way i approach pedal building have changed because of that, in the past i always used Wima caps, avoided electrolites in the signal, used the best opamps etc. now i want to use "sh*tty" parts to get more analog weirdness out of the pedals.

I actually thought about building an "Analogizer", it would be a preamp of sorts where the signal would go through 1-2 tube buffers, 1-2 transistor buffers, a linear tonestack, 1-2 transformers and stuff.  This way the signal could "collect" some analog mojo in the device, but for this i would probably need a really good and expensive interface because most of the time the D/A A/D conversion is causing more harm which renders the analoge outboard gear useless.

[WhenBoredomPeaks]

Installed the two 10k resistors, the filter got much more usable.

With the filter wide open and the resonance knob pushed into oscillation you can hear the pitch through a guitar amp so if you wanna use this filter on instruments with more high harmonics you can go lower than 10k on those resistors, i might gonna try some values between 1-5k there in the future.

BTW it doesn't change the bottom end of the filter (which is pretty low, it might go down all the way till "0hz") just the high end.

[Taylor]

You might want to be careful there, as you could potentially send too much current into the Iabc pins with something below 10k or so - you'd kill the 13600. But it's worth experimenting with if you have a few extra chips and don't mind if you kill a couple.

[mrslunk]

Here's what i'd try:

put a trimpot wired as a variable resistor between +9v and Pot pin 1.
put a trimpot wired as a variable resistor between GND and Pot pin 3.
(each trimpot say 1/2 or 1/4 value of the pot)

connect pot pin 2 to the gate of some jfet
connect the drain of the fet to +9v
connect the source of the jfet to the 2 10k resistors feeding into the OTA's

the top trimpot will control the highest frequency
the bottom trimpot will control the lowest
you'll need to futz around to get past the cutoff voltage for your jfet.

the IAbc pin should be around 0.7v if i remember correctly

if you find you're still not getting enough range out, swap the 10k's for say 6.8k
but don't go any lower than say ~5k or you'll burn out your OTA (trust me i've got about 8 OTA's that only 1 half works cos i stuck too low of a resistor in there)


once you've tuned your trimpots, you can swap them for fixed resistors
for real fun, stick it in a wah pedal shell.

[Strategy]

This was just posted on Eric Archer's DIY site.

Perf a 9V LM13700 filter. Again the headroom limitations apply but it sounds pretty cool.

http://ericarcher.net/devices/diy-lpf/

[artifus]

not so sure about being full range and this is prolly old hat around here but have been having fun with ray wilsons odd filter from the wsg recently:

wsg full scheme

filter filter section minus input cap

have a small diylc stripboard layout if anyone is interested. (quite anal about small and neat layouts...)