FUNKKONTAKT: a simple (and slightly unconventional) funky envelope filter

[Fancy Lime]

Hi all,

so I finally got around to recording a little demo. Just going through some finger style funk starting with highest Q setting, ending with the lowest Q, all while playing around with the Base and Range controls.

http://www.aronnelson.com/DIYFiles/up/Funkkontakt.mp3

I also modified the current limiting resistors of the LDR LED's in order to get slightly higher maximum settings on the Range control. And I moved the small filter cap from VR to V- to save 1 switch-to-board connection, which are always a pain in the posterior. The sound sample is for this version 1.3:



Anyway, I noticed that the two small caps to ground in the filter, C4 and C5, seem to introduce some noise (slight white noise-like background, a bit lower than a "hiss", more of a "shhhhh"). When either one of them is bypassed, the noise disappears, it is only there when both caps are in series. This noise was not there in the parallel configuration of version 1.1. Any idea what that might be? I was thinking that either the combined value of 688p is too small (but I don't know too small for what), thus causing the noise. Or that maybe the series resistance/non-ideality of the caps causes some sort of problem because it adds up. In the latter case I would try to solve it by bypassing the two caps with single lower value cap with good ideality, like a 100p silver mica (but I don't have one of those sitting around at the moment). Does that make any sense? I use greenies at the moment for C4 and C5, for what it's worth.

Thanks for any input,
Andy

[rankot]

Cool sound!

[Mark Hammer]

I agree with Rankot that it sounds great.  As I'm fond of repeating, a lowpass with higher Q yields a sound with all the benefits of both lowpass (no thinning out) AND bandpass (a clear point of focus).

If, in fact the series-cap arrangement IS yielding noise, then an alternative is to go back to parallel, but stick a higher-value resistor (3M3-10M) in series with the cap that is to be added on, and use the toggle to shunt the resistor, when you want to change ranges.  The resistor provides a path for the cap to bleed off, but leaves the additional cap functionally out of circuit.  Gibson used the same thing for switching between caps in the Varitone circuit.

[Fancy Lime]

Thanks for the kind words, guys!

As I'm fond of repeating, a lowpass with higher Q yields a sound with all the benefits of both lowpass (no thinning out) AND bandpass (a clear point of focus).

Right? I find it quite astonishing how few commercial units use resonant lowpasses compared to bandpasses (tinny) and peakers less quacky/focused and noisier). It is a bit more complicated to implement but really only a little. And with things like the DOD FX-25 it makes extremely little sense to me why they chose to use the bandpass out of the SVF instead of the lowpass.

Thanks for the resistor tip! I'll try that tonight and see if it helps. I suspect it will. If not, then I'm going to get some higher quality caps and see it the HiFi enthusiast ravings on this particular topic may have some merit to them.

Cheers,
Andy

[Fancy Lime]

Hi there,

since Ranko has revived the Quackmire, I thought I'd repost the missing schematic of it's big brother:

https://postimg.cc/fSCy97Hj

Cheers,
Andy

[rankot]

Do I smell challenge? 

[Mark Hammer]

Is decay time set by a resistance in parallel with C10, or by altering the resistance of R10?

As well, how is the resonance altered?  I'm having a hard time spotting contacts 3 and 6 for the switch in the diagram.

[Fancy Lime]

Is decay time set by a resistance in parallel with C10, or by altering the resistance of R10?

As well, how is the resonance altered?  I'm having a hard time spotting contacts 3 and 6 for the switch in the diagram.

Attack and decay are fixed in this one. Making R9 a pot (2k-5k) would change attack times, R10 as a pot (25k) would give you a decay pot. However, as you yourself have pointed out in some old post I read ages ago, using these resistors as controls is a bit hit and miss because they are very interactive (also with the sensitivity setting) and have limited range. I find it much more useful to change C10 to 1u and add a on/off/on switch that can add 4u7 or 22u. Much bigger range and easier to use, yet sufficiently precise if used together with the Sensitivity and Floor controls. The main "innovation", if you want to call it that, on this thing is the Floor or Base control, which controls where the filter sweep starts. Setting this low means the envelope detector has to rise quite a bit before reaching the point where it starts opening the filter. Because this movement is slowed down by the time cap, that means that a low Floor also increases the attack time and shortens the decay time (because Q1 pulls the detector down harder). So it acts kind of like a "short attack and long decay <<>> long attack short decay" control. Together with a switchable time cap, which makes both simultaneously longer or both shorter, I find this an extremely versatile and easy to use control. Very easy to dial in some earth-shaking WHOOP, if you want. Or set the filter to start above the fundamentals, so you get a nice quack without blowing out windows.
The Q is set by switching the caps in the sallen-key filter but I have been pondering changing that to a continuous pot by giving the sallen-key some controllable gain (instead of just being buffer based). Problem is that variable-gain sallen-keys are awfully touchy and have not yet figured out how to design it so that it works well and reproducibly when considering all part tolerances.
Contacts 3 and 6 of the switch are not connected to anything.
BTW, you can add Rx to make the Q get smaller as the filter opens, or reduce R5 to make Q smaller at the bottom of the sweep. Or add more pots here and have an 8 pots, 2 switches version. At that point I would also ad a UV meter, some tubes (just for the light effect and the advertising) and a wifi connection. Everything needs a wifi connection, or so I'm told.


@rankot
Let's put it this way: I certainly would't object if you thought you absolutely had to design a PCB for that. But maybe hold your horses for a bit,. since I still have some issues with the filter section. I would really like to get the continuous-Q control working but that will take some time.


Cheers,
Andy

[Fancy Lime]

Just a quick re-post of the schematic.

https://postimg.cc/fSCy97Hj

Cheers,
Andy