Envelope Filter Suggestions

[Qfunk]

Hey,

I realy want to build a good envelope filter.  I have a vintage seamoon funk machine (i posted about this before) that i was going to clone, but compared to some newer filters, it kinda sounds like crap.....basically, its just got some mojo.

ANYWAY......i really like the Q Tron (which is similar to the mu-tron, right?), but that seems a bit out of my league.  Anyone know of another filter that works well for bass?  I hear the DOD 440 was good for bass, and it looks like one i can put together. 

Thanks in advance for your thoughts!

-sam :-)

[oldrocker]

The EV's I've built are
Dr. Quack (A fairly easy build)
Nurse Quacky (Stripped version of the Dr. Quack)
Phuncgnosis (Found on Tim E's Circuit Snippets website)
And tonepads FSH-1 (Filter Sample and Hold)
The only one I actually tried with the bass was the FSH-1.  It worked ok although not an easy build by any means. 
I think there could be some mods to the Dr. Quack that would make it better for a bass.
I've been meaning to try a DOD 440 but haven't got around to it yet.

[Mark Hammer]

The Dr Quack works reasonably well ( http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=118&Itemid=26 ).    The primary difference between it and the Nurse Quacky is that the Quacky omits the input buffer and one of the two filter modes.  What it retains from the Dr Quack is the ingenius bias adjustment that Jack Orman came up with to permit use of a broader range of dual op-amp chips than the LM1458 used by the original Dr Q.  If you've tried a Dr Q in the stores, my guess is you'll be disappointed with it, the reason being that it is not optimized for bass, and uses a lesser-quality op-amp (though that is probably the least of its flaws).  It also lacks the input buffer that Jack Orman added to the Dr. Quack.  One of the filter modes also tends to sweep fairly high and takes a while to settle down, remaining thin-sounding for too long, making it more suitable for rhythm guitar than for bass.

So, how would you optimize it for bass?

First, I'd build myself a Dr. Quack but use only one of the filter modes, as the Nurse Quacky does.  That will let you use the PCB layout for the Dr Quack.  The next step is to shift the range of the filter downwards to suit the bass.  Indeed, the only difference between Guyatone's wah-rocker for guitar and bass is that the bass model has its range lower.  To make that shift, you need to increase the value of the two capacitors that determine the range.  In the original DQ schematic, these are shown as .005uf, though they are more likely to be .0047uf (a standard value).  A suitable shift is to double or triple their value to .01uf or .015uf.  You can even go down as far as .022uf, though if you play a 5 or 6-string bass and need something that will still show off the upper notes on the upper strings, I'd suggest sticking with .01-.015.

The next step in bass-adapting it is to allow the filter to sweep back down quickly.  Where there is often a need for a guitar being filtered to sweep a little more slowly on the "backswing", bass players generally seem to want that filter to zip back in place like a yo-yo ready for the next note, and that requires a fast attack and decay.  The stock DQ schematic will often show a 100R attack-determining resistor, and 10uf decay-determining capacitor.  Often, I recommend to folks to increase the value of that cap, but that advice is primarily directed at folks who want the longer decay but do not want to hear the ugly envelope ripple that can sometimes accompany it.  If your attack and decay is going to be fast enough, though, the sweep will come and go at a speed where any envelope ripple that occurs will be barely (if at all) noticeable.  Essentially, the stock 10uf value is right in the "trouble zone".  My advice would be to drop the 100R down to 47-68R, which will increase the attack speed, and either swap out the 10uf cap for 4.7uf or even 2.2uf, or install a variable decay-time control in parallel with it to have something suitable for different "moods".  The variable decay would be essentially something like a 33k fixed resistor, in series with a 250k-500k pot.  As the combined resistance increases, it will take longer for the cap to discharge and the decay will be slower.  Make the resistance low and the filter will sweep back down nice and quick for more percussive effects.

The final step is to possibly blend in some clean signal with the filter.  This is a matter of taste, I suppose.  A great many bass players have a real need to provide the solid foundation for the band, and if the filter sweeps too high, their tone gets thin and the foundation gets kind of rickety.  It is possible to have the best of both worlds by combining some clean tone in with the filtered sound.  Indeed, many commercial bass filter pedals have this feature.  The simplest way to combine some clean tone with your filtered tone would be to take advantage of the JFET input buffer's ability to feed several different paths at once, and run something like a 10uf cap and perhaps a 10k fixed resistor (in series) between the source of the JFET (i.e., where the existing 10uf cap connects to) and the output, where the .05 (.047 in reality) output cap and 470k terminating resistor.  There are more sophisticated ways to do it, I suppose, but that one will get you there.  If there's still too much clean, make the resistor a little higher in value.

There are plenty of other excellent, and buildable envelope filters that can also be adapted nicely for bass, and plenty which - to my ears anyways - sound better.  This one uses cheap and available parts, and is dead easy to debug, so I think that makes it a reasonable suggestion for a first DIY filter pedal.  Once you get the hang of it, I'll leave it to you to decide whether a different build, like the Mu-Tron or MXR Envelope Fiter,or even the Meatball, is right for you and your skills.

Note that the LEDs indicated in the schematic need to be both red.  One will light up all the time, and the other may or may not light up with your playing, depending on how bright that particular one is.  Some can provide lots of nice eye-candy and warrant mounting on the chassis, but others which barely light up will still do the job electronially.