ReneDSR: Fastest Envelope in the West: Zen and the Art of Tau

Hello again, just finished a new module based on a custom PCB.  The original design seems pretty well known in the DIY synth audio community: Rene Schmitz's "Fastest Envelope in the West".

Find his original page for this design here--scroll to the bottom of the page....

FADSRITW Prototype. No LazerTran front Panel Art yet, still testing it.

The Why in AudioDIWHY: I needed more ADSR's in my rig. What I have now works but doesn't always give me a super quick snappy decay.

I saw Rene's Fastest envelope schematic and thought--this will work" 

To me "fastest envelope" sounds postal; the acroymn is FEITW ("feeet-oo"?) is odorous, so I am calling it "fastest ADSR" or FADSRITW or even FDITW "Fastest Decay in the West". Whatever.

I will use acronyms going forward.

 Here it is:

Eagle BRD and SCM files, wiring diagrams, PDFs used for my take on FADSRITW are available on my website, click here.

About the module: Rene describes a couple of variations of his design--one with 2.2M A,D,R pots, another with 1M pots. Increase the the cap value if you decrease the pot value. To me, with analog envelope devices, the size of the cap creating the "TAU" (which, my EE brother explained to me, is simply the R*C value used as the heart of the ADSR) matters.

This is an RC curve--but what are RC curves, really?  Why does its output "curve"?

Warning: Math zone ahead. 

ReneADSR decay here's the formula to compute your decay control voltage:

V = S*(e^(-t/RC))

where:

V is the voltage at any given time--what we are after.
e is Eular's number (about 2.718), so, a constant
^ means "raised to a power of what follows", so, e raised to -t divided by R*C
-t is (time * -1).
RC is the resistance * capacitance.  That's TAU, baby!
S is the voltage at time = 0

If you plug this into a graphing program like this very cool one you get beautiful curves:

You take that voltage changing over time, and plug it into an appropriately designed VCA or VCF (such as this one, Matohiko Takeda's "Farm VCA") And: Aaaahhhh! The classic Moog/Roland analog synth sound.

A good sounding ADSR > VCF curve makes me very happy indeed.

Another aside: TAU is a Greek Letter and looks like the PI symbol that is missing a leg or pi cut in half:

It  means something different in mathematics, or biology, or astrophysics, or whatever; ditch the RC crapola if you're not a physicist because Tau means way different things to different scientists. This confused the hell out of me for a bit. You learn something every day eh?

OK back to it: from the equation above, it shouldn't matter if R gets big as long as C gets small. Right? Within reason perhaps. That's basic math. But again, how it sounds is way more important than however things look on paper. So what sounds best, really?

Guess we'll find out.

I built my first FDITW using 2.2M pots I found on Ebay (linear taper). 2.2M pots are not at all easy to find, but I found 'em, they arrived via eBay from China and 5 minutes later were soldered into the prototype. Yep! Worked first time! Update: I looked around for 2M or 2.2M audio taper--forget it, I am not sure they even exist?

Honking big 2M2 linear taper pot. The US dime to left, for nonUSAers playing along at home, is approx. 17.9mm in diameter. 

obligatory on the bench shot.

Can't find the pots? In an ElectroMusic forum post, DIY mensch donpachi lays down a cool trick for getting 2M out of a dual gang 1M audio taper pot:

I put this on the bench, and it works, but, if there is a +/- 20% error on each pot you might have -40% error overall, giving you a < 2M pot not 2M, so choose your pots accordingly.  You may have to try a few 1M AT dual gangs to get some with a rough value of 2MB overall. I dug some 1M dual gang audio tapers out of my junk box and pachi'd them in, replacing the linear taper honkers. To me, the FADSRITW sounds more musical with audio taper pots for A, D and R. I also jammed in a DPDT switch to select 2.2uF and 10uF for C3 to switch-a-tau. Yep that works, and you could use a DPTP switch here with 3 values for C3, a rotary with X values, a 4066 controlled by an MPU, or whatever else works for you. Overall another fun build. You can get the Eagle stuff, etc. here.

One more tid-bit: For some reason, I got the circuit to work w/ a standard 555 IC, but not a CMOS 7555. No idea why. I will build more of these and see if I can track that one down.

Going forward: I went online and bought some 1M audio taper pots from Small Bear. Waiting for those as I write this post.  I will build another FASDRITW with 1M, 100 ohm resistors vs. the 220's and maybe some other TAU-values (taulues?).  I'll also post some sound samples.  But no, no scope snaps. My eyes always confuse matters like these. The question: What sounds best, really? I figure the different RC combos will sound exactly the same but who knows.  Update: they do all sound pretty much the same, use 1MB single gang pots, since they are a lot easier to find online.

Part II of this post is here, where I go into variations on the RC parts of Rene's design.

Until then, DBTF.  See ya.