A fast/slow mod for things driven by half-wave recifiers.

[Mark Hammer]

Here's a simple little thing that occurred to me on the way home. I'm surprised it never occurred to me before.  It lands squarely in the spirit of mid-to-late 70's E-H and MXR effects by collapsing two functions into one switch.

Those of you who have made either compressors, or any sort of envelope controlled device using a half-wave rectifier (HWR), such as found in the Dr. Q, or the Orange Squeezer, may know that I frequently recommend to people to play with the "time constants" of such devices.  This is basically a resistor and capacitor, configured like a low pass filter (R in series, cap to ground) immediately after a diode.  The value of the resistor and cap sets the attack or onset time (or if it's a filter, the time taken for the initial direction of the sweep), and the value of the cap and any resistance that might be in parallel with it sets the time for the decay or "settling/recovery time".  

As the series R gets bigger, it takes more time for the sweep to kick in motion.  As the value of C gets bigger, the decay takes longer.  Longer decays are nice with HWR because they help to smooth out the envelope ripple that often comes with it.  Longer attacks are nice too, and add a different feel.  Shorter attack feel more "urgent" when it comes to swept filters.  Faster response also allows you to provide greater emphasis on the beat.  At the same time, slower response time allows for a lazier feel that can sometimes better suit rhythm work, especially where you don't want it to distract too much.  Slower response time also makes the filter a little less directly responsive to picking style (as if the filter is saying "Yeah, hold on a sec, I'm almost done.  Okay, now what was it you wanted?")

So, there are reasons to have the HWR respond quickly, and reasons to have it respond sluggishly.  I often recommend to people to stick in a pot in series with the resistor, to vary attack/rise time, and a toggle switch to select cap values for different decay/settling times.  That's fine, except that it requires a pot, a toggle, and a knob, AND more panel space, something which not everybody has in every chassis.  It is also the case that sometimes, people don't want to tinker with a pot that doesn't make HUGE differences with small amounts of rotation, and it is also true that the series resistance and cap value interact.

So, instead of having separate attack/decay functions, I thought "What if somebody only wanted 'slow' and 'fast'?"  That is, they wanted to be able to make an envelope cycle "happen" quickly or slowly.  Could I easily build it into a single toggle switch function?

The answer, happily, is yes.  The schematic can be seen here:  http://hammer.ampage.org

This uses a SPDT toggle to simultaneously change the attack AND decay parameters.  IN one position both the attack and decay times are short.  In the other position, they are both lengthened.  What they OUGHT to be depends on you I suppose, but I'm going to suggest at least a three-fold change as a minimum detectable change.  So, if the series R was 47 ohms and the cap to ground was 10uf for "fast", "slow" would be a resistance of 150R and a capacitance of 30uf.

In the diagram, the centre-lug of the switch is tied to the junction of a fixed R and the cap. Leading up to this point are two series resistors.  In parallel with the decay cap is a second larger value cap.  When the switch is in position 1, it shorts the extra resistor and lifts the path to the extra cap.  So, the series resistance is less and the capacitance is at minimum - "fast".  In position 2, the extra resistor is "unshorted", producing a larger series resistance, and the extra cap is connected, producing a larger parallel capacitance to ground and longer decay time - "slow".

The connecting and unconnecting of a polarized cap is not without its price.  You WILL hear a pop when reconnecting the additional cap.  That is easily overcome by only doing the fast-to-slow switching when the pedal is in bypass mode.  

Another thing you need to accommodate when inserting additional series resistance is the loss of current charging up the cap.  That series resistor acts as a current limiting resistor.  The bigger it gets, the less envelope sensitivity you will seem to have.  What I do, is simply add more gain to the envelope follower gain stage.  So, for instance, the Anderton Bi-Filter Follower has a 470k feedback resistor - I stuck in 820k.  The Dr. Q started out with 2.2meg - I put in 3.3meg.  If you REALLY wanted to get deluxe, you could use a DPDT switch and use the second set of contacts to switch in/out additional gain-setting resistance so that sensitivity was kept relatively constant across fast/slow settings.  But that would make the wiring more complex so you're forgiven if you overlook it.

That's it.  You should now be able to flick a switch and get a big change in character of the pedal, from one which is generally lazy to one which is generally twitchy.

[R.G.]

Very nice indeed Mark.

[troubledtom]

damn
   - tom

[moosapotamus]

simple... elegant... cool... thanks, Mark!

~ Charlie

[Mark Hammer]

Thank you gentlemen.  Just finished repacking up a Bi-Filter Follower that I modded this way, and I like it very much.  Two very different characters now.

I suspect that in the wide wide world of DIY synths something like this must have been implemented out there already.  I know that envelope generators often have "multipliers" or switch-enabled changes that change ALL time constants by some fixed proportion or amount (i.e., something like a global x2 time change), but I was unaware of anything in the HWR zone that did something similar.  Like I say, though, DIY synth is extremely vast and hard to keep up with unless dedicated to it.  In that respect, I'll claim credit for describing and illustrating it so a pedal-phile could implement it, but no more than that.

[onboard]

"unshorted"

Classic.  


Great write-up! "At the flick of a switch" comes in mighty handy.

[ExpAnonColin]

Three cheers for attack/decay control

-Colin