Bassballs Sweep

[Ghode]

I'm actually modifiying my russian bassballs with a some of the mods found here (external filter pots, attack/decay, filters mix, diodes for more consistent fuzz, CV input, change envelope opamp and gain) and I was wondering if it was possible to invert the sweep of the bassballs (wah->haw). I've seen these mod on son envelope filters, but they use optical triggers, so I'm not sure if this mod can be made or not for the BB (without changing the envelope to optical).

Thnx!!

[Mark Hammer]

Reversing the sweep normally involves imposing some sort of starting voltage, and then subtracting the envelope from that starting voltage, instead of starting out with some basic DC voltage and adding the envelope to that.

Alternatively, one can use control elements that function in opposite ways to positive-going voltages, and switch between those elements.  Tim Escobedo's Gargler prototype is an example of that.  The positive-going voltage from the same envelope follower/rectifier makes the NPN bipolar transistor produce a smaller resistance, and makes the J176 P-channel FET produce a larger resistance.  Figure out how to replace the bipolars in the BB with P-channel JFETs, and you're in business.

Alternatively, one of the interesting things about the unit is that if the attack time is VERY fast - fast enough so you don't even hear the rise - it sounds very much like it only sweeps down, and not up-then-down.  ty replacing the 100R resistor with 33R and see if that suits your tastes.

[Ghode]

Thanks Mark, cool solution! that's what I was looking for, though I thought it would be changing something in the follower rather than the filter. I wasn't familiar with that design, I only knew the gargletron but it didn't apply in this case. I'm guessing the FET would need some fine tuning in order to get a nice weep since I would leave the 10k before it (it's now a pot), and I'm not sure if I can get that FET in my country, haven't heard of it. Guess the option of turning into an optical follower doesn't sound so bad now haha

Wouldn't chaning the PNP at the output of the follower have the same effect, like inverting the whole sweep? Maybe it's a dumb question, I seriously have no idea.

The fast attack and quick decay gives me a really cool synth sound which was what I was looking forward to invert, so the quick attack solution won't work for me, though it is an awesome sounding setting!

[Mark Hammer]

You may have a tricky time sourcing that particular P-channel JFET, but you should be able to get some sort of P-channel device.  And from there it may simply be a matter of a parallel resistor here and series resistor there.

Considr what would happen if you reversed the polarity/orientation of the diode in the rectifier section.  It only allows the half cycle appropriate to its orientation to pass (that is, after all, how we get a changing DC voltage, proportional to signal amplitude).  So, instead of a shifting DC that goes more positive, and then decays, you get something that is the mirror image.  But what is the offset?  That is the challenge: you would need to establish an offset DC voltage that the envelope could be subtracted from.

And keep in mind that while it is relatively easy to set a "resting level" for a filter, and adjust high much higher than that the filter will sweep, using the trimpots and the Sensitivity pot, downward sweeps are a little different.  The starting point is not simply the "opposite" of whatever the starting point for upward sweep is.  It usually has to be set a little lower than that.  As well, because it isn't likely to "travel" quite as far, one needs to adjust the sensitivity, and maybe the attack time.  I don't know if you've ever built the MXR Enevelope Filter, but Stephen Giles came up with a brilliant mod for it, some years back, using one of the spare invertors that was already on-board.  It works, but sounds terrible if you simply flip the up/down switch in the other direction.  You must tweak the attack time and sensitivity.  Somewhere out there is a version of that mod using a more complex toggle switch that reverses direction and adjusts the "starting point" of the sweep, simultaneously.