Q For Those Of You Who Have Built Anderton's VPD

[Paul Marossy]

On the Craig Anderton "Volume Pedal De-Scratcher" circuit that I built, when the volume is supposed to be turned down all the way, there is still some guitar coming thru. I have checked to make sure that the pot is actually turned all the way CCW, and it is (I've got an Ernie Ball with a 1M pot in it). I haven't used this volume pedal for a while, and I noticed this last night.

So, anyhow, my question is - is that how the circuit normally would work or does something need tweaking? Does that diode between the volume pot and ground prevent it from completely dumping the signal to ground or something?

Here's the schematic: http://www.diyguitarist.com/PDF_Files/vpr.pdf

[Paul Perry (Frostwave)]

What are the chips there? If the first one is a CA3080 then it is surely going to need some tweaking..

[Mark Hammer]

I'm a novice in such matters, but doesn't the diode to ground on the pot present a sort of resistance, making the voltage divider never reach "zero"?  That is, it acts a bit like the small value resistor one sometimes sees on the ground leg of gain controls in amps and some pedals.

Incidentally, while we're busy attracting the attention of folks interested in volume control, has anyone ever attempted to use the Motorola MC3340 attenuator chip?  I understand there is some CV leakthrough, but if the CV is in the seriously subaudio range (how fast can YOU move your volume pedal foot back and forth?) perhaps it's not such an issue.  I'm asking because these things can provide a ridiculously simple remote volume control.

[Paul Perry (Frostwave)]

has anyone ever attempted to use the Motorola MC3340 attenuator chip?  I understand there is some CV leakthrough, but if the CV is in the seriously subaudio range (how fast can YOU move your volume pedal foot back and forth?) perhaps it's not such an issue.  I'm asking because these things can provide a ridiculously simple remote volume control.

They aren't very hi-fi, I suspect they are much like a 3080 internally.
I used them successfully in a recreation of an experimental synthesiser designed by Percy Grainger ((interrupting a light beam varied light reaching a photocell, that was amplified & controlled the MC3340).
personally, I have been very unhappy with the MC3340, and the feedthrough isn't just 'minor', it's damn near as big as the signal :shock:  :x
BTW Mark, shouldn't you be posting en français?

[Paul Marossy]

If the first one is a CA3080 then it is surely going to need some tweaking..

Yeah, the first IC is the CA3080... I'm thinking that maybe R4 could be a trimpot and I could adjust that until there is no signal coming thru with the volume pot at full CCW. Whaddya think?!

I'm a novice in such matters, but doesn't the diode to ground on the pot present a sort of resistance, making the voltage divider never reach "zero"?

I was intuitively thinking something along the same lines. Anderton says that the diode is there to counteract a "dead spot" in the CA3080's response. Maybe that is a side effect?

I guess it's OK like it is, I was just more curious than anything else...  

[puretube]

if so, make R4/R5 a trimpot (50k) and "balance out";

D1 sits there, coz the Iabc node "sits" at 1 diode drop
above ground, and that is exactly the voltage you`d want to have for min/zero volume;
(similarly, in a smallstone, the LFO`s minimum voltage "floats" above ground, for that reason);
You could try a diff. diode, with lower drop than 1n4001.

Or increase R9 (e.g.330k) - if the max volume is too low afterwards,
then increase R6 in turn.

[Paul Marossy]

Thanks puretube!  

[puretube]

err, Paul: that`s estimation paired with a little experience from similar circuits - haven`t built that schemo per se...
(but give it a try!  :wink: )

[Paul Marossy]

I know puretube, but I'm sure that you would "hit the mark" before I would.  :wink: