Help with Single Pot LFO on Neovibe

[ChristoMephisto]

Just finished up a Neovibe, and worked the first time!!
Managed to house it in an old Boss EV10 expression pedal, find the sweep longer.
Got a nice orange glow using the stock parts, with the trimpot and r48 around 200ohms.
Used a rotary switch and added the Tremolo mod like the Reisly RT18
Since it came with 10k pot at the foot treadle, went with using the Single Pot LFO mod
http://www.geofex.com/circuits/ldrlfo.htm
The problem is when it gets to about half way back, the rate drops off. Is this normal?
Using a 2N5089, a 4k7 trim pot and the voltage is from the Neovibe at 15.2V.
Found a calculation in another posting, (voltage - led voltage)/.005Au, (15.2v-2v)/.005Au=2k64
does this sound right?
Would I be able to extend the Rate's sweep if i added a small resistor between the pot and the diode, or maybe between the pot at lug 2 and the transistor's base?
The pot on it's own measures 8k6.

[Mark Hammer]

Ran into the OP over lunch in a store, and after listening to "the symptoms", one of the possibilities we discussed was that maybe there IS variation in sweep speed between halfway back and fully back, but it's just inaudible.

That's one of the quirks of vibe-type units.  The wide-n-shallow dips it produces are not as audible as the sharper notches produced by phasers.  As a result, unless the LFO rate is fast enough, there's simply not enough going on for the human ear to detect, and it can sound like it's not doing anything at all.  The same sweep rate with 3 or 4 notches would be quite audible as a satisfyingly long tonal change.  Subtlety, on the other hand, has to come faster to be as noticeable.  One of the reasons why, if one is converting a phaser board to vibe specs, and the speed pot listed is 500k, a 250k or even 100k may well do you for speed modulation, since the larger resistance is for slower speeds....that you won't be able to hear.

The suggested change, then, is to use some additional resistance between the ground side of the pot, and ground, such that when the foot treadle is pulled fully back, it won't have entered that zone where the speed is slow enough to produce undetectable effect.  The good news is that the full rotation of the existing pot, via the foot treadle will now be usable, and the little in-betweenie speed settings will be easier to dial in with your foot.

[ChristoMephisto]

Yeah, was lucky to run into you at the music store.
As soon as I got home, i placed a 4k7 trimpot between the pot and the diode, and seems to have cured some of the problem
Too much resistance and the top speed it too fast, sounds warblerly.
Gonna have to play with some more to tweek it some more, but it's going in the direction i wanted

thanks mark!!

Trolling through the Colorsound Vocalizer thread, found a link to 100k dual stereo pots that should fit in my pedal
so i can hopefully do away with the Single pot lfo mod...
http://www.ebay.com/itm/Dual-Gang-Potentiometer-100K-Logarithmic-RP286-/160655699633?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2567d36eb1

[Mark Hammer]

AFAIC, it's a crapshoot as to which tracks better, a dual-ganged pot, or a dual optoisolator.  I have no basis for pointing to one as offering better matching than the other.  Dual-ganged pots are rarely well-matched (i.e., the resistance on each side of the wiper is exactly the same on each section at any point in the rotation), which is why high end audiophile preamps will often use multi-position rotary switches with 1% resistors for things like treble and bass controls, rather than chance it with dual-ganged pots.

On the other hand, LDRs are not always assured of being faithful replicas of each other either.  To some extent, at least you know that two LDRs in the same sealed component will not be subject to as muchbetween-unit variation as to exposed LDRs installed on a board near the same light source.  But whether they will be MORE matched than a dual-ganged pot is, as I say, anybody's guess.

Certainly if anyone has more knowledge on the area, PLEASE chime in.  But for the moment, I'm not seeing any strong reason to switch from the one method to the other.

Finally, do note that not all foot-treadle shells easily accommodate dual-ganged pots, and you're not always guranteed the gear fro the rack and pinion system will a) come off your existing pot and b) fit well over the replacement.

[ChristoMephisto]

Here's a gut shot showing the pot, it's a 16mm with a 30mm shaft

That little board has the Sinlge pot LFO, Millennium Switch 2, and elements of the tremolo mod, the other board is a Charge Pump
The Vactrol is upside down, but wired correctly.
The only reason I got the pot off of evilbay is in case I decided I needed it, but I'm so close, really don't think I'll need it
Also I have a wah pedal that'll it'll fit in, and spare parts on hand is handy.
Going to play with it some more today, have to lay it open in it's side to work on the insides...

[ChristoMephisto]

Seems to work best with the trimpot set to about 160ohms between the pot and the diode to ground
Still wondering if the trimpot for the Single pot LFO is set right, lower than 2k64 and the minimum rate gets shorter,
Meaning when the Rate is all the way back, the effect kicks in as you turn the trimpot lower.

Don't know if this is any relevance, but measuring pads C&D on the Neovibe, it read 200k, but nothing with pad E.
It does take a dual stereo 100k pot wired in a weird fashion. Pads C-D-E are for the rate pot.