This Week on the Breadboard: ROG Tri-Vibe

[Chuck D. Bones]

Yes, that could work, but I don't know how well. The LDR resistance range and rate of change are highly variable. You need to select just the right LDR for the job. Seems a bit kludgy to me.

The STOMPLFO eliminates all of the unknowns, and by replacing the LFO circuit with a single chip, uses fewer parts.

 

[Diynot]

Shit, already built a harbinger 2, but a tri-vibe in a wah shell would be amazing and apparently there is one manufacturer out there that makes a 500k ICAR pot (Joe Gagan). Is there a way to get the same speed spread with a 200k, or at least adjust to shift the speed to a higher or lower speed range? Is it as simple as putting these caps on a switch (from the Aion schem):

Or would you put R27/R28 on a switch instead with values increased to accommodate use of a 200k pot?

 

[Chuck D. Bones]

This points out another advantage of the STOMPLFO: the RATE pot can be any value and the range remains the same.

Increasing C9 & C10 is one way to accommodate a smaller RATE pot value. You'd better use tantalum because aluminum caps are leaky and might cause the LFO to stall.

There is another way to connect the RATE pot, which accommodates different pot values without fiddling C9/C10.
Remove R27.
Connect pin 3 of the RATE pot to IC3-7.
Leave RATE pin 2 connected to R28.
Connect RATE pin 1 to one end of a resistor and the other end of that resistor to Vb.
Select the values of the new resistor and R28 based on the RATE pot value and the desired speed range.
Another advantage of this connection is by carefully selecting R28, we can obtain a broad range of effective tapers.
(See the EQD Space Spiral's LFO for an example of this connection)

I have a question for you: would you actually use the full-range of the Tri-Vibe's speed?

 

[Diynot]

Awesome! Thanks for the info. I will check out the dark rift schem. As far as using the entire speed range? Maybe. I definitely liked almost the entire range of speeds on my tri-vibe build. That and since there are 3 modes I feel like they each sounded best at slightly different speed ranges.

 

[Chuck D. Bones]

I mention the speed range because it's much easier to select a speed with a knob compared to a treadle. The larger the range, the harder it is to hit a specific speed. The UniVibe did not have a huge speed range.

 

[Diynot]

Gotcha. The draw for me wouldn’t be for hitting a specific speed, but doing the Leslie speaker tones/ramping like @cooder mentioned.

 

[cooder]

Here's my functionality wishlist:
two footswitches, bypass (obviously) and ramp up/down
four pots: depth/ speed (low) / speed (high) / ramp speed
toggle switch for the three modes

Which sounds pretty much like what @Diynot said.

 

[Chuck D. Bones]

That's what I had in mind also. Once Steve's New Years Design Contest is over, I'll get on it.

 

[Diynot]

Happy birthday Mr Wizard! Once the raging CDB bday extravaganza is over, will there be some movement towards developing this sweet sweet circuit? Purty please!

 

[aquataur]

I had not spotted the huge electrolytic on the output of rail splitter OPA. This is indeed commonly advised against.
The R/C in front plus the OPA's driving stage take care of the purity of this rail.

My unit exhibited quite a bit of switching noise, which may partially due to the cheap footswitch they delivered with the kit. If I were to build a new one, I would order a Gorva - much higher quality.

I remembered Mr Black's tips against switch pops (to be found on the internet) and added a 4.7k in series with the input, 2.2M against ground, 1k in series with the output and 100k to ground right at the board. With those resistance dimensions the order is of little importance.

Mr. Black thinks the input grounding resistance can be omitted by the grounding action of the true bypass switch, but since he is nit-picking about DC voltages building up we might as well consider the switch when it is in transition during the change of state. (I noticed on some recently available kits some of those resistors are already included).

This took care of the switching pops.

The Tri-Vibe works very well after dirt, unlike the uni-vibe. I wish the whirl position (the one opposite of the phaser sound-alike) were a bit more pronounced. It reminds me of a rotating speaker.