This Week on the Breadboard: ROG Tri-Vibe

[Chuck D. Bones]

You can read about this on Runoff Groove. A nice OTA-based Phase-Shifter / Vibrato.
I built with this four minor mods:
1. I used a TL064 in place of two TL062s.
2. I added a THROB LED.
3. I replaced the back-to-back 10uF caps in the LFO with a pair of 2.2uF film caps in parallel.
4. I ditched the 10uF cap on the output of the Vref buffer. Loading an opamp with a large capacitor is bad practice and not necessary here.

Good vibrato and pseudo-UniVibe tones. Not quite as intense as a UniVibe, but pretty close. More headroom than the Grand Orbiter.
ROG, in their usual cleverness, designed an LFO that makes something close enough to a sinewave and then passes it thru an exponential converter to get just the right sweep. Unlike LDR and JFET-based phase shifters, there is nothing to select or tweak. This circuit is truly plug-n-play. I'm going to try adding another LM13700 to get four phase-shifter stages and see how that sounds.
Highly recommended.

Left knob: SPEED - Right knob: DEPTH.
LFO, exponentiator & Vref buffer on the left, phase-shifter OTAs in the middle, input buffer & mixer on the right.

 

[jubal81]

I liked this one, but I seem to remember it had a lot of hiss. Always wondered about swapping the OTAs for THAT Corp VCAs but never got around to trying that.

 

[Chuck D. Bones]

Not getting any hiss with mine; using an LM13700 I bought on Amazon.
That 10uF cap on the output of the Vref buffer might be the culprit. If it causes the Vref buffer to oscillate at high freq, that could sound like hiss.

 

[jimilee]

I really like mine, too. I built a couple of them, I forget what happened to the first one.

 

[music6000]

You lost me right here ''passes it thru an exponential converter''
What is that?

 

[Chuck D. Bones]

They say a picture is worth 1K words, so here goes:

This is a generic exponential curve:


The exponential circuit in the Tri-Vibe is a crude approximation of the lower part of the exponential curve above, but it is close enough.


Many things in nature and human experience are exponential. In music, the octave scale is exponential. By passing the LFO signal thru an exponential converter, the phase-shifters move at a constant octave rate. I.e., it takes the same amount of time to sweep thru the octave from 100Hz to 200Hz as it does the octave from 750Hz to 1.5KHz.

This is the effect of sweeping the LFO voltage going into the Tri-Vibe's exponential converter in 100mV steps. Notice that the notches are more-or-less evenly spaced.

Without the exponential converter, the notches would be spread far apart in the bass region and bunched together in the treble region.

 

[Chuck D. Bones]

Latest Tri-Vibe mod:
I added two more stages and stagger-tuned them to more-or-less mimic the Uni-Vibe's phase-shifters. I used these phase-shifter capacitor values: 15nF, 2.7nF, 560pF, 100pF. These yield a nearly constant phase slope across the audio frequency range. There are two sets of notches, with the lower freq set approx the same as the Tri-Vibes notches. Does it sound any different? Not really. The 2nd set of notches are all above 2KHz. My opinion is the two additional stages do not affect the tone enough to be worth the additional effort and expense.

The notch depth can be maximized by tweaking the balance going into the mixer. I increased Rmix from 47K to 49.2K by inserting a 2.2K resistor in series with Rmix and obtained deeper notches. whether the difference is audible is open to debate.



The only other mod I'm considering is replacing the LFO and exponentiator with a STOMPLFO chip with a custom waveform that replicates the control waveform, something like this:

 

[Archimedes]

Many thanks for your thoughts on this circuit. Mine works quite well without too much hiss but it is really bright do other folks find this? If so any ideas to make the tone a bit more mellow?

 

[Chuck D. Bones]

Mine is not. This circuit has pre-emphasis & de-emphasis to minimize high-freq noise. If one or more of the components in those networks is wrong, then the freq response could end up bright. Check the 15K & 4.7nF caps on the input and output stages. C2, R2, C6, R20 in the LTSpice circuit above.

 

[Archimedes]

Mine is not. This circuit has pre-emphasis & de-emphasis to minimize high-freq noise. If one or more of the components in those networks is wrong, then the freq response could end up bright. Check the 15K & 4.7nF caps on the input and output stages. C2, R2, C6, R20 in the LTSpice circuit above.

Thank you I’ll check that out

 

[Archimedes]

Thanks for the tip, I think there was a poor connection on the de emphasis side, tonality is much more normal now albeit still a little thin

 

[Chuck D. Bones]

Have you tried running it in Vibrato mode (100% wet)? The freq response should be absolutely flat.

 

[BryGuy]

Really like mine. No hiss at all. Maybe a slight tone shift to slightly brighter but it’s pretty minor.

 

[Archimedes]

Thank again Chuck D. Vibrato side is also a bit weak. I think I need to audio trace everything through and see if it is happening in the region of the TLO72 or the 13700. It’s not my best build it seems but I am determined to get there in the end as it‘s just a great little circuit. You won‘t hear from me for a bit as I am away for a while now but thanks for the pointers.

 

[G.G.]

I may be the only person who would ever want this, but I for my Tri-Vibe I wanted something like the Leslie Speed Switch that would slowly ramp up or down when switched between fast and slow speeds.

 

[Bio77]

I was wondering: has anyone ever implemented an exponential converter in a chorus LFO?

 

[Chuck D. Bones]

I may be the only person who would ever want this, but I for my Tri-Vibe I wanted something like the Leslie Speed Switch that would slowly ramp up or down when switched between fast and slow speeds.

I was wondering: has anyone ever implemented an exponential converter in a chorus LFO?

Both of these could be accomplished with the STOMPLFO chip.

 

[cooder]

I think the ramp switch with slow to fast speed would be great I'd love to see that. And build that.

 

[Chuck D. Bones]

I can think of two options right off the bat:
1. Build the Tri-Vibe into a Wah pedal housing and use the treadle to vary the speed.
2. Use the STOMPLFO chip, where the speed is voltage-controlled. Three knobs: FAST, SLOW and RAMP RATE (how quickly the speed changes). Use a stomp switch to select between the FAST & SLOW speeds.

 

[temol]

Is it possible to implement LERA as a fast/slow switch here?