Adding Depth to the Univibe

[bob p]

I've started listening to Trower again, so I've been thinking about 'vibe circuits lately. One of the things that I've been contemplating has been deepening the tone of a UniVibe type circuit by adding stages to the phase shifting array. Basically, I've thought about cascading two 'vibe type phase circuts in series, using the LFO from one to control both.

Of course, we all know that great minds think alike. When I was doing my research I stopped by GEO and took at look at RG's "Technology of the UniVibe" article. He had already contemplated doing exactly what I'd been thinking about. He called it the "UltraVibe", but alas, his article only mentioned that he had tried building the circuit, but didn't say much more about it.

Has anyone else given any thought to this sort of project? It sounds like RG's given a spin but he hasn't written much about it. I'm wondering where the best place would be to tap into the LFO output to control the second board, and what kind of impedance matching (if any) might be needed to have one LFO control two vibe circuits without shifting the response of the controls.

Has anybody else thought about this?

[R.G.]

The eight stage adaptation was interesting, but not hugely compelling for the cost of nearly doubling the size of the circuit.

As for tapping into the LFO, you don't need to. You just stick four more LDRs into the same light chamber as the original. There's plenty of light to go around.

Frankly, the best bang for the buck in deepening the effect of a univibe is to put in the balancing pot that lets you adjust the dry and phase delayed signals for the deepest notches.

[bob p]

Thanks, R.G. for the prompt response!

The eight stage adaptation was interesting, but not hugely compelling for the cost of nearly doubling the size of the circuit.

As for tapping into the LFO, you don't need to. You just stick four more LDRs into the same light chamber as the original. There's plenty of light to go around.

Well, as it turns out I'm sitting on a bunch of those inexpensive little poultry-vibe variants that came out of China by Danelectro. I was thinking that it wouldn't be too costly of an experiment to take one of them, disconnect its LFO, slave the light chamber circuit off of the first board's LFO, and then run the boards in series. Then, if the experiment shows merit, it might be worth putting the time/effort into a better circuit.

Am I correct in interpreting your post to say that the results from your cascade experiment weren't really worth the effort? Was that before or after you did the cap swaps to bring out the hiding notches?

Frankly, the best bang for the buck in deepening the effect of a univibe is to put in the balancing pot that lets you adjust the dry and phase delayed signals for the deepest notches.

You're referring to the segment of your Tech paper entitled, "Trim the Phase Mix", right?


Back to the idea of tying the LFOs together: one other thing that I had considered trying was tying the phase circuits together with a common LFO, and then moving one or both of the segments back/forth in the signal chain to position the phasor segments before or after, or surrounding a distortion stage.

As you know, the combination of a vibe and distortion can sound very different, depending on placement order in the signal chain. In addition to a simple juggler type implementation to move things around, I had thought it might be interesting to hear how it sounds to place a pair of sync'd 'vibe circuits both before and after a distortion stage. To do that though, you'd really need to sync the LFOs on each 'vibe. It would be kind of like using common LFO #1 to drive both Phasor A and Phasor B on a BiPhase, and sandwiching a distortion stage between one phasor's output and the other's input. I was wondering if you've ever given this a try, or if you had any thoughts on whether or not it might be sonically justifiable to make this kind of project worth the effort.

[52 Bill]

I've always felt that the lack of feedback in the Univibe was a real limiting factor here. Twenty plus years ago I modded one for a guy by adding a 470K (I think) feedback resistor from the output back to the first stage. The depth was much improved and the owner was thrilled.

[Mark Hammer]

Then it likely wasn't functioning as a Uni-Vibe.

Mike Irwin ran off some graphs for me some 8 years ago, showing the frequency response with standard uniform cap values over 4 stages, and Uni-Vibe cap values. The thing about Uni-Vibe (or comparable) values is the "notches" are barely that. They are broad. The kind of broad that if you had a 31-band EQ, you'd use 4 or 5 adjacent sliders to make one of them.

This is what makes the difference between a phaser and a Uni-Vibe. The notches on a phaser ARE notches, and are focussed and confined to a more specific portion of the spectrum. As such, adding regeneration/resonance/feedback makes the dip and adjacent peaks more pronounced. Add regeneration to a Uni-Vibe and the broad shallow dips produced effectively suck the volume out of the signal, as well as any constant harmonic content. There is really NO reason to include regeneration in a Uni-Vibe-like device, and that is precisely why you've never seen it on any commercial product.

The classic application of a Uni-Vibe is before a distortion pedal, or going into an amp dialed in for distortion. The reason why it sounds so good like that is precisely because it HAS no particular resonant focus, but moves the proximity of the signal to the clipping point around for broad segments of the signal. The result is what many would describe as an "animated" sound, rather than an obviously filtered one, as you get the enhancement and taming of harmonic content over broad regions.

[52 Bill]

Well I stand corrected.

[Mark Hammer]

Bob's original desire to have variable depth IS a reasonable addition to any Uni-vibe since there is a general principle in modulation effects that faster speeds demand narrower sweep width, and slower speeds demand more.