Does the Uni-Vibe Preamp...

[Scruffie]

Intrinsically have some overdrivey fuzz to it? Or does it just not bias very well at 9v... or have I just missed an error on my breadboard.

[R.G.]

It doesn't bias well at 9V. Even if it biased well at 9V, the amount of clean swing is quite small, so it's easy to overdrive.

All of the phase stages share this issue.  The maximum theoretical swing is 1/4 of the 9V supply, or about 2.25V peak. In practice, with real transistors and other parts, it's much smaller.

Running a univibe at 9V requires some sincere rethinking of the design.

[Scruffie]

I did wonder, but not quite grasping the Preamp I wasn't sure. I wasn't going for the whole thing on 9v, just the Preamp just to play about with as it's an odd design, but it seems no matter what I do I can't get that to not be fuzzy even at 18v.

Think i'll just back away from it slowly...

[R.G.]

I did wonder, but not quite grasping the Preamp I wasn't sure. I wasn't going for the whole thing on 9v, just the Preamp just to play about with as it's an odd design, but it seems no matter what I do I can't get that to not be fuzzy even at 18v.

Think i'll just back away from it slowly...

You're probably running into the limited range of the available voltage. The hallmark of that preamp design is that it produces not only gain and buffering, but two equal-and-opposite outputs at the same time. The third transistor has to produce both outputs together in the middle on one peak and far away at the ends of the power supply on the opposite peak. So it has to saturated precisely in the middle of the available voltage, and turn fully off to get the most undistorted range. That means it has to be biased so it's sitting with its collector at 3/4 of the available power supply and its emitter at 1/4 of the available power for the biggest undistorted range. 

You can tinker the resistor values to get that to be true with 9V instead of 18V. I think Roger Mayer did a 9V design of a "vibe", but he used opamps to do the phase splitting and some of the other stuff to get around the available voltage limitations as I remember.

[Scruffie]

I did wonder, but not quite grasping the Preamp I wasn't sure. I wasn't going for the whole thing on 9v, just the Preamp just to play about with as it's an odd design, but it seems no matter what I do I can't get that to not be fuzzy even at 18v.

Think i'll just back away from it slowly...

You're probably running into the limited range of the available voltage. The hallmark of that preamp design is that it produces not only gain and buffering, but two equal-and-opposite outputs at the same time. The third transistor has to produce both outputs together in the middle on one peak and far away at the ends of the power supply on the opposite peak. So it has to saturated precisely in the middle of the available voltage, and turn fully off to get the most undistorted range. That means it has to be biased so it's sitting with its collector at 3/4 of the available power supply and its emitter at 1/4 of the available power for the biggest undistorted range. 

You can tinker the resistor values to get that to be true with 9V instead of 18V. I think Roger Mayer did a 9V design of a "vibe", but he used opamps to do the phase splitting and some of the other stuff to get around the available voltage limitations as I remember.

Or you shouldn't play with electronics at 1AM, misread the positioning of the 1.2k resistor and had it coming straight of Q3s Emitter

Assuming you're referring to the Roger Mayer Voodoo Vibe having a look the audio path has been redesigned to run on 9v yes although still all transistor, the LFO is OpAmp and a waveform generator chip.

The UniVibe is near a gain of 4 thereabouts if I remember correctly so are we saying it could just be replaced with this (each set for a gain of 4).



Or drive this with a non-inverting preamp set up once again for a gain of 4.



And that basically it's just a complicated way of achieving the same thing at a time when OpAmps were cost prohibitive.

Since you brought up the phase stages, how come we still use a pair of transistors rather than just a darlington package these days, is having a separate collector resistor on the second transistor important for proper gain/impedance/drive or is it just to keep it original, if so this might be an alternative to some other transistor phase shift stages.

[ch1naski]

Aren't there Darlington mods floating around out there for the vibe? Forum vibe?

Edit: not sure that they are for the phase stage, tho..don't have a  schematic in front of me

[R.G.]

The UniVibe is near a gain of 4 thereabouts if I remember correctly so are we saying it could just be replaced with this (each set for a gain of 4).

Or drive this with a non-inverting preamp set up once again for a gain of 4.

And that basically it's just a complicated way of achieving the same thing at a time when OpAmps were cost prohibitive.

Yes. If I were redesigning the vibe front end these days, I'd probably use an input unity gain buffer for one phase, an inverter for the other phase, and run it at unity gain through the whole thing.

Since you brought up the phase stages, how come we still use a pair of transistors rather than just a darlington package these days, is having a separate collector resistor on the second transistor important for proper gain/impedance/drive or is it just to keep it original, if so this might be an alternative to some other transistor phase shift stages.

It's all about keeping it original. An integrated darlington works fine. The original univibe was designed at a time when *transistors* were around $1 each, darlingtons in one package were prohibitively expensive, and IC opamps hadn't yet been introduced to the market at all.

I posted the first free-for-everyone-to-make layout for what was later named the Neovibe on usenet, before there was a world wide web. When I did that it was greeted with enthusiasm, and a great deal of booing from a few people. The boos were because I put the non-original voltage regulator in there. In today's effects world where we get first posts looking for mods, it seems odd to think that there were actually people who would write down that changing to a clean power supply would make the univibe circuit not sound like the original.

It was an education for me!  

[R.G.]

Aren't there Darlington mods floating around out there for the vibe? Forum vibe?

It's not even really a mod. The connections inside the univibe circuit are darlingtons; it's one connection to minimize the saturation voltage of the second transistor, but a darlington still.

I used single package darlingtons in the Son of UVICS pretty extensively.

[ch1naski]

Thanks for the clarification, Mr keen.

[Scruffie]

The UniVibe is near a gain of 4 thereabouts if I remember correctly so are we saying it could just be replaced with this (each set for a gain of 4).

Or drive this with a non-inverting preamp set up once again for a gain of 4.

And that basically it's just a complicated way of achieving the same thing at a time when OpAmps were cost prohibitive.

Yes. If I were redesigning the vibe front end these days, I'd probably use an input unity gain buffer for one phase, an inverter for the other phase, and run it at unity gain through the whole thing.

I did wonder why it had quite so much gain up front, my experience with any phase design has been that usually equals unpleasant distortion.

Since you brought up the phase stages, how come we still use a pair of transistors rather than just a darlington package these days, is having a separate collector resistor on the second transistor important for proper gain/impedance/drive or is it just to keep it original, if so this might be an alternative to some other transistor phase shift stages.

It's all about keeping it original. An integrated darlington works fine. The original univibe was designed at a time when *transistors* were around $1 each, darlingtons in one package were prohibitively expensive, and IC opamps hadn't yet been introduced to the market at all.

I figured that might be the case, i'm of the mind set of if I can save 4 (5 if you count the PSO) transistors I will, I doubt i'll hear the change in tone, but if in 10 years i'm not Hendrix well I guess i'll know why 

I posted the first free-for-everyone-to-make layout for what was later named the Neovibe on usenet, before there was a world wide web. When I did that it was greeted with enthusiasm, and a great deal of booing from a few people. The boos were because I put the non-original voltage regulator in there. In today's effects world where we get first posts looking for mods, it seems odd to think that there were actually people who would write down that changing to a clean power supply would make the univibe circuit not sound like the original.

It was an education for me!  

Clean power? But the tone is in the hum!

I never realised the Neovibe was quite so old! That must have been a bit of a labour of love back in those days..

[ch1naski]

RG was way ahead of the curve.

[tubegeek]

RG was way ahead of the curve.

Still is. But back then, the curve was all-original!