Univibe's phase stage bias

[Danich_ivanov]

I've been messing around with phasing lately and at some point decided to give univibe's stage a try, and it left me with some questions. What confuses me is that it is not a Darlington per se, yet it is biased like one, and it got me thinking that alternatively you can bias first transistor to 1/2 supply, just like a regular buffer, which will give 1/3 on the emitter and 2/3 on the collector of the second transistor, which as far as I'm concerned is fine for a phase splitter. Another way would be to bias second transistor as a regular phase splitter, about 1/3 supply on first transistor's emitter/second transistor's base, which should also be fine. What are you'r thoughts on stock vs alternative biasing configurations? Is stock biasing perfect and they got it right, or is it debatable?


Edit: Oh, and i forgot to mention that stock biasing is about 1/3 supply on the first transistor base.

[antonis]

I have to admit you've puzzled me a bit.. 

As I see it, it's a bootstrapped Emitter follower driving a phase spliter..

Second BJT Emitter is biased at about 1.5V ( 9*47/147 - 100k*Base current - 2V_BE ) so Collector is biased at about 7.5 Volts..

That leaves an "unexploited" headroom of about 3Volts but, maybe, 3V_pp for each phase is more than enough..

[Danich_ivanov]

I have to admit you've puzzled me a bit.. 

As I see it, it's a bootstrapped Emitter follower driving a phase spliter..

Second BJT Emitter is biased at about 1.5V ( 9*47/147 - 100k*Base current - 2V_BE ) so Collector is biased at about 7.5 Volts..

That leaves an "unexploited" headroom of about 3Volts but, maybe, 3V_pp for each phase is more than enough..

Right. What you're saying is correct. However it is not what I'm asking. My question is: "what do you think about alternative biasing configurations?" First one being - bias emitter follower's base to 1/2 supply, just like a regular buffer. Second one being - bias emitter follower's emitter to 1/3 supply, so that second transistor is biased like a regular phase splitter. The reason why i'm curious about this is because it is not Darlington, which inevitably makes me think of other options in terms of how it can be biased. And, you know, tell me if i'm wrong about any of this, maybe stock biasing is perfect and there is no reason to question it. Hopefully i'm not being puzzling this time 

[highwater]

On *9v power*, 1/3-supply plus two Vbe-drops is going to be conveniently close to 1/2-supply. Equal-value bias resistors would do nicely... both input and output would have the most headroom at nearly the same bias point. As a bonus, it wouldn't even require stocking additional parts (helpful if you were building them by the 100s-1,000s).

The original Univibe runs on 16v DC, though. That extra 7v makes headroom *much* less of a headache... chances are, they just didn't care (or even notice) that it was only close-enough.

[Danich_ivanov]

On *9v power*, 1/3-supply plus two Vbe-drops is going to be conveniently close to 1/2-supply. Equal-value bias resistors would do nicely... both input and output would have the most headroom at nearly the same bias point. As a bonus, it wouldn't even require stocking additional parts (helpful if you were building them by the 100s-1,000s).

The original Univibe runs on 16v DC, though. That extra 7v makes headroom *much* less of a headache... chances are, they just didn't care (or even notice) that it was only close-enough.

Yes sir, this is exactly what i'm thinking! Equal resistors, which is always a good thing to have, everything is a bit more consistent, less boost at the collector, meaning wet signal will be closer in levels to the dry signal, and can be further balanced by playing around with caps and collector/emitter resistors, which is what you want in a phaser situation in order to get deeper notches, better consistency. The reason why I'm thinking in ratios is simply to minimise confusion, since univibe's audio part of the circuit runs at ~ 15v, at the same time everyone is used to 9v, and so do i, and i don't want to water things down with voltages, trying to keep it simple. Basically just want to be sure that i'm not over my head with this ideas, and/or missing something important about stock arrangement.

[antonis]

Basically just want to be sure that i'm not over my head with this ideas,

Absolutely not..

Just add an Emitter resistor on 1^st BJT and move bootstrap cap there..