Understanding Univibe Cap Changes

[Big Monk]

Reading through some older Vibe posts:

https://www.diystompboxes.com/smfforum/index.php?topic=82717.msg686644#msg686644

R.G., can you clarify the tonal impact you found from increasing the series LDR caps and bootstrap caps?

[Big Monk]

Just want to bump this to pose an additional question:

Could you sub MPSA13 for the discrete darlington pairs of the UniVibe circuit?

[antonis]

Only for Q11/Q12 pair..
(Q4/Q5, Q6/Q7 & Q8/Q9 can't be considered Darlingtons 'cause their Collectors aren't directly connected..)

[Big Monk]

Only for Q11/Q12 pair..
(Q4/Q5, Q6/Q7 & Q8/Q9 can't be considered Darlingtons 'cause their Collectors aren't directly connected..)

I'm admittedly not a huge circuit analysis guy but I ran a copy of the first phase stage of the UniVibe into Multisim and put a switch to take the first transistor collector to power or connected just downstream of the 4.7k and they seem to have equivalent performance:

[PRR]

...(Q4/Q5, Q6/Q7 & Q8/Q9 can't be considered Darlingtons 'cause their Collectors aren't directly connected..)

True; but here it may actually work "1% better": we want equal-but-opposite signal from C and E, the straight transistor with hFE near 100 has 1% "error", the Darlington is 0.01% error.

I don't think it makes a dang bit of difference in a musical phaser, so I'd go for what is cheap or easy (and at this cost scale,I like easy).

[antonis]

I ran a copy of the first phase stage of the UniVibe into Multisim

Not a presice copy..

Bias configuration is by far wrong..
(check Univibe Q4 Base connection in relation with R13/R14 & R12..)

That also results into scrappy input impedance bootstrapping..
(Univibe stage bootstraps both bias divider resistors (via C7 & R12) where your sim bootstraps 47k only)

[antonis]

we want equal-but-opposite signal from C and E,

Ouch..

( loosely considered LDRs grounded..)

[Big Monk]

Thank you both for the replies.

I guess a goal statement is in order: I'm trying to look for places to eliminate or consolidate components to get a Univibe project in a 125B enclosure. Having all Darlingtons saved for the input stages saves me 5 or 6 transistors.

[Big Monk]

I ran a copy of the first phase stage of the UniVibe into Multisim

Not a presice copy..

Bias configuration is by far wrong..
(check Univibe Q4 Base connection in relation with R13/R14 & R12..)

That also results into scrappy input impedance bootstrapping..
(Univibe stage bootstraps both bias divider resistors (via C7 & R12) where your sim bootstraps 47k only)

I'll make sure to correct and run it again.

[AtomicRob]

I guess a goal statement is in order: I'm trying to look for places to eliminate or consolidate components to get a Univibe project in a 125B enclosure.

If size is the main concern you can probably fit two SOT23 transistors in the space of one TO-92. Those aren't hard to hand-solder. Same goes for resistors, SMT and/or really tiny through-hole parts can save a lot of space. Just saying it might be easier to go small versus changing the circuit.

[Big Monk]

I guess a goal statement is in order: I'm trying to look for places to eliminate or consolidate components to get a Univibe project in a 125B enclosure.

If size is the main concern you can probably fit two SOT23 transistors in the space of one TO-92. Those aren't hard to hand-solder. Same goes for resistors, SMT and/or really tiny through-hole parts can save a lot of space. Just saying it might be easier to go small versus changing the circuit.

It's not all size based. Some of the changes are just modern improvements on the vintage circuit.

I'm still in the design stage though so once things are laid out I can sub in SMT in place of through hole if need be.