Univibe Slower speed

[Mcentee2]

I can see a fair few threads mentioning how to speed up a classic Univibe, just lowering those 4.7k in the LFO, but I can't find anything that easily says how to slow it down, is that just as easy?

I can appreciate there are limits to which the LFO will just stop at either end,but I want to investigate what the slow end is on my current vibe.

Simple resistor changes should be fine, if it's swapping out the 3 1uF caps I might realise they am happy with its current lowest speed after all!

[duck_arse]

you'd have to show us a circuit, but ...... it's the caps. there is plenty of threads discussing operation of phase shift oscillators hereabouts.

[Mcentee2]

you'd have to show us a circuit, but ...... it's the caps. there is plenty of threads discussing operation of phase shift oscillators hereabouts.

Thanks, it's a univibe like the original circuit:

[PRR]

Change the caps, all the same direction and ratio.

Changing resistors "works", up to a point, and then it stalls. If someone has tried it, copy them. If you are on your own, change capacitors. They only affect frequency, not bias or swing.

[Mcentee2]

Change the caps, all the same direction and ratio.

Changing resistors "works", up to a point, and then it stalls. If someone has tried it, copy them. If you are on your own, change capacitors. They only affect frequency, not bias or swing.

Thanks, I'd like to try the resistors, but am not sure which ones to change and to higher or lower?

[antonis]

I'd like to try the resistors, but am not sure which ones to change and to higher or lower?

LFO

[Rob Strand]

Change the caps, all the same direction and ratio.

Changing resistors "works", up to a point, and then it stalls. If someone has tried it, copy them. If you are on your own, change capacitors. They only affect frequency, not bias or swing.

Thanks, I'd like to try the resistors, but am not sure which ones to change and to higher or lower?

The simplest change is to lift the two 220k resistors that are across the speed pot/4k7 region.  If the oscillator stops working at the lowest speed try increasing the 2M2 to 3M3.

If you want to make large changes to the frequency, like half the speed, then you would be best changing the caps.  If for some reason you want to leave the existing caps in you could tack another set of 1uF caps on the back of the PCB.

[Mcentee2]

Change the caps, all the same direction and ratio.

Changing resistors "works", up to a point, and then it stalls. If someone has tried it, copy them. If you are on your own, change capacitors. They only affect frequency, not bias or swing.

Thanks, I'd like to try the resistors, but am not sure which ones to change and to higher or lower?

The simplest change is to lift the two 220k resistors that are across the speed pot/4k7 region.  If the oscillator stops working at the lowest speed try increasing the 2M2 to 3M3.

If you want to make large changes to the frequency, like half the speed, then you would be best changing the caps.  If for some reason you want to leave the existing caps in you could tack another set of 1uF caps on the back of the PCB.

Ah, I see, I was wondering what those 220k were actually doing!

Given each one wraps across the fastest 4.7k and the 100k pot, these change the range from 4.6k - 70.94k.

I can see removing them pushes that 70.94k back up to 104.7k, so that must be the slow end if travel!

If that stops the LFO I can look at choosing another pair at different sizes, mind you even 1M only pushes it up to 94.78k, hmmmm

Those 220k probably change the taper as well so that could also be a designed function, or just luck, changing them too much might kill that too.

I'll take a look at that 2M2 to 3M3 as well, thankyou.

[Rob Strand]

Ah, I see, I was wondering what those 220k were actually doing!

Yep, you got the idea now!

Those 220k probably change the taper as well so that could also be a designed function, or just luck, changing them too much might kill that too.

Don't worry about the taper it's not going to be noticeable or do anything bad.   Also remember you are changing the speed compared to the original anyway so the speed for each pot position *has* to be different to the original.  The fast speed is will be virtually unchanged after pulling those 220k's.

[Mcentee2]

Ah, I see, I was wondering what those 220k were actually doing!

Yep, you got the idea now!

Those 220k probably change the taper as well so that could also be a designed function, or just luck, changing them too much might kill that too.

Don't worry about the taper it's not going to be noticeable or do anything bad.   Also remember you are changing the speed compared to the original anyway so the speed for each pot position *has* to be different to the original.  The fast speed is will be virtually unchanged after pulling those 220k's.

Thankyou, I'll unsolder one end of each and see how it goes

[amptramp]

If you can only find log taper and not reverse taper, you can relabel the frequency control as a "period" control.  If you prefer to keep the original frequency control with log taper, there is always this solution:




One gear for the control shaft and the other for the pot shaft.