UNIVIBE question

[Coony]

Hello!

I’ve build the Univibe and it seems to work, except that I notice a volume-drop when in the effect-mode. Mind, I don’t have true bypass, just the original ‘cancel’ switch, which is why I think something is wrong (with true bypass, it would be normal!). The above happens when in the ‘chorus’ mode, in the ‘vibrato’ mode there isn’t a volume-drop.
Anyone who cloned the ‘vibe had the same problem?

BTW, it does sound good on its own I think, it's just the volume-drop!

Best Regards,
Koen

[Coony]

Nobody knows?

[R.G.]

I’ve build the Univibe and it seems to work, except that I notice a volume-drop when in the effect-mode. Mind, I don’t have true bypass, just the original ‘cancel’ switch, which is why I think something is wrong (with true bypass, it would be normal!). The above happens when in the ‘chorus’ mode, in the ‘vibrato’ mode there isn’t a volume-drop.
Anyone who cloned the ‘vibe had the same problem?

OK, let's reason about this. Something happens in the "chorus" setting (actually phasing) that does not happen in "vibrato" setting, and it makes the volume come out lower.

If you've read "The Technology of the Univibe" at GEO, you know that both of these paths involve the phase-shifted signal, but only the chorus path mixes phase shifted and dry signals. Since the vibrato one is about the right volume, that implies that the phase shifted signal by itself is OK. We are left with something about the dry signal that's mixed to the phase shifted signal, or the mixing itself.

I personally would go over all the resistors, component values, etc. on the dry path from the preamp to the output resistors mixer and out to the chorus output on the selector switch. I believe you have a problem with either the value of the chorus mixing resistors or a soldering/wiring problem with the "chorus" path.

[Eb7+9]

Nobody knows?

... : ... if the vibrato mode works ok then the filter stages are probably all ok also, so it's likely a problem in the mixing ... make sure the direct feed is capacitively coupled to the right place in the front - try lifting the direct-path 100k to see if you hear any change, if not that's where the problem's coming from ... let us know ...

jc

[Coony]

Thanks to all for your help.
Yes RG, I read your info. Thank you for making such an informative website!

I checked all the connections and measured the mixing R's and they all seem fine.

I should add that the volume changes even in bypass when switching the Chorus-Vibrato-switch; the vibrato-setting is louder than the chorus. Anyway, I talked it through with a friend and he thought that the differences in the voltage-divider in the mixer were responsable.
If I'm right the chorus-setting has a 100k/78k and the vibrato-setting a 47k/68k.
Could this be causing it?
What voltages could I expect on the emittors of both the transistors before the mixer? Perhaps this will lead to a solution!

Thanks for helping me out here!

Best Regards,
Koen

[R.G.]

If I'm right the chorus-setting has a 100k/78k and the vibrato-setting a 47k/68k. Could this be causing it?

Huh? The chorus setting has two 100K's, one each from the dry and phased signals, going into the 100K volume pot. Where are you getting 78K?

The Vibrato setting has a 47K/220K divider going into that same 100K volume pot.  I guess you mean that the 47K/68K is the divider of the 220K parallel with the 100K pot. That's probably OK, but I can't see the 78K on the chorus side. Both dry and phased signals to the 100K/100K/100K mixer are fed from low impedance outputs, so the loading on the 47K side is not going to be an issue.

You should find about 5.4V on the emitter of Q10 and between 3 and 6 volts on the emitter of Q3.

[Coony]

If I'm right the chorus-setting has a 100k/78k and the vibrato-setting a 47k/68k. Could this be causing it?

Huh? The chorus setting has two 100K's, one each from the dry and phased signals, going into the 100K volume pot. Where are you getting 78K?

(100k+47k+220k)//100k=78k
I checked these calculations by measuring R between the top of the output pot and ground; I get about 80k and 69k...

You should find about 5.4V on the emitter of Q10 and between 3 and 6 volts on the emitter of Q3.

I've got 5.2V at Q10 and 3.4 at Q3, so that seems fine.

Really, even with no effect (= the cancel switch) I hear a small volume drop when I switch from vib to chorus... Perhaps with a compressing amp it isn't an issue? Has nobody noticed this???

Best Regards,
Koen

[Rob Strand]

What you are experiencing could be normal.  From what I can see in vibrato mode the level should be about right with the cancel switch in either position.    In the chorus+cancel position there should be a slight increase in gain, ad in chous+active there is a slight loss.    This means when you click on the cancel pedal in and out you will hear a noticeable change in level in chorus mode.  However if you switch between chorus mode and vibrato mode you should hear a small increase with cancel on and a small decrease with cancel off (ie. active).  The vibrato level is the base-line level and the gains in the circuit are such that they allow for the non-bypassed connection.  It seems the *contrast between* the two slightly off gains in chorus mode is what you see as a problem.