Univibe Output Impedance

[Big Monk]

Running a prototype of my modernized vibe into my Fuzz Face and the impedance of the Univibe is causing issues.

What is the output impedance of the Univibe and what is it determined by? Is it easier to tweak it directly or to add a switchable buffer?

[antonis]

What is the output impedance of the Univibe and what is it determined by?

It's complicated a bit 'cause signal through C18 goes to an Emitter follower practically getting R36 & R37 out of the game..
If you consider A = ( R35 // VR3 // next effect input impedance ), signal attenuation is A / (A + R34)..
(actually, a bit more complicated 'cause next effect input impedance should be considered in parallel with VR3 wiper-lug1 and the parallel combination of VR3 wiper-lug1 and next effect input impedance is series with VR3 wiper-lug3 should be considered in parallel with R35 - but let it be..)

[Big Monk]

What is the output impedance of the Univibe and what is it determined by?

It's complicated a bit 'cause signal through C18 goes to an Emitter follower practically getting R36 & R37 out of the game..
If you consider A = ( R35 // VR3 // next effect input impedance ), signal attenuation is A / (A + R34)..
(actually, a bit more complicated 'cause next effect input impedance should be considered in parallel with VR3 wiper-lug1 and the parallel combination of VR3 wiper-lug1 and next effect input impedance is series with VR3 wiper-lug3 should be considered in parallel with R35 - but let it be..)

R36/37 are not in the circuit.

Im not using the vibrato mixer or a C/V switch.

The next pedal on this case is a Fuzz Face. Let’s assume 5K.

How would I them calculate the output impedance of the Vibe?

[PRR]

my Fuzz Face and the impedance of the Univibe is causing issues.

The FuzzFace will not like ANY amplifier in front. It depends heavily on sucking against a guitar pickup and pot, something not easily approximated in another pedal.

There is a whole bunch of Pickup Simulators for this purpose. Often a $3 transformer and a 25c cap.

[antonis]

Without a specific schematic, I'd suppose around 100K..
(considering vibrato circuit completely out of the game..)

What might be of concern should be the output HPF raised corner frequency..

IMHO, you could try to lower R34 value (down to 10k, say) in the risk of loading Q3 Emitter..

P.S.
As PRR said above, using a buffer will not solve your issue..
(after all, by minimizing R34 value, you approach output impedance to that of an Emitter follower..)

[Lino22]

Oops, wrong post.

[Big Monk]

I guess I was envisioning this as the same scenario as running a wah friendly buffer before the Fuzz.

[PRR]

Yes.

[Big Monk]

Yes.

In my final revision that I’m sending off to JLCPCB this week, I’ll likely include a switchable, Wah friendly type buffer on the output.

Thanks!

[Eb7+9]

Buffering implies added noise …
Each emitter feed has at most 4k7 dpi
So, no real need to go super highZ in the mixer