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## Univibe Output Impedance

Started by Big Monk, November 23, 2022, 07:54:38 AM

#### 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?
"Beneath the bebop moon, I'm howling like a loon

#### antonis

Quote from: Big Monk on November 23, 2022, 07:54:38 AM
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..)
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

#### Big Monk

Quote from: antonis on November 23, 2022, 09:25:22 AM
Quote from: Big Monk on November 23, 2022, 07:54:38 AM
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?
"Beneath the bebop moon, I'm howling like a loon

#### PRR

Quote from: Big Monk on November 23, 2022, 07:54:38 AMmy 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.
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#### 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..)

"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

#### Lino22

#5
Oops, wrong post.
When the core started to glow and people started yelling, he promptly ran out the door and up a nearby hill.

#### Big Monk

I guess I was envisioning this as the same scenario as running a wah friendly buffer before the Fuzz.
"Beneath the bebop moon, I'm howling like a loon

Yes.
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#### Big Monk

Quote from: PRR on November 23, 2022, 05:08:29 PM
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!
"Beneath the bebop moon, I'm howling like a loon