Help me understand the Micro Vibe

[PRR]

[jlo]

Thx for that.  Ok now I get it.  Apologies to Antonis and Duck Arse if I was being obtuse.   I didn't realize that that R18 and R20 are in series.  Thats the part I was confused about.

Thx for everyone's help and being patient with me

[jlo]

C6 on the schematic is to reduce noise/op amp stability?   Does it affect the sound?  In the Photonvibe/Lovepedal Vibronaut the value is 100pF and is in the feedback of U1-2,U2-1 and U2-2.  Is that due to running at 18V?

https://www.pedalpcb.com/docs/PhotonVibe.pdf

Also revisiting Wampler's mods he recommends replacing R2 (3K3) with a jumper.  Thats negative feedback to the oscillator and jumpering increases output from the oscillator?   

I see different values for R1 (Microvibe) in various schematics for the univibe and its clones.  4K7 vs 6K2,  that also changes the oscillator output?   

Finally running 15v vs 18v through the LFO.  Any big difference? 

[duck_arse]

C6 on the schematic is to reduce noise/op amp stability?   Does it affect the sound?  In the Photonvibe/Lovepedal Vibronaut the value is 100pF and is in the feedback of U1-2,U2-1 and U2-2.  Is that due to running at 18V?

https://www.pedalpcb.com/docs/PhotonVibe.pdf

Also revisiting Wampler's mods he recommends replacing R2 (3K3) with a jumper.  Thats negative feedback to the oscillator and jumpering increases output from the oscillator?   

I see different values for R1 (Microvibe) in various schematics for the univibe and its clones.  4K7 vs 6K2,  that also changes the oscillator output?   

Finally running 15v vs 18v through the LFO.  Any big difference?

the cap across the feedback resistor is usually selected to do a job without affecting the tone/sound. the resistor looks and acts like 33k until a [rising] frequency is reached, at which point the cap starts making the resistor look like a lower value. this means that stage gain reduces as freq increases.

how to find the cap value? same old formula, shuffle the unknowns about .... 1/(2*pi*R*C) gives the frequency, so 1/(2*pi*f*R) gives the cap value for the -3dB point. supply voltage has no part in the formula.

[jlo]

C6 on the schematic is to reduce noise/op amp stability?   Does it affect the sound?  In the Photonvibe/Lovepedal Vibronaut the value is 100pF and is in the feedback of U1-2,U2-1 and U2-2.  Is that due to running at 18V?

https://www.pedalpcb.com/docs/PhotonVibe.pdf

Also revisiting Wampler's mods he recommends replacing R2 (3K3) with a jumper.  Thats negative feedback to the oscillator and jumpering increases output from the oscillator?   

I see different values for R1 (Microvibe) in various schematics for the univibe and its clones.  4K7 vs 6K2,  that also changes the oscillator output?   

Finally running 15v vs 18v through the LFO.  Any big difference?

the cap across the feedback resistor is usually selected to do a job without affecting the tone/sound. the resistor looks and acts like 33k until a [rising] frequency is reached, at which point the cap starts making the resistor look like a lower value. this means that stage gain reduces as freq increases.

how to find the cap value? same old formula, shuffle the unknowns about .... 1/(2*pi*R*C) gives the frequency, so 1/(2*pi*f*R) gives the cap value for the -3dB point. supply voltage has no part in the formula.

So 47k and 33nF gives 102.6hz
47k and 100 gives 33.8hz
So when the frequency reaches the cutoff you get a -3db reduction?   And whats the reasoning behind selecting the value/frequency?   Does it matter which opamp is used?

[jlo]

Is it to prevent oscillation?   Related to slew rate of opamp?

[antonis]

Stephen accused me as an hard guy but he didn't take a look to himself..

He never told you the role of NFB loop shunt cap which is simply a part in LOW pass filter, formed with feedback resistor..
It actually cuts (dominates) gain of frequecies above corner frequency, which action has to do both with slew rate & oscilation prevention (in this order..)
Both of the above mentioned roles fall short of the main role which is cut highs simply for audio purpose..

P.S.
No problem at all, but I presume mixing "elementary" queries in a rather complex (compared to queries level) circuit results into mind trouble..

[ElectricDruid]

how to find the cap value? same old formula, shuffle the unknowns about .... 1/(2*pi*R*C) gives the frequency, so 1/(2*pi*f*R) gives the cap value for the -3dB point. supply voltage has no part in the formula.

So 47k and 33nF gives 102.6hz
47k and 100 gives 33.8hz
So when the frequency reaches the cutoff you get a -3db reduction?   And whats the reasoning behind selecting the value/frequency?   Does it matter which opamp is used?

That's right, except that the value of C6 is 33*p* not 33*n*. Which gives a way-higher 100KHz.

The general rule that I learned about designing op-amps was "don't amplify anything outside the bandwidth of interest". So if you're designing an audio amp, you don't want your amp to also amplify radio frequencies, or even ultrasonics. That's what that cap does - reduces the gain outside the bandwidth of interest. Personally, I think 33p is pretty small there, and I'd have used 100p (which brings the -3dB down to a still-not-audible 33KHz). But you could even stick 220p in and not really hurt anything.

In the same way that AC-coupling caps should roll-off unwanted mains hum at the bottom end (guitars don't play 50/60Hz notes - basses are a different story), you should have a scattering of caps in the circuit to limit ultrasonics and anything higher. You choose what you regard as the necessary top-end of audio. 15KHz, 18KHz, 20KHz, 22KHz, 25KHz are all figures I've seen given as guidelines at various times and in various situations.

Finally, no, it doesn't matter what op-amp is used. The op-amp's gain is so high the theory regards it as infinite. That's clearly an exaggeration, but the theory still works in practice. Any half-decent op-amp has *more than enough* gain for our sort of frequencies for the final circuit's behaviour to be determined entirely by the components we put around it, and not by the op-amp itself. You can design (for example) an inverting op-amp stage and try ten different op-amps in it, and it'll work exactly the same (same gain, same frequency response) with each one. The only time this starts to not be true is when you use either very old op-amps which were much more limited, or your design starts to push the op-amp towards its limits, or both.

[duck_arse]

Stephen accused me as an hard guy but he didn't take a look to himself..

So 47k and 33nF gives 102.6hz
47k and 100 gives 33.8hz
So when the frequency reaches the cutoff you get a -3db reduction?   And whats the reasoning behind selecting the value/frequency?   Does it matter which opamp is used?

102 Hz, yes, and that's all your guitar range gorn, nearly.
100? we don't allow nude 100's around here, you must tell us what you have one hundred of. MUST!
reasoning behind selecting your frequency - 102 Hz low cut filter, for example cuts a lot of guitar, which 'starts' at 86 Hz.
you tell the opamp what to do with your parts selection, don't let it tell you. no, the opamp type number does not appear in the equation.

me? hard?

[jlo]

Stephen accused me as an hard guy but he didn't take a look to himself..

So 47k and 33nF gives 102.6hz
47k and 100 gives 33.8hz
So when the frequency reaches the cutoff you get a -3db reduction?   And whats the reasoning behind selecting the value/frequency?   Does it matter which opamp is used?

102 Hz, yes, and that's all your guitar range gorn, nearly.
100? we don't allow nude 100's around here, you must tell us what you have one hundred of. MUST!
reasoning behind selecting your frequency - 102 Hz low cut filter, for example cuts a lot of guitar, which 'starts' at 86 Hz.
you tell the opamp what to do with your parts selection, don't let it tell you. no, the opamp type number does not appear in the equation.

me? hard?

I made an order error as been pointed out earlier.  The caps are in the pF range and so the frequency is 100Khz

[jlo]

I thought the higher the slew rate the more potential for ringing and oscillation.  The Micro Vibe use the TL062 and the Photonvibe/Vibronaut uses the TL072. 

[antonis]

I thought the higher the slew rate the more potential for ringing and oscillation. 

Intreresting point of view..
So, we have to decide/choose between frequency depended potentially distortion (low slew rate) and ringing/oscillation ..

You have to admit you don't tolerate us many options..

[ElectricDruid]

I thought the higher the slew rate the more potential for ringing and oscillation.

Sort of true, in a way. The reality is that the op-amps gain will start to drop off at some frequency or other (bandwidth isn't actually infinite either). The higher that is and the faster the op-amp can react, the more possibilities you've got for spurious RF pickup (that circuit you just built - it's an antenna!). This is why thinking about those extra caps to make sure we're only amplifying what we *want* to be amplifying are important.

[jlo]

Any comment on R41/42 on the Univibe schematic.  These resistors are absent on the Micro Vibe

[PRR]

R41/42 limit the minimum speed of the LFO. It will certainly work without them.

[jlo]

R41/42 limit the minimum speed of the LFO. It will certainly work without them.

So without it the LFO can go slower?  It affects the taper I'm assuming as well?

[PRR]

Invest the 24 cents and try it.

That's an utterly odd oscillator. I know it works, and which-way does what, but am not inclined to cudgel my brain figuring values when 4 quick tack-solders will tell all.

[jlo]

Invest the 24 cents and try it.

That's an utterly odd oscillator. I know it works, and which-way does what, but am not inclined to cudgel my brain figuring values when 4 quick tack-solders will tell all.

Fair enough.   I did try JC's mod and switched the 2M2 resistor going to the speed pot to 4M7 but I found that the LFO went too slow.  But thats without the 220K resistors.  I'm trying to figure out if its worth trying 2M7,3M3 or 3M9 vs 2M2 and the 220Ks.  I know I could just switch things and experiment but I'm also curious and want to learn the hows and whys.   Going back to R.G.s univibe tech article  I realized that he mentions the 220K resistors but I'm not sure if anyone has played around with tweaking them

[antonis]

Just figure out how R41 & R42 shunt VR1+R43/44 respectively (which resistors are there just to limit series minimum value for not shorting LFO caps to Q12 Emitter when pot set all the way down)

So, a 220k in parallel with a 2k2 can't do many things..

[jlo]

Just figure out how R41 & R42 shunt VR1+R43/44 respectively (which resistors are there just to limit series minimum value for not shorting LFO caps to Q12 Emitter when pot set all the way down)

So, a 220k in parallel with a 2k2 can't do many things..

Do I calculate parallel resistance of 220k and 102.2K?  Which is 69.8?  And then parallel 69.8K and 69.8k to get 34.9K?