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DIY Stompboxes => Building your own stompbox => Topic started by: jlo on June 30, 2020, 08:53:36 AM

Title: Help me understand the Micro Vibe
Post by: jlo on June 30, 2020, 08:53:36 AM

(https://i.postimg.cc/N5BSZtvx/0-B478244-6779-45-B8-BB2-B-A0014-BC1-A9-DB.jpg) (https://postimg.cc/N5BSZtvx)


(https://i.postimg.cc/Q9LhTz9n/2344-C241-CDA6-4-BCE-A880-3269877-E649-E.gif) (https://postimg.cc/Q9LhTz9n)

(https://i.postimg.cc/r0ScC95y/C93-B2-EF1-8381-4008-9955-15-F665-BA90-DC.jpg) (https://postimg.cc/r0ScC95y)

in the Univibe R11,R17,R23 R29 is 4K7.  Ive read that the resistors in series to the LDRs will affect the light resistance.   In the Microvibe are R10,12,14,16 (47K)  in series with the LDRs and can that value be lowered to get a lower light resistance? 

I understand that Im comparing op amps to a discrete circuit.   In the Easy Vibe it looks like 10K is the value chosen.  Whats the reasoning behind all of this?


Title: Re: Help me understand the Micro Vibe
Post by: antonis on June 30, 2020, 09:23:04 AM
Maybe just because we don't deal with identical optocouplers..?? :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on June 30, 2020, 10:35:07 AM
in both the microvibe and the easyvibe, those resistors are not in series w/ the ldrs. the values are chosen as a match for the opamp feedback resistors to limit gain to unity each stage, and to work against the cap at the other input. in the univibe, the 4k7's .... dunno, balance out the ldr's and limit their swing?
Title: Re: Help me understand the Micro Vibe
Post by: jlo on June 30, 2020, 11:17:21 AM
in both the microvibe and the easyvibe, those resistors are not in series w/ the ldrs. the values are chosen as a match for the opamp feedback resistors to limit gain to unity each stage, and to work against the cap at the other input. in the univibe, the 4k7's .... dunno, balance out the ldr's and limit their swing?
Thx for the explanation.  So if I change opamps do I need to change the resistors?  The stock Microvibe has the TL062.   What if I sub in a TL072 or OPA2604 or OPA2134?
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on June 30, 2020, 11:38:17 AM
opamps is opamps. you tell them what to do with the resistors, and they do it, or go in the bin.

no, no need for opamp part number dependant resistors.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on June 30, 2020, 11:41:38 AM
Yes I misunderstood.  The resistor at the output matches the feedback resistor.   And what effect would lowering or raising the value of the resistor have?  Comparing the Micro Vibe and the Easy Vibe why was 47k chosen vs 10k
Title: Re: Help me understand the Micro Vibe
Post by: antonis on June 30, 2020, 01:32:28 PM
And what effect would lowering or raising the value of the resistor have?

None, by maintaining equal resistor ratio..
Actual resistors values only counts  for noise & off-set issues..

@ Stephen: 4k7 resistors in the univibe are actually set inside NFB loop but are placed on Darligton Emitters rather than Bases for not upsetting bias bootstrapping..
(rough guess..) :icon_redface:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on June 30, 2020, 03:30:49 PM
Ok. Things are starting to make sense.  Now how about the gain staging?  According to RG Keen the preamp of the Univibe has a gain of 4.  And the phasing stages are unity? 

For the Micro Vibe what is the gain staging?   U1-1 is the input buffer?  Is the gain 2 if the input and feedback resistors are 47K? Thats for U1-1/2,U2-1/2 and U3-1 each have a gain of 2?   And then what happens with U3-2? 


Title: Re: Help me understand the Micro Vibe
Post by: antonis on June 30, 2020, 05:29:03 PM
It's a bit complicated topology..

e.g. U3-2 is wired as non-inverting configuration with its own stage gain of (1 + R19/R21) BUT actuall signal comming into pin5 is what comes out of U3-1 divided with R22/[(R20//(R18+R9+R8)] (or multiplied with the reverse fraction - to be more true-blue voltage divider formula..)

U1-2, U2-1, U2-2 & U3-1 are wired as inverting configuration with unity gain BUT their non-inverting inputs are biased via LDRs and toghether with respective capacitors (C5, C7, C8 & C9) high-pass filter configuration form all-pass filters..
At high frequencies caps are shorts so we have unity gain voltage buffers (no phase lead)..
At low frequencies caps are open circuits and we have unity gain inverting amplifiers (180o phase lead)
At corner frequency of HPFs (0.159/R*C) we have 90o phase lead..
(I let you guess the reason for it.. :icon_wink: hint: at corner frequency LDRs resistance and capacitors impedance are equal..)

LDRs resistance varies with light intensity which variation is militated by lower right circuitry (LFO) resulting into LFO frequency depended "moving" phase lead..

Title: Re: Help me understand the Micro Vibe
Post by: ElectricDruid on June 30, 2020, 06:24:14 PM
The resistor at the output matches the feedback resistor.   And what effect would lowering or raising the value of the resistor have?  Comparing the Micro Vibe and the Easy Vibe why was 47k chosen vs 10k

No, the resistor at the *input* matches the feedback resistor. I know it looks like that because it's connected to the output of the previous op-amp, but you're dividing the parts up wrong. That pair of resistors are an inverting op-amp arrangement.
If you look at the equation for the gain of an inverting op-amp, it's

gain = -Rf/Rin

and since 10K/10K = 1 and 47K/47K = 1, both give unity gain and unless you only change *one* of the resistors, it doesn't much matter what the value is within reason, at least not for the gain.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on June 30, 2020, 07:03:32 PM
It's a bit complicated topology..

e.g. U3-2 is wired as non-inverting configuration with its own stage gain of (1 + R19/R21) BUT actuall signal comming into pin5 is what comes out of U3-1 divided with R22/[(R20//(R18+R9+R8)] (or multiplied with the reverse fraction - to be more true-blue voltage divider formula..)

U1-2, U2-1, U2-2 & U3-1 are wired as inverting configuration with unity gain BUT their non-inverting inputs are biased via LDRs and toghether with respective capacitors (C5, C7, C8 & C9) high-pass filter configuration form all-pass filters..
At high frequencies caps are shorts so we have unity gain voltage buffers (no phase lead)..
At low frequencies caps are open circuits and we have unity gain inverting amplifiers (180o phase lead)
At corner frequency of HPFs (0.159/R*C) we have 90o phase lead..
(I let you guess the reason for it.. :icon_wink: hint: at corner frequency LDRs resistance and capacitors impedance are equal..)

LDRs resistance varies with light intensity which variation is militated by lower right circuitry (LFO) resulting into LFO frequency depended "moving" phase lead..
Thank you for the detailed explanation.  Yes its complicated to someone like me that has little to no electronic knowledge. 
What about U1-1?  How does it compare in gain to the Univibe preamp?   I see that Wampler recommends changing R8 from 47k to 39K. 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 01, 2020, 06:01:16 AM
What about U1-1?  How does it compare in gain to the Univibe preamp?   I see that Wampler recommends changing R8 from 47k to 39K.

U1-1 is wired as non-inverting configuration (that's when signal goes into + marked Input)
Gain of such a configuration is what Tom said above, without minus sign (-) plus unity.. (1+Rf/Rin with no phase reversal - although Rin should be named Rgain 'cause there isn't any input signal through it..) :icon_wink:
So, U1-1 stage gain (ignoring R18 & R20) is 2 (1 + 47k/47k).. By making R8 39k you just raise gain 10%.. (from 2 to 2.2)

P.S.
Any "complicated" circuit consists of a number of more "plain" blocks so you firstly have to locate (isolate) them, understand their function and then cascade their individual facilities..
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 01, 2020, 07:29:11 AM
What about U1-1?  How does it compare in gain to the Univibe preamp?   I see that Wampler recommends changing R8 from 47k to 39K.

U1-1 is wired as non-inverting configuration (that's when signal goes into + marked Input)
Gain of such a configuration is what Tom said above, without minus sign (-) plus unity.. (1+Rf/Rin with no phase reversal - although Rin should be named Rgain 'cause there isn't any input signal through it..) :icon_wink:
So, U1-1 stage gain (ignoring R18 & R20) is 2 (1 + 47k/47k).. By making R8 39k you just raise gain 10%.. (from 2 to 2.2)

P.S.
Any "complicated" circuit consists of a number of more "plain" blocks so you firstly have to locate (isolate) them, understand their function and then cascade their individual facilities..
Ok thats what I thought re U1-1.  But the original Univibe preamp has a gain of 4.   So the gain in the MV is made up in U3-2?  Which has a gain of 2.27 “ BUT actuall signal comming into pin5 is what comes out of U3-1 divided with R22/[(R20//(R18+R9+R8)] (or multiplied with the reverse fraction - to be more true-blue voltage divider formula..)”.  What does that mean? 

Also does it matter where the gain is in the circuit?  I get that the Univibe is not true bypass vs the Micro Vibe which is.
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 01, 2020, 07:57:30 AM
So the gain in the MV is made up in U3-2?  Which has a gain of 2.27 “ BUT actuall signal comming into pin5 is what comes out of U3-1 divided with R22/[(R20//(R18+R9+R8)] (or multiplied with the reverse fraction - to be more true-blue voltage divider formula..)”.  What does that mean?
Maybe I talk (write) too much resulting into listener (reader) confusing.. :icon_redface:

U3-2 STAGE (isolating it from whatever is set in front of it) gain indeed is 2.27..
For shake of simplicity, ignore any resistor on the left side of R20 upper leg..
Signal coming out of U3-1 (pin1) reaches U3-2 pin5 by the half of its previous amplitude due to voltage dividing effect..
(Vpin5 = Vpin1 X R20/R22)
Now, if you substitute everything goes to AC GND for the single R20 you have the equivalent resistane of voltage divider lower leg.. :icon_wink:

Also does it matter where the gain is in the circuit?

Hmmm.. That's a, more or less, philosophical query.. :icon_lol:
(in the mean of: "Better attenuate then amplify" or vice-versa..??)
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on July 01, 2020, 10:46:46 AM

@ Stephen: 4k7 resistors in the univibe are actually set inside NFB loop but are placed on Darligton Emitters rather than Bases for not upsetting bias bootstrapping..
(rough guess..) :icon_redface:

are they? circuits like the magnavibe don't have the series resistor [or, usually, the darlington], I thought it was just a mixing function.
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 01, 2020, 10:56:25 AM
It depends on particular point of view..

You may also face it as a variable impedance Emitter decoupling cap for individual Darlington stage variable Gain..
(wired on Collector for more effective "grounding" - apparent LDR + 4k7 value lowered by stage gain..)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 01, 2020, 11:14:46 AM
Ok.  So the gain of U3-2 is 2.27 but the signal coming from U3-1 has been halved.  Is this because we are combining the dry and wet signals?  So in the end the whole thing is unity?   The gain of U1-1 is 2 but we are splitting the signal into the wet and dry paths.  Phase stages are unity.  Am I on the right track?

Thx again for all the detailed explanations.  They may be over my head at first but it is helping me learn alot!
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 01, 2020, 02:56:43 PM
Ok.  So the gain of U3-2 is 2.27 but the signal coming from U3-1 has been halved.  Is this because we are combining the dry and wet signals?  So in the end the whole thing is unity?   The gain of U1-1 is 2 but we are splitting the signal into the wet and dry paths.  Phase stages are unity.  Am I on the right track?

No..!!
(with no further explanation..)  :icon_lol:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 01, 2020, 08:38:52 PM
 :'(
Ok.  So the gain of U3-2 is 2.27 but the signal coming from U3-1 has been halved.  Is this because we are combining the dry and wet signals?  So in the end the whole thing is unity?   The gain of U1-1 is 2 but we are splitting the signal into the wet and dry paths.  Phase stages are unity.  Am I on the right track?

No..!!
(with no further explanation..)  :icon_lol:
:'( :'( :'(
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 02, 2020, 05:30:02 AM
Oops I mean a gain of 2.27.  When combining the dry and wet signals we are back to unity
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on July 02, 2020, 10:43:43 AM
that antonis, he's too hard.

jlo, observe the non-invert in of U2-2 [2 small for my eyes] - there is a 100k resistor to bias. this sets the stage input impedance. now there is two other 100k resistors feeding signal into that (+)in - so, what happens [to those original signals] when you have a 100k series feeding a 100k-to-ground [what the input impedance looks like]?
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 02, 2020, 11:02:22 AM
U2-3 if you use magnifier tool.. :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 02, 2020, 11:48:35 AM
that antonis, he's too hard.

jlo, observe the non-invert in of U2-2 [2 small for my eyes] - there is a 100k resistor to bias. this sets the stage input impedance. now there is two other 100k resistors feeding signal into that (+)in - so, what happens [to those original signals] when you have a 100k series feeding a 100k-to-ground [what the input impedance looks like]?
You mean U3-2?
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on July 02, 2020, 12:01:49 PM
I have no idea.

you can quote me.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 02, 2020, 01:04:19 PM
(R18 + R9 + R8) is in parallel with R20?   So the net resistance is 66.6k.   So the voltage coming out of U3-1 is 0.398 of its input? 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 02, 2020, 02:59:47 PM
(R18 + R9 + R8) is in parallel with R20? 

Who knows..  :icon_mrgreen:
(Hard enough, Stephen..??) :icon_wink:

(https://i.imgur.com/rwqpvjH.png)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 02, 2020, 05:38:21 PM
Im clearly confused.  Is the input signal divided into wet and dry paths and then mixed at the end like the Univibe?  If so I can’t see where the two separate paths are.  I thought the signal split at U1-1 output.  Wet going through the various phasing stages and the dry was going through R18 going L to R on the schematic and then mixed into the + input of U3-2 along with the output of U3-1. 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 02, 2020, 05:50:56 PM
Im clearly confused.

You're not the only one..  :o

Last attempt: Are we talking about dry/wet mix or about R22 right leg signal attenuation..??
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 02, 2020, 06:33:01 PM
In the univibe circuit, the input signal is split.  The wet path undergoes phase shifting and is mixed in with the dry signal in chorus mode.  In vibrato mode, the dry signal is not mixed in.  Micro Vibe only has chorus.  So does it have the same architecture of splitting the signal and mixing the two at the output? 
Title: Re: Help me understand the Micro Vibe
Post by: ElectricDruid on July 02, 2020, 06:48:40 PM
In the univibe circuit, the input signal is split.  The wet path undergoes phase shifting and is mixed in with the dry signal in chorus mode.  In vibrato mode, the dry signal is not mixed in.  Micro Vibe only has chorus.  So does it have the same architecture of splitting the signal and mixing the two at the output? 

If it has chorus, then yes definitely it splits and mixes. The difference between "chorus" and "vibrato" modes is whether the dry signal is mixed in. If it is, then you have chorus (dry signal + modulated "wet" signal") or if not vibrato (just the modulated "wet" signal).
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 02, 2020, 06:56:41 PM
In the univibe circuit, the input signal is split.  The wet path undergoes phase shifting and is mixed in with the dry signal in chorus mode.  In vibrato mode, the dry signal is not mixed in.  Micro Vibe only has chorus.  So does it have the same architecture of splitting the signal and mixing the two at the output? 

If it has chorus, then yes definitely it splits and mixes. The difference between "chorus" and "vibrato" modes is whether the dry signal is mixed in. If it is, then you have chorus (dry signal + modulated "wet" signal") or if not vibrato (just the modulated "wet" signal).


Right so then where is the dry signal path?  I thought it was from U1-1 output across R18 and then mixing just after R22 into +input of U3-2?
Title: Re: Help me understand the Micro Vibe
Post by: ElectricDruid on July 02, 2020, 07:37:44 PM
If you're talking about this schematic:

https://postlmg.cc/N5BSZtvx (https://postlmg.cc/N5BSZtvx)

Yes, you're right. R18 takes the output from the input buffer U1-1 to the output mixer U3-2. Interestingly, it's a passive mixer into a non-inverting op-amp, instead of the (perhaps more common) inverting active mixer. Not that it makes a huge amount of difference when this is a phase shifter circuit - phase is going to get scrambled either way.

Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 02, 2020, 08:31:20 PM
If you're talking about this schematic:

https://postlmg.cc/N5BSZtvx (https://postlmg.cc/N5BSZtvx)

Yes, you're right. R18 takes the output from the input buffer U1-1 to the output mixer U3-2. Interestingly, it's a passive mixer into a non-inverting op-amp, instead of the (perhaps more common) inverting active mixer. Not that it makes a huge amount of difference when this is a phase shifter circuit - phase is going to get scrambled either way.
So thats a different path that Antonis is describing. 
(R18 + R9 + R8) is in parallel with R20? 

Who knows..  :icon_mrgreen:
(Hard enough, Stephen..??) :icon_wink:

(https://i.imgur.com/rwqpvjH.png)
Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 02, 2020, 10:27:08 PM
Right so then where is the dry signal path?

That's what I see too.

(https://i.postimg.cc/947xnsK1/Micro-Vibe-wet-dry-42.gif) (https://postimg.cc/947xnsK1)
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 03, 2020, 07:36:52 AM
Making things clear now you can see that both signals (Wet & Dry) are divided on R18/R20/R22 junction.. :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 03, 2020, 09:54:34 AM
Making things clear now you can see that both signals (Wet & Dry) are divided on R18/R20/R22 junction.. :icon_wink:
Ok so now revisiting the gain structure.  Are both wet and dry signals halved and then combined before tge U3-2 input? 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 03, 2020, 09:58:00 AM
It depends on paricular point of view..

What's your documented viewpoint..??
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 03, 2020, 10:09:59 AM
It depends on paricular point of view..

What's your documented viewpoint..??
Can you please clarify your question?  Im having trouble understanding
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on July 03, 2020, 10:30:41 AM
Making things clear now you can see that both signals (Wet & Dry) are divided on R18/R20/R22 junction.. :icon_wink:

thanks antonis - this was the point I was trying to drive at.
Title: Re: Help me understand the Micro Vibe
Post by: ElectricDruid on July 03, 2020, 11:18:02 AM
Making things clear now you can see that both signals (Wet & Dry) are divided on R18/R20/R22 junction.. :icon_wink:
Ok so now revisiting the gain structure.  Are both wet and dry signals halved and then combined before tge U3-2 input? 

Yes. For both wet and dry, you've got a 100K/100K divider, so the level is halved. And then the amp provides some make-up gain. 150K and 118K (118K?!? Seriously? I don't believe it) gives 150/118+1 = x2.27, so there's a very slight boost overall.

Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 03, 2020, 09:08:15 PM
(https://i.postimg.cc/CztR1qZz/Micro-Vibe-wet-dry-sum-42.gif) (https://postimg.cc/CztR1qZz)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 04, 2020, 08:11:47 AM
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
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 04, 2020, 09:40:18 AM
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? 
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on July 04, 2020, 10:33:20 AM
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.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 04, 2020, 12:08:41 PM
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?
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 04, 2020, 03:03:21 PM
Is it to prevent oscillation?   Related to slew rate of opamp?
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 04, 2020, 03:24:44 PM
Stephen accused me as an hard guy but he didn't take a look to himself.. :icon_mrgreen:

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.. :icon_wink:

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..
Title: Re: Help me understand the Micro Vibe
Post by: ElectricDruid on July 04, 2020, 03:33:41 PM
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.
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on July 05, 2020, 11:24:06 AM
Quote
Stephen accused me as an hard guy but he didn't take a look to himself..

Quote
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?
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 05, 2020, 11:34:35 AM
Quote
Stephen accused me as an hard guy but he didn't take a look to himself..

Quote
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
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 05, 2020, 11:37:50 AM
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. 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 05, 2020, 01:27:04 PM
I thought the higher the slew rate the more potential for ringing and oscillation. 

Intreresting point of view.. :icon_wink:
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.. :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: ElectricDruid on July 05, 2020, 02:20:28 PM
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.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 05, 2020, 10:47:15 PM
Any comment on R41/42 on the Univibe schematic.  These resistors are absent on the Micro Vibe
(https://i.postimg.cc/t7yyDQ04/681-DA144-3-EB8-4-AE2-93-E3-82-BB267-F59-C7.gif) (https://postimg.cc/t7yyDQ04)
Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 06, 2020, 01:27:55 AM
R41/42 limit the minimum speed of the LFO. It will certainly work without them.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 06, 2020, 08:03:57 AM
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?
Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 07, 2020, 12:59:31 AM
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.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 07, 2020, 08:35:32 AM
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
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 07, 2020, 08:45:18 AM
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.. :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 07, 2020, 09:21:29 AM
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.. :icon_wink:
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?

Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 07, 2020, 09:39:21 AM
Yes, the former..
No, the later..
(guess why.. no hint) :icon_mrgreen:

Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 07, 2020, 09:51:31 AM
Yes, the former..
No, the later..
(guess why.. no hint) :icon_mrgreen:
Does it have to do with the diodes?
I don’t understand why its a dual pot in the first place...
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 07, 2020, 10:03:58 AM
For exactly the reason you can't consider both gangs in parallel..

We deal with an LOW frequency oscillator.. This means Caps DO exhibit resistance (or else, RC time constants should be zero and also Diode pair should be shorted..)

Diodes, as they are, don't allow C20 cap plates voltage to be raised over 600mV, either on charging or discharging mode..
So, LFO can't swing Q12 Emitter higher or lower than this..
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 07, 2020, 10:36:32 AM
For exactly the reason you can't consider both gangs in parallel..

We deal with an LOW frequency oscillator.. This means Caps DO exhibit resistance (or else, RC time constants should be zero and also Diode pair should be shorted..)

Diodes, as they are, don't allow C20 cap plates voltage to be raised over 600mV, either on charging or discharging mode..
So, LFO can't swing Q12 Emitter higher or lower than this..
I see.  So the diodes are oriented opposite each other to affect charging AND discharging
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 07, 2020, 11:14:26 AM
Revisting R.G.’s tech article. 
“The two diodes across the center capacitor limit the size of the LFO output waveform. An oddity of this particular way of building a Univibe is that the LFO amplitude goes up with increasing speed, even with the diodes there.“

So is it a dual pot because each pot controls each side of the waveform?  Each with its own upper and lower time constraints?
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 07, 2020, 12:02:49 PM
I've noticed when there isn't an answer on time (clearly specified by yourself) you retrace well established texts..
(making me to set a delay of 48 hours, at least, for my answers..)  :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 07, 2020, 12:36:29 PM
I've noticed when there isn't an answer on time (clearly specified by yourself) you retrace well established texts..
(making me to set a delay of 48 hours, at least, for my answers..)  :icon_wink:
Apologies.  I never expect an answer and I do truly appreciate anytime someone takes the time out of their busy life to help.   Its probably my learning style that doesn’t translate well in the format of posts.   A lot of times I’m thinking “out loud”.   
Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 08, 2020, 12:02:12 AM
I have never seen quite that oscillator anywhere else. (Maybe one of the gurus here has.)

An oscillator is generally an amplifier and a time-network. A Sine oscillator is a little tougher.

This one is notable that the amplifier is unity voltage gain. (High current gain though.) Compare with a Wien which needs voltage gain of 3, or a phase-shift which generally needs voltage gain of 27(?).

A sine oscillator generally wants at least two reactances. This can be an L and a C. Coils are brutal in audio so we try with 2 or 3 Cs and some Rs. Wien and phase-shift are popular. The string of 2 or 3 identical caps and two ganged variable resistors is obviously setting the frequency here, but it is an odd duck. There are much odder ducks. Life is too short.

These oscillators normally start on random noise. (Performance LFOs may add a kick-start.) If the amplifier gain exceeds the reactance-network loss the output WILL build up "to infinity". Before infinity the amplifier clips, which makes the *average* gain somewhat less, and limits the final amplitude. Either by clipping or by de-biasing the amplifier to a lower-gain condition.

Rather than clip to the supply rails we can clip with diodes, which may be less traumatic to the amplifier.

And there is the Becham/Hewlett lamp (a slow limiter) and much fancier limiters. These can give point-oh THD but LFOs usually do not have to be so sweet.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 08, 2020, 11:24:19 AM
Ok I think Im understanding the LFO a bit better.  So does the Darlington create a 180 phase shift and then each RC creates 90 + 90 so that the output is in phase with the input with a full 360 shift.  When its fed back it creates/maintains the oscillation.  If we want to change the frequency we use the dual pot so that the 90 degree shifts are equal?
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 08, 2020, 11:32:22 AM
Emitter followers (Darlington or not) are not famous enough for phase shifting.. :icon_wink:
(maybe that's the reason for also called "Voltage followers"..)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 08, 2020, 01:40:06 PM
Emitter followers (Darlington or not) are not famous enough for phase shifting.. :icon_wink:
(maybe that's the reason for also called "Voltage followers"..)
I see.  That 180 shift is with a common emitter .   So then the output of the Darlington is in phase with the input but gets shifted 180+180? 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 08, 2020, 02:20:01 PM
So then the output of the Darlington is in phase with the input but gets shifted 180+180?

Philosophical brainstorm: Can you point any reason for shifting 360o the output..??
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 08, 2020, 02:34:19 PM
So then the output of the Darlington is in phase with the input but gets shifted 180+180?

Philosophical brainstorm: Can you point any reason for shifting 360o the output..??
I was just inferring based on RC phase shift oscillator.  But that uses a common emitter transistor...
Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 08, 2020, 07:15:09 PM
We do not need phase shift. A "follower" with gain of +1.1, connect out to in, it will scream.

The oddity is that this amplifier has gain like 0.98. They found voltage gain in a passive R-C network. I've modeled it and yes, there's a sliver of voltage gain (and larger impedance dip) at one frequency.
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 08, 2020, 10:19:59 PM
We do not need phase shift. A "follower" with gain of +1.1, connect out to in, it will scream.

The oddity is that this amplifier has gain like 0.98. They found voltage gain in a passive R-C network. I've modeled it and yes, there's a sliver of voltage gain (and larger impedance dip) at one frequency.
Can you please explain the RC network and the dual pot?   And how do you get the additional gain?
Title: Re: Help me understand the Micro Vibe
Post by: jatalahd on July 09, 2020, 02:10:57 AM
Can you please explain the RC network and the dual pot?   And how do you get the additional gain?
Yes.

As a pre-requisite you need to understand the RC phase-shift oscillator in its more common form (feedback from collector to RC stage). This infomation is available in the internet (just search by "RC phase-shift oscillator"). The following information presented here is top secret and will be buried in this thread and never found again :)

There are four different feedback topologies in amplifier design: series-shunt, shunt-series, shunt-shunt, series-series (use internet search if you want to know more). The "normal" RC phase-shift oscillator uses the shunt-shunt feedback model. The emitter-follower circuit (with only bias and emitter resistor) is already a series-series feedback amplifier on its own, where the FEEDBACK voltage is 180 degrees out of phase from the OUTPUT. See the diagram below:

(https://i.postimg.cc/5HnT0hKZ/cc.png) (https://postimg.cc/5HnT0hKZ)

In this image, we have the signal source added and the feedback voltage Vf is summed as "inverted" to the input. Please ignore the RC/RL, since this model can be applied to the common-emitter amp as well but not now. Hopefully this is clear so far. In the oscillator, the signal source can be removed, since the input signal is taken 100% as a feedback signal from the output.

Next we draw the Uni-Vibe RC network on top of the same feedback diagram (NOTE: redrawing circuits in standard form is a huge help to understand them)

(https://i.postimg.cc/rD6KnbjX/resize-ff.jpg) (https://postimg.cc/rD6KnbjX)

Now it is starting to look as the "normal" RC phase-shift oscillator. So we have 180 degree shift already at the input side of RE and another 180 shift from the RC network. This sums up either as 180 - 180 = 0 OR 180 + 180 = 360 = 0 at the frequency of oscillation. When there is positive feedback (0 degree shift) the thing will oscillate. But wait...

There is also a theory called the "Barkhausen stability criterion". It says that for oscillation to happen, the LOOP GAIN of the circuit must be equal or larger than -1. There is so much false information floating around saying that the gain of the circuit must be so-and-so much for oscillations to happen (to make up the loss of the RC feedback network). This is not so. The basic emitter follower circuit can easily have a LOOP GAIN of over 100 (although the voltage gain is ALWAYS less than 1), and there is no magic related to this. The loop gain is calculated differently than "normal" gain. So in this case the Barkhousen stability criterion is fullfilled and also overly ensured by the darlington's high current gain (these days a single high-Beta tranny like BC549C would be enough here).

By adjusting the resistances, you adjust the cut-off frequencies of the RC-pairs, thereby affecting the oscillation frequency. Not any different than in the normal RC phase-shift oscillator.

And if your next question is how to calculate the loop gain for this specific circuit, then I am sorry to say but you are not ready for it. I did it using numerical math tools and utilizing matrix algebra. There is no simple equation to give.
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 09, 2020, 06:17:23 AM
There is no simple equation to give.

Additionally to the above, there isn't a compact formula for BOTH necessary & sufficient oscillation criterion..
(Barkhausen's criterion is necessary  but not sufficient condition where Nyquist one is the opposite - in the mean of instability..)
Logical enough, taking into account that both mathematical conditions developed aiming for ensuring stability criterions..

To make long story short, you can never be sure for a working oscillator without breadboarding it.. :icon_wink:

@jatalahd: I presume hybrid-π model analysis of CC amp confuses rather than enlighten OP..
(with no intention for knowledge underrating, jlo..) :icon_redface:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 09, 2020, 09:27:18 AM
There is no simple equation to give.

Additionally to the above, there isn't a compact formula for BOTH necessary & sufficient oscillation criterion..
(Barkhausen's criterion is necessary  but not sufficient condition where Nyquist one is the opposite - in the mean of instability..)
Logical enough, taking into account that both mathematical conditions developed aiming for ensuring stability criterions..

To make long story short, you can never be sure for a working oscillator without breadboarding it.. :icon_wink:

@jatalahd: I presume hybrid-π model analysis of CC amp confuses rather than enlighten OP..
(with no intention for knowledge underrating, jlo..) :icon_redface:
Im confused most of the time! :)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 09, 2020, 09:39:40 AM
Emitter followers (Darlington or not) are not famous enough for phase shifting.. :icon_wink:
(maybe that's the reason for also called "Voltage followers"..)
But the feedback is 180 shifted?
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 09, 2020, 10:55:43 AM
Emitter followers (Darlington or not) are not famous enough for phase shifting.. :icon_wink:
(maybe that's the reason for also called "Voltage followers"..)
But the feedback is 180 shifted?

Should we remane topic "Endlessness is not enough"..?? :icon_wink:

How can come 180o phase shift between 2 IN PHASE signals without anything to lead/lag them..??
Title: Re: Help me understand the Micro Vibe
Post by: jatalahd on July 09, 2020, 11:45:02 AM
Quote
How can come 180o phase shift between 2 IN PHASE signals without anything to lead/lag them..??
Please forgive me Antonis :)

This is the reason I presented the hybrid-pi model, because it is a "standard" model and it shows that the 180 degree shift is obtained by taking the signal to the feedback loop from ground potential and the emitter of the transistor is the ground for the "input" part of the feedback loop. This is the same as "inverting" the signal, multiplying by -1, or applying 180 degree shift. But now I am not sure if we are even talking about the same thing.

I just hoped that my drawing would have been enough :)
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 09, 2020, 11:54:24 AM
But now I am not sure if we are even talking about the same thing.

Definately not... :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 10, 2020, 10:48:55 AM
Emitter followers (Darlington or not) are not famous enough for phase shifting.. :icon_wink:
(maybe that's the reason for also called "Voltage followers"..)
But the feedback is 180 shifted?

Should we remane topic "Endlessness is not enough"..?? :icon_wink:
How can come 180o phase shift between 2 IN PHASE signals without anything to lead/lag them..??

Where’s the dead horse emoji?  I’m sure you’ve had enough of me.  Just going by what was posted earlier:  “The emitter-follower circuit (with only bias and emitter resistor) is already a series-series feedback amplifier on its own, where the FEEDBACK voltage is 180 degrees out of phase from the OUTPUT. See the diagram below:”
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 10, 2020, 11:10:23 AM
You're free to go anywhere might be convenient to your understanding.. :icon_wink:

As I see it, Emitter follower circuit represents Voltage-series feedback topology with 100% negative feedback..
(entire output voltage is fed back in series with input..)

As Paul already told you, the issue here is (less than unity) gain rather than phase shift.. :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 13, 2020, 11:34:19 AM
Quote from JC I found from another thread
“ The Univibe oscillator belongs to the phase-shift oscillator family - where a combination of greater than unity loop-gain and at least 180degree phase shift in the loop is required for the circuit to start and maintain itself ... each cap produces 90deg of shift so three caps gives a total maximum potential of 270deg ... this kind of principle is used in Fender amp Tremolo oscillators for example - except in the Vibe case the voltage gain of a Darligton follower is less than unity, so the oscillator loop must be operating in the current variable - with a (current) gain of Beta^2 - and capacitors providing phase shift in the same way they would in a voltage-mode oscillator ... in theory phase-shift oscillators exhibit exponentially increasing Amplitudes with time and so require amplitude limition to keep the output signal steady, hence the two diodes clamping the middle shifting cap ... using LED's or Germanium diodes will alter the output size indirectly ...”

I thought you needed a 360 shift for oscillation to occur?
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 13, 2020, 11:58:08 AM
But you just need enough shift so that the loop gain is sufficient to cause oscillation?
Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 13, 2020, 05:16:03 PM
The usual amplifier in the Phase-Shift is an inverter. We need another inversion in the network.
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 14, 2020, 05:02:45 AM
But you just need enough shift so that the loop gain is sufficient to cause oscillation?

https://en.wikipedia.org/wiki/Electronic_oscillator (https://en.wikipedia.org/wiki/Electronic_oscillator)
https://www.youtube.com/watch?v=XVS8Puf4tiw (https://www.youtube.com/watch?v=XVS8Puf4tiw)
https://www.youtube.com/watch?v=aJAZHPqEUKU (https://www.youtube.com/watch?v=aJAZHPqEUKU)
https://www.electrical4u.com/what-is-an-oscillator/ (https://www.electrical4u.com/what-is-an-oscillator/)
https://learnabout-electronics.org/Oscillators/osc10.php (https://learnabout-electronics.org/Oscillators/osc10.php)
https://electronics.howstuffworks.com/oscillator.htm (https://electronics.howstuffworks.com/oscillator.htm)
https://www.elprocus.com/different-types-of-oscillator-circuits-its-applications/ (https://www.elprocus.com/different-types-of-oscillator-circuits-its-applications/)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 14, 2020, 07:31:26 AM
“As any single high pass filter can produce a phase change of up to 90°, it would seem that, in theory, only two such networks, would be needed. However, using two filters with each producing a 90°phase shift would mean that, as the phase graph in Fig. 3.1.1 shows, the phase response curve is quite flat at and above 90°, so any drift in frequency would have little effect on the 180° phase shift produced. This would mean that if the frequency of the oscillator changes, due to a change in temperature for example, there would be hardly any change in the amount of phase shift, so frequency stability would be poor. It can be seen from Fig. 3.1.1 that at 60° or 45° the phase response curve is much steeper, and so with three filters producing 60° each, or four filters providing 45° each, to make up the required 180°, frequency stability will be much better.“

From https://www.learnabout-electronics.org/Downloads/Oscillators-module-03.pdf
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 14, 2020, 08:34:25 AM
Can you please explain the RC network and the dual pot?   And how do you get the additional gain?
Yes.

As a pre-requisite you need to understand the RC phase-shift oscillator in its more common form (feedback from collector to RC stage). This infomation is available in the internet (just search by "RC phase-shift oscillator"). The following information presented here is top secret and will be buried in this thread and never found again :)

There are four different feedback topologies in amplifier design: series-shunt, shunt-series, shunt-shunt, series-series (use internet search if you want to know more). The "normal" RC phase-shift oscillator uses the shunt-shunt feedback model. The emitter-follower circuit (with only bias and emitter resistor) is already a series-series feedback amplifier on its own, where the FEEDBACK voltage is 180 degrees out of phase from the OUTPUT. See the diagram below:

(https://i.postimg.cc/5HnT0hKZ/cc.png) (https://postimg.cc/5HnT0hKZ)

In this image, we have the signal source added and the feedback voltage Vf is summed as "inverted" to the input. Please ignore the RC/RL, since this model can be applied to the common-emitter amp as well but not now. Hopefully this is clear so far. In the oscillator, the signal source can be removed, since the input signal is taken 100% as a feedback signal from the output.

Next we draw the Uni-Vibe RC network on top of the same feedback diagram (NOTE: redrawing circuits in standard form is a huge help to understand them)

(https://i.postimg.cc/rD6KnbjX/resize-ff.jpg) (https://postimg.cc/rD6KnbjX)

Now it is starting to look as the "normal" RC phase-shift oscillator. So we have 180 degree shift already at the input side of RE and another 180 shift from the RC network. This sums up either as 180 - 180 = 0 OR 180 + 180 = 360 = 0 at the frequency of oscillation. When there is positive feedback (0 degree shift) the thing will oscillate. But wait...

There is also a theory called the "Barkhausen stability criterion". It says that for oscillation to happen, the LOOP GAIN of the circuit must be equal or larger than -1. There is so much false information floating around saying that the gain of the circuit must be so-and-so much for oscillations to happen (to make up the loss of the RC feedback network). This is not so. The basic emitter follower circuit can easily have a LOOP GAIN of over 100 (although the voltage gain is ALWAYS less than 1), and there is no magic related to this. The loop gain is calculated differently than "normal" gain. So in this case the Barkhousen stability criterion is fullfilled and also overly ensured by the darlington's high current gain (these days a single high-Beta tranny like BC549C would be enough here).

By adjusting the resistances, you adjust the cut-off frequencies of the RC-pairs, thereby affecting the oscillation frequency. Not any different than in the normal RC phase-shift oscillator.

And if your next question is how to calculate the loop gain for this specific circuit, then I am sorry to say but you are not ready for it. I did it using numerical math tools and utilizing matrix algebra. There is no simple equation to give.
Ok I think I understand now.  The diagrams were very helpful.  The voltage from the collector in an emitter follower is 180 out of phase with the input.  The RC network flips it 180 back into phase and results in oscillation when fed back with the input.  Is that correct?   

Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 14, 2020, 09:37:09 AM
The voltage from the collector in an emitter follower is 180 out of phase with the input.  The RC network flips it 180 back into phase and results in oscillation when fed back with the input.  Is that correct?

NO..!!! :icon_mrgreen:

There isn't any RC circuit connecting Collector to anywhere so there isn't neither feedback from Collector nor any phase shift..
Title: Re: Help me understand the Micro Vibe
Post by: duck_arse on July 14, 2020, 10:40:56 AM
an emitter follower ....

(https://wps.prenhall.com/wps/media/objects/416/426098/10fig1.gif)
Title: Re: Help me understand the Micro Vibe
Post by: jatalahd on July 14, 2020, 10:51:45 AM
Quote
Ok I think I understand now.  The diagrams were very helpful.  The voltage from the collector in an emitter follower is 180 out of phase with the input.  The RC network flips it 180 back into phase and results in oscillation when fed back with the input.  Is that correct?   
The 180 degree phase-shift from the emitter-follower amplifier stage is here due to using the emitter as ground potential and ground as "signal". That makes the amplifier an inverting amp, you simply switch + and - potentials to invert the signal to the feedback loop. This you can do for any circuit that carries a sine wave. By measuring the signal between + and - using - as ground you see the "in-phase" signal. By measuring the signal between + and - using + as ground you see the same signal, but inverted (out of phase by 180).

After this "trick", the rest is generic RC phase-shift oscillator theory, which you can find using the links that Antonis kindly provided. Without the diodes connected, there is absolutely nothing special in this circuit. It is a plain old RC phase shift oscillator and all the general theories related to it hold also in this circuit.
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 14, 2020, 01:10:50 PM
The 180 degree phase-shift from the emitter-follower amplifier stage is here due to using the emitter as ground potential and ground as "signal". That makes the amplifier an inverting amp, you simply switch + and - potentials to invert the signal to the feedback loop. This you can do for any circuit that carries a sine wave. By measuring the signal between + and - using - as ground you see the "in-phase" signal. By measuring the signal between + and - using + as ground you see the same signal, but inverted (out of phase by 180).

It'a pitty that nobody thought this very dealing for an CE amp to get both amplified signal and IN PHASE...  :icon_smile:

To be more serious, Jarmo: :icon_wink:
True and Correct all the above but let's stay on a more conventional acceptance about potential difference points definition..
(or else, we might be seriously accused for OP committed suicide complicity..) :icon_redface:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 14, 2020, 01:19:40 PM
Quote
Ok I think I understand now.  The diagrams were very helpful.  The voltage from the collector in an emitter follower is 180 out of phase with the input.  The RC network flips it 180 back into phase and results in oscillation when fed back with the input.  Is that correct?   
The 180 degree phase-shift from the emitter-follower amplifier stage is here due to using the emitter as ground potential and ground as "signal". That makes the amplifier an inverting amp, you simply switch + and - potentials to invert the signal to the feedback loop. This you can do for any circuit that carries a sine wave. By measuring the signal between + and - using - as ground you see the "in-phase" signal. By measuring the signal between + and - using + as ground you see the same signal, but inverted (out of phase by 180).

After this "trick", the rest is generic RC phase-shift oscillator theory, which you can find using the links that Antonis kindly provided. Without the diodes connected, there is absolutely nothing special in this circuit. It is a plain old RC phase shift oscillator and all the general theories related to it hold also in this circuit.
So is there some semantics involved in terms of whether a phase shift happens or not? 
Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 14, 2020, 02:02:03 PM
“As any single high pass filter can produce a phase change of up to 90°....

Wrong. "up to almost 90°". Or "90° at infinity, which is out of reach."
Title: Re: Help me understand the Micro Vibe
Post by: jatalahd on July 14, 2020, 03:50:56 PM
So is there some semantics involved in terms of whether a phase shift happens or not? 
Regarding the phase shift in the amplifier there is no special semantics involved, if an emitter resistor exists without a parallel bypass capacitor, there is always negative feedback from output to input for AC-signals. Negative feedback through emitter resistor is present both in an emitter follower and a CE amplifier, if the emitter resistor is not bypassed for AC-signals.

May I ask why is it so important for you to understand this oscillator thing?
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 14, 2020, 05:09:31 PM
So is there some semantics involved in terms of whether a phase shift happens or not? 
Regarding the phase shift in the amplifier there is no special semantics involved, if an emitter resistor exists without a parallel bypass capacitor, there is always negative feedback from output to input for AC-signals. Negative feedback through emitter resistor is present both in an emitter follower and a CE amplifier, if the emitter resistor is not bypassed for AC-signals.

May I ask why is it so important for you to understand this oscillator thing?
Its not that important.  And I appreciate everyone’s help and patience.  I fell down a rabbit hole trying to understand it but most likely because of my lack of knowledge I was getting confused with a lot of the posts.   This had me going in circles.  I read up and understood the RC phase shift oscillator concept.  But didn’t understand the implementation in the UV LFO because of the use of emitter follower vs common emitter.   In my mind I was getting contradictory information.   Time for all of us to move on!   I have more questions but lets put this phase shift stuff to bed.
Title: Re: Help me understand the Micro Vibe
Post by: PRR on July 14, 2020, 09:58:49 PM
> emitter follower vs common emitter

It's common emitter. Just connected funny.
(https://i.postimg.cc/rz4SKmBc/Micro-Vibe-LFO-redraw-42.jpg) (https://postimg.cc/rz4SKmBc)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 22, 2020, 05:14:55 PM
Thx for everone’s help so far.  How about C13 on the schematic.  Its a 33uF cap
(https://i.postimg.cc/sMhLhMgY/8-ED3-CA83-0-E5-E-4983-94-A0-A2-EABCA5-BD3-F.jpg) (https://postimg.cc/sMhLhMgY)
This is for decoupling?  How is that value chosen?  If I want to run the pedal at 18V and use a 40mA bulb and opamps with a higher current draw, will I need to increase this? 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 22, 2020, 05:31:26 PM
It's just a power supply ripple voltage reservoir cap..
It could do better job (just like C11 does for +4.5V supply) if a relatively small value resistor was placed between D3 cathode & cap positive plate..)

For 18V, its voltage rating must be raised up to 25V..
Its capacity has nothing to do with supply voltage but a lot to do with supply current..
(you can see C11 been more than ten times smaller 'cause it has to smooth much less current..)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 22, 2020, 09:52:42 PM
So assuming the caps are rated 25V, what would you change in the power supply section to run at 18V.  So if the TL062 has a current draw of 200uA  vs TL072 1.5mA vs OPA2604 10.5mA  that would affect the value you would use? 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 23, 2020, 06:25:32 AM
Without any info about new power supply, I shouldn't change anything at all.. :icon_wink:
I should raise R31 value up to 2k2..
I should change that  :icon_evil: :o >:( C15 tantalum cap with an electro at any cost..
(or even better, I should get rid of the whole by-pass configuration..)
I should add 100nF ceramic caps as close as physically possible between pins 8 & 4 for each of U1, U2 & U3..


But shouldn't be more convenient to start a new thread about power supply basics..?? :icon_cool:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 23, 2020, 06:53:54 AM
Wasn’t sure if I should start a new thread for each question... I can change this if the mods feel like i should

Power supply is Dunlop 18v 500mA

I did add caps to each opamp

Why change C15? 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 23, 2020, 07:01:29 AM
Why change C15?

Because it's my personal habbit to hang around with a desoldering pump desperately looking for a tantalum cap..!!
(and a cut plier for browbeating the tough ones)
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 23, 2020, 08:55:18 AM
So with an 18V 500mA power supply and 40mA bulb and OPA2604s would c13 or c11 need to be changed? 
Title: Re: Help me understand the Micro Vibe
Post by: antonis on July 24, 2020, 07:41:17 AM
Maybe yes, maybe no..
(it stricktly depends on power supply regulation specifications..)

I let you calculate total circuit current requirement, take in mind worst case scenario (e.g. mains lower amplitude), determine your PS total circuit configuration (e.g. simple full-wave rectifier with a reservoir capacitor of known(?) value or the former followed by linear regulator with appropriate caps) and decide if respective caps need to be changed..

P.S.
I'd suggest you to study power supply basics, 'cause if you don't, you'll keep coming for any slightly different case of the one already answered/pointed (but not understood..) :icon_wink:
Title: Re: Help me understand the Micro Vibe
Post by: jlo on July 24, 2020, 09:01:20 AM
Maybe yes, maybe no..
(it stricktly depends on power supply regulation specifications..)

I let you calculate total circuit current requirement, take in mind worst case scenario (e.g. mains lower amplitude), determine your PS total circuit configuration (e.g. simple full-wave rectifier with a reservoir capacitor of known(?) value or the former followed by linear regulator with appropriate caps) and decide if respective caps need to be changed..

P.S.
I'd suggest you to study power supply basics, 'cause if you don't, you'll keep coming for any slightly different case of the one already answered/pointed (but not understood..) :icon_wink:
Will definitely do some reading.  Thx.  The specs on the power supply say 5% regulated.  Not sure what that means.  Is that 18v +/- 5%?