Revv G3 & Revv G4 - Power supply

[LaloFP]

Hi guys!

  Im looking at the Revv Gs schematics on pcbguitarmania.com and I am intrigued about the Power Supply.

  FIRST, the Revv G4:



  You see there that the VRef that supplies almost all the circuit is being buffered buy an Opamp.

  So, QUESTION A: Is that really needed? My assumption is that its not, BUT the circuit needed 5 opamps... so why not use the "extra" opamp at hand to give a steady Vref. Am I right?

  But THEN, the Revv G3:



  In G3 they add 1 extra gain stage Opamp AND anoother Vref just for the final EQ part.

  QUESTION B: they could just repurpose the "extra" opamp that we had before (in G4) and make it the new gain stage opamp added now and have VREF not buffered by an opamp (like the common other pedals). Instead, they wanted to preserve the first VREF ("VB" on schem) so they now had anoother extra opamp with no use, so they made it a VREF Buffer ("VC" in schem) and used it just once. Am I right?

  QUESTION C: Does the stability added by the VREFs buffers really have an impact in this scenario? I see regular Opamps and filters (am I wrong?).

  QUESTION D: Can it be that because the original amp in which are based this pedals were made that way because of "amp like specific needs" and wanted to preserve the original topology (didn't check that, and I dont have experience on amps)?

  What do you think?

Thanks :D:D

[antonis]

They presume a buffered Vref is needed - period..!!!

[Rob Strand]

  What do you think?

Thanks :D:D

I think it's overly complicated without any obvious reason.   I suspect you could use a 10k + 10k + 100uF cap without any buffered Vrefs and it would work fine.  If the designer made an account of the reasons behind it you would know why it is like that and make your own judgements.

The reasons for not buffering are:
- There is no DC current being fed into any VREF
- There is no significant AC fed down VREF.
  The lowest resistances are R6 (33k) and R29 (10k). 
  R33 is fed through small caps which will increase the AC impedance.
  R29 only applies to small signal AC as large swings go though 470k which injects little AC current into Vref.

Designers can have a habit of buffering Vref without justifying it.  Perhaps even inflicting what they see as better onto the design. In fact my list does that to some degree.   Designers can also have design problems during development and change areas of the circuit, when the problem "goes away" they leave the last changes in.   Sometimes those fixes aren't the solution and it is actually changes to the layout and wiring that really fixed it.   With weird issues it can be a tough job proving a fix does or doesn't actually work.   It's even possible there's an obscure reason for the crazy Vref configuration on the Revv's  - I just can't see it.   Bottom line - changing stuff comes at a risk but you can't assume everything you read is true so you also have to trust your experience and better judgement.

  So, QUESTION A: Is that really needed? My assumption is that its not, BUT the circuit needed 5 opamps... so why not use the "extra" opamp at hand to give a steady Vref. Am I right?

  But THEN, the Revv G3:


  In G3 they add 1 extra gain stage Opamp AND anoother Vref just for the final EQ part.

I think you are right to suspect something.

Note also the extra stage on the Revv G3 makes the pedal non-inverting overall whereas the Revv G4 inverts.  (That can be a conscious design choice.)

  QUESTION B: they could just repurpose the "extra" opamp that we had before (in G4) and make it the new gain stage opamp added now and have VREF not buffered by an opamp (like the common other pedals). Instead, they wanted to preserve the first VREF ("VB" on schem) so they now had anoother extra opamp with no use, so they made it a VREF Buffer ("VC" in schem) and used it just once. Am I right?

I think they worked out the extra VC opamp isn't required and they could rejig things to remove a whole dual opamp.   A good idea.

  QUESTION C: Does the stability added by the VREFs buffers really have an impact in this scenario? I see regular Opamps and filters (am I wrong?).

Got serious doubts.   I'd call it lower impedance Vref.  stability can mean oscillation, low supply impedance, freedom from ripple, or the line regulation (which isn't helped by a Vref buffer).

  QUESTION D: Can it be that because the original amp in which are based this pedals were made that way because of "amp like specific needs" and wanted to preserve the original topology (didn't check that, and I dont have experience on amps)?

Got doubts here too.   I'd have to look at the original circuits.

[LaloFP]

AWE SOM reply Rob!!

I'd call it lower impedance Vref.  stability can mean oscillation, low supply impedance, freedom from ripple, or the line regulation (which isn't helped by a Vref buffer).

I now realise I was using the "stability" term vaguely because my understanding of the buffered vref was vague. THANK YOU

<3 <3 <3 !!

They presume a buffered Vref is needed - period..!!!

Hi antonis! Its not much about the circuit. Its more about recognizing a (maybe-small) cognitive dissonance between the circuit and my actual knowledge and poking there to see if there is something more to learn Its maybe the most potent way for me to learn (if its not tied to a specific part of a build im pursuing, I forget what I understand)

[LaloFP]

EDIT: Ignore it! Silly question