op amp bias voltage divider - resistor values?

[JTEX]

Yes, but I believe @thunderaxe is planning to use that extra opamp for something else eventually. And in this particular, simple circuit, there's no real benefit from an opamp-buffered VREF, since nothing draws or sinks significant current to/from it.

 

[thunderaxe]

yes, that fourth op amp will definitely be used, for either an active baxandall tone section, or a clean blend buffer. speaking of the latter, would a separate buffer for the clean blend still be necessary if i instead took the clean signal from after the first gain stage?

 

[thunderaxe]

Could also save a component and do this...
View attachment 87366

if i were to use VREF in all the places you suggested, but wanted to create an asymmetrical clipping control that changed the bias voltage on the clipping stage op amp, i would need to create a separate voltage divider with its own conditioning cap and a pot to vary one half of the voltage divider, right?

also, would the clipping LEDs prevent the signal from hitting the rails no matter how far in one direction or the other i go?

 

[Paul.Ruby]

if i were to use VREF in all the places you suggested, but wanted to create an asymmetrical clipping control that changed the bias voltage on the clipping stage op amp, i would need to create a separate voltage divider with its own conditioning cap and a pot to vary one half of the voltage divider, right?

also, would the clipping LEDs prevent the signal from hitting the rails no matter how far in one direction or the other i go?

No. The clipping diodes are clipping at voltages that surround vref by 0.6v (depending on choice of diode). Vref is not relevant to the clipping symmetry. Asymetrical clipping would be by using two different diodes (or multiple diodes in series) to create the asymettry.

 

[darwin999]

if the 10uF capacitor on the sour grape's bias voltage is smallish, isn't the 220nF one on the dream fuzz tiny by comparison?

No - the Dream Fuzz's 1M in parallel with 220nF gives a ~0.7Hz filter corner.
The Sour Grape's RC filter corner is ~1.7Hz.
So quite comparable.

 

[darwin999]

The sour grape has a vref buffer and doesn't use it!

Not true, Vref is used in Q3 on the noninverting input.

And, the sour grape has a vref supply and then doesn't use it as the reference to the first op amp???

Hmm, that would give a very low input impedance at audio frequencies, not good.

 

[darwin999]

i noticed something interesting about the original op amp big muff -- the clipping stage op amp is biased by a voltage divider consisting of two deliberately mismatched resistors - 1M on the ground side and 820K on the V+ side, creating a slightly asymmetrical offset and, presumably, a slightly asymmetrical clipping as a result.

In both these circuits, the clipping comes from the diodes in the Q3 opamp's feedback loops - not from hitting the opamp's rails. Many/most opamps don't clip in the most audibly pleasant way when they hit their rails, and some even suffer from momentary latchups/etc.

 

[Paul.Ruby]

Not true, Vref is used in Q3 on the noninverting input.


Hmm, that would give a very low input impedance at audio frequencies, not good.

An op amp is shown buffering Vref and not using it. But the op amp is going to be doing something else later... Temporary schematic. I don't understand your low impedance comment. Of course it's low impedance. It would pass through a 1M resistor at each point it is used for bias. I drew this in the schematic snippets.

 

[darwin999]

An op amp is shown buffering Vref and not using it.

Semantic difference here, I guess, I view Vref as providing a reference voltage that is used by Q3.

I don't understand your low impedance comment. Of course it's low impedance. It would pass through a 1M resistor at each point it is used for bias. I drew this in the schematic snippets.

Exactly, it needs a large added resistance. So agreed.