Blues Breaker style

[Flying]

I’m looking at the various blues breaker inspired circuits, and I’m confused, the inverting I/P (pin 2) on the IC is sometimes, via a resistor and capacitor, referencing ground and other times Vref.

Referenced to ground on:

Parthenon
Paragon Overdrive

And the Vref on

Blue Breaker
Glory Hole

From the schematics on the web I think the original Blues Breaker is referencing ground.

What is the effect of referencing Ground or Vref in this situation?

 

[phi1]

I’ve seen it both ways on drive pedals so many time that I haven’t given it much thought. After thinking about it a little more, the capacitor should block all DC, only AC flows through, and Vref and gnd are both 0v AC. So I believe it doesn’t matter.

The more important difference, which I’m sure you’ve seen, is the value of those resistors (33k & 27k vs 3k3 & 4k7). These affect the tone. They originate from two different versions of the Marshall blues breaker pedal (the bluebreaker on pedalpcb is the newer version with 3k3 & 4k7). I prefer the larger values, which give a warmer tone.

 

[Flying]

Thanks phi1, my electrical knowledge is enough for me to notice differences, but not to work out how they will effect the sound.

 

[Chuck D. Bones]

In my mind, GND is the better place because Vref will never be as clean as a good ground. One reason to use Vref is that the DC voltage across the cap will be very close to zero, which minimizes leakage if you use aluminum electrolytics. If you use tantalum or film, then leakage is not a concern.

 

[HamishR]

I always wonder why some pedals like Tubescreamers connect Volume 1 to Vref rather than Ground. Would Ground work just as well?

 

[Chuck D. Bones]

I always wonder why some pedals like Tubescreamers connect Volume 1 to Vref rather than Ground. Would Ground work just as well?

Pin 1 is returned to Vref because pin 2 needs to be biased to Vref, at least in the soft-switching buffered bypass originals. With the LGSM it's ok either way because there are caps blocking DC at pins 2 & 3.

 

[HamishR]

Well, when you put it like that... Nope, still don't get it! That's ok. I don't get why there are magnets in my microwave either.