Circuit design - where Vref is needed

[bipedal]

Greetings,

I've had good luck with a number of pedal builds and mods, and I think I'm starting to grasp the theory of what's happening "under the hood" in a number of these circuits.  As such, I thought I'd challenge myself by puzzling my way through my first mutt circuit design for an overdrive pedal.

I am using the 'Son of Screamer' as a building block for the clipping/gain stage, because there's plenty of info available on that circuit's topology and because I've got a few spare 4558 chips and some silicon and germanium diodes laying around.  I'm still deciding what to use for the tone shaping section (maybe some variation of the Big Muff tone, such as the AMZ dual pot tone + body control).  Obviously, I won't be reinventing the wheel with this gadget, but I'm hopeful I can coax some decent sounds from the final product.

My question:

There are several points in the Tube Screamer & Son of Screamer circuits that connect to Vref (+4.5v from the voltage divider) -- both prior to and after the gain stage -- typically with some resistance in line.  I'm unclear how one determines where this voltage in "push" is needed.

I have a basic understanding of how to calculate voltage drop.  Do I simply look for points in the circuit flow where the voltage drops below a certain threshold, and that will indicate where the Vref ins are necessary?  If so, what is that threshold range?

Thanks in advance for any tips (or suggested references that address this) -

- Jay

[Auke Haarsma]

in my experience Vref often comes around when working with IC's and transistor who need the Vref at a certain pin to operate as designed.

[PerroGrande]

Vref, typically 1/2 the supply voltage, is almost always required when running op-amps from a single supply.  Op-amps are designed to operate from two different potentials, one positive and one negative, relative to a reference point.   With a true bipolar supply, the reference point is 0v -- the midpoint of the supplies.  When an op-amp is operated from a single supply (ground and 9v, for example), the reference point cannot be ground.  The reference point needs to be raised so the op-amp sees the two different potentials.  The supply mid-point (Vref) provides this reference point.  The side-effect of doing this, however, is that the overall headroom of the circuit is reduced.

Other circuits will use a Vref to bias a transistor (of some type) so that it will remain in its conductive region for the entire range of incoming signals (or, perhaps not, depending on the circuit). 

[bipedal]

So, if it's an AC source, 0v is reference point (halfway between +9v and -9v); with a 9v DC source, +4.5 v is that mid point.  Makes sense.

I'm guessing a basic datasheet on the 4558 would specify which pins need to to "see" a supply mid-point (i.e. reference point) voltage?  Any recommended sources for that info?

- Jay

UPDATED
Never mind: I just noticed Mouser has PDF versions of IC datasheets -- seems to have what I need...

[Mark Hammer]

One of the things that can happen in circuits is that one can sometimes "borrow" the Vref from an eariler stage, assuming that it hasn't been corrupted in some manner.  That is, V+/2 is inserted into the signal path and retained for a bit further along the signal path.  Often, however, what starts out as Vref can become inaccurate.  In those instances, the "slate" has to be wiped clean by a DC-blocking cap followed by a re-bias to Vref all over again.

[bipedal]

Thanks Mark,

Would this 'wiping the slate clean' mean a cap placed in the signal path, followed by a tie to Vref?

As a reference example, I'm looking at the schematic for the Crank circuit.  I notice two .1uf caps prior to IC pins 3 & 5, with links to Vref (+ 220K resistors) between the cap and the IC.  Are those examples of DC-blocking caps?

And I suspect the resistance is needed between the Vref +4.5v source and the IC to set proper input bias, which would vary from chip type to chip type...  I see Orman's Son of Screamer uses a 470K resistance where Vref leads to pin 3 on the 4558.

Thanks for the info walking me through this.  I'm sure you can hear the gears turning in my brain.   

- Jay

[Mark Hammer]

The answer is Yes.

When one looks at high-end "instrumentation" circuits, you'll often see trimpots used to provide compensatory offsets.  It bears noting that these circuits are rarely cheesy 9v-battery-powered devices, but rather devices with good bipolar supplies.  Why would they need such correction?  Because few op-amps are perfect, and regularly introduce DC offsets of their own that need to be counteracted.  This is especially true if the signal path is supposed to be DC and what gets produced at the tail end of the circuit is 0vdc when it says so.

[Minion]

Also how you use the VRef is also determined by the Typs of Opamp Config you are useing , IE: Inverting and Non-Inverting...... You will sometimes see the + Input tied to Vref in Inverting circuits and see the - input at Vref in Non-Inverting curcuits (between the Small gain resistor and Vref) and you will sometimes see both the +/- inputs referanced to Vref as well as the outputs depending on how the opamp is used......

This here is a Good read on Useing opamps off of a Single supply.....

http://www.eng.yale.edu/ee-labs/morse/compo/sloa058.pdf



Cheers

[bipedal]

Ah, great reference, Minion.  Lots of answers there, including some to questions I haven't even formed yet.

Thanks!

- Jay