Am I right about this? MXR Dist +

[GuitarLord5000]

I recently built the MXR Distortion Plus from Tonepad with a 1M drive pot.  I get very little (no?) audible distortion for over half the range of the pot.  The last bit has a decently smooth sweep over the clipping range.  Now if I put in, say, a 100k pot in its place, I would still have the maximum range of distortion available right?  I'm thinking max distortion is achieved at zero resistance?  With a smaller value pot, I'd just have a smoother sweep over the actual clipping range of the circuit, correct?  

The reason I ask this is, I've always believed this to be true, but recently I've read a few articles that say something like "if you want more distortion, change the drive pot to a larger value", which if I am correct would only give you a larger RANGE of distortion, and no extra actual clipping.  Somebody gimme a clue please.
Thanx

[RDV]

I recently built the MXR Distortion Plus from Tonepad with a 1M drive pot.  I get very little (no?) audible distortion for over half the range of the pot.  The last bit has a decently smooth sweep over the clipping range.  Now if I put in, say, a 100k pot in its place, I would still have the maximum range of distortion available right?  I'm thinking max distortion is achieved at zero resistance?  With a smaller value pot, I'd just have a smoother sweep over the actual clipping range of the circuit, correct?  

The reason I ask this is, I've always believed this to be true, but recently I've read a few articles that say something like "if you want more distortion, change the drive pot to a larger value", which if I am correct would only give you a larger RANGE of distortion, and no extra actual clipping.  Somebody gimme a clue please.
Thanx

You would not have the same range of distortion at all, in fact I don't think you'd have much at all as you would be greatly reducing the gain. The reason for the gain being bunched up at the end of the rotation is the taper of the pot you're using. A reverse audio pot would help a great deal as it would go up in resistance really fast early in it's rotation, then slow down, so the distortion would come up more smooth and gradual...

HTH

RDV
RDV

[Phorhas]

Think about - when you have a 1M resistor in the FB loop and a 1M drive pot going to ground, at half rotation you only get 1000k/500k + 1 = 3 which isn't much. if you'll put a 100k pot, the minimum gain will be 1000k/100k+1 = 11.

tha calculations aren't correct to the letter because I didn't consider the 4k7 resistor is series to ground after the pot - but you get the picture

since the use of the gain pot for most of us is to set a kind of "saturation control" and not "level control" and thing can use a regular linear pot.

Dan.

[GuitarLord5000]

Quote: "I would still have the maximum range of distortion available right?"

Okay, I had a slight brain fart here.  What I meant to say is, with the 100k pot, would I still have the maximum AVAILABLE distortion?  But now I'm confused.

RDV said "in fact I don't think you'd have much at all as you would be greatly reducing the gain. "

Phorhas said "when you have a 1M resistor in the FB loop and a 1M drive pot going to ground, at half rotation you only get 1000k/500k + 1 = 3 which isn't much. if you'll put a 100k pot, the minimum gain will be 1000k/100k+1 = 11."

These statements seem to contradict themselves.  RDV, are you saying that I will be limiting the AMOUNT of gain, or the RANGE of gain?  Sorry if I wasnt clear in the previous post.  What I have is basically too much range right now.  I want less range, but still be able to use all the gain available in this circuit.
I'm not really thinking about putting a 100k pot in the Dist +.  I was just kinda thinking out loud.  But if I am still able to get maximum available clipping, I will definately try a 500k pot in place of the 1M.

MAN, sorry if this is extremely slow witted of me.  But thanx for the replies fellas.

[Phorhas]

to calculate gain in a noninverting opamp the equesion is this - (R1/R2)+1 = gain

R1 represents the resistance inthe FB loop - between the output pin and the inverting input pin
R2 is resistance from inverting pin to geound or Vbias.

So, in CKTs such as the Dist+ where the pot is R2, decresing the pot value raies the minimum gain but MAXIMUM gain is kept, whereas in CRKTs like the tube screamer where the pot is R1, minimum gain is ratained but the makximun gain is lowered.

my english is not my native tounge, so feel free to ask more if I made a mess in your head

[RDV]

That's what happens when I answer questions @ 4:00am. I've got to stop that. :roll: With a 500k pot, you'll get a range from pretty much clean to full distortion. If you're looking to limit the range, a 100k pot may do exactly what you want, but a 1meg is certainly going to have a lot of range that does virtually nothing.

Sorry

RDV

[GuitarLord5000]

Alright!  Thats what I wanted to know.  It makes a lot of sense now.  And Phorhas, your english is great.  You guys gave me all the info I needed.  Thanx for the responses.

Peace

[Dave Z]

I agree w/above, Phoras, that was the most useful post I've seen on opamp gain - at least for a opamp Neanderthal like myself...nice to have it in practical terms...

[Mark Hammer]

It seems the misinformation received about increasing the pot's resistance value confused the role of resistance when gain is set by the feedback resistance in a non-invertng gain stage (such as the TS-9 , SD-1, and Rat, as opposed to the ground leg resistance. of the same sort of gain stage.  Increases to feedback resistance will increase gain, but it is decreases to resistance in the ground leg that will boost gain.

A hybrid control that is often used in gain stages intended to cover a wide range involves a single pot used to alter both feedback and ground-leg resistance at the same time.  Here, the wiper of the pot is tied to the inverting ('-') pin of the op-amp.  One leg of the pot is in series with some value of feedback resistor, and is added to the feedback resistance.  The other leg of the pot is in series with a value of fixed resistor to ground.  Turn the pot in one direction and the feedback resistance gets smaller as the ground resistance gets bigger (big downward shift in gain).  Turn it in the other direction and the feedback resistance gets bigger as the ground resistance gets smaller (huge increase in gain).