Altering Usable Range of Drive/Gain Pot - Newbie Question & Introduction

[holmburgers]

Hi All,

I've just joined the forum and this is my first post. It's great to be here!

So I just completed my first ever DIY pedal kit, a Breamb (BB Booster) from Musikding, and all went off without a hitch! I am very happy and it sounds great.

However, I wanted to pose my question because (a), I might consider making a modification, but (b) I also just want to learn more. I figure a concrete example is a great jumping off point.

I bought the BB clone more because of it's purported "clean boost to overdrive" reputation than it's ability for high gain. The whole range of gain is honestly a bit much for me, and the first 25% where I want to live is a little too touchy. There's a sharp transition from clean to dirty, and then I'm trying not to breathe on the knob too hard once I've found the sweet spot, for fear of it changing on me!

I'd probably be happy with the first 50% of the pot's throw, or even less.

Here is the schematic and it's the 1MB gain pot in question.

So my question is... in what ways could I alter the range of the gain pot? A few different cases to imagine...

1) I want to change it so that the whole throw of the new pot is just sweeping the first 25% (or 50%) of the current pot. So I lose the high gain options, but I have more control in the cleanish-to-dirt range.

2) I want to keep the whole range (cleanish to high gain) as it is, but I want more control in the early stage where I will be spending 99% of my time.

3) Let's say we do some combination of 1 & 2, but we throw in a toggle switch for controlling the stage.

Any ideas? Your input on this topic will be very helpful! Your answers, even if I don't fully understand them at first, will be points that I can dive into and investigate further.

I guess the truth is, I don't fully understand the role of a variable resistor (pot) in the gain circuit, or how it functions to affect the drive level. Perhaps this is the beginner's lesson I am truly seeking...

Many thanks!

[ElectricDruid]

I've just joined the forum and this is my first post. It's great to be here!

Welcome!

1) I want to change it so that the whole throw of the new pot is just sweeping the first 25% (or 50%) of the current pot. So I lose the high gain options, but I have more control in the cleanish-to-dirt range.

The simplest way would be to swap the pot for a smaller value. 470K/500K would halve the max gain. 330K would give you a third, 250/220K would give you only the bottom quarter.

2) I want to keep the whole range (cleanish to high gain) as it is, but I want more control in the early stage where I will be spending 99% of my time.

That's more tricky! I notice the pot is marked as "1M B" which is usually linear. You could try a log pot instead and see if that helps. It should, a bit.

3) Let's say we do some combination of 1 & 2, but we throw in a toggle switch for controlling the stage.

If you had (say) a 470K pot in series with a 470K resistor, you could add a SPST toggle across the extra resistor. That would give you the first 50% of the current range with the resistor shorted out, and the upper 50% of the current range with the switch open and the resistor in place.

I guess the truth is, I don't fully understand the role of a variable resistor (pot) in the gain circuit, or how it functions to affect the drive level. Perhaps this is the beginner's lesson I am truly seeking...

Well, that stage is a non-inverting op-amp, so you can look up how the gain equation for those works, and you'll see that the overall "shape" of the stage is the same. This example has caps and diodes added to it for tone shaping and clipping, but it's still fundamentally the same thing. The pot takes the place of the feedback resistor (conventionally called "Rf") and lets you change the feedback ratio and hence gain.

HTH

[antonis]

Additional to what Tom well said..

https://www.electronics-tutorials.ws/opamp/opamp_3.html

You see that IC1A gain is set by Gian pot + R6 divided by R7 (plus unity..)
Another way to set the gain is to make R7 variable (wired CCW) but it should alter R/C4 HPF corner frequency..

[holmburgers]

Thanks for your reply!

So then referring to the Breamb schematic linked above and comparing it to this description of a non-inverting op amp, which components make up the "Rf-R2 voltage divider network"?

Is it P1 and R6? Or am I totally off here...

What I'm gathering from the referenced description and your reply is that the amount of signal we "feedback" into the "inverting input terminal", controls gain, in this case. The higher the resistance, the more overdrive.. or I'm way off? But if so, why?

Ok here's another newbie question... is the stated resistance of a potentiometer achieved when it's at "maximum" or at "minimum"? In other words, am I getting 1M ohms resistance with the knob turned down or the knob turned up?!

OMG... I really feel like a child learning to speak for the first time, so forgive me if I'm trying anyone's patience! ;-)

By the way, I really like your suggestion for option 3 - a toggle switch for hi & lo gain could be perfect!

Your suggestion for option 2 (linear to log pot) raises yet another question for me. I have a boss BE-5 and the overdrive/distortion section, which is essentially an OS-2 I guess, has the exact same issue for me... (schematic) The usable zone of drive (for me) is in the first few degrees, and then it just goes nuts beyond that. However, this pot (VR006) is 250kA (already logarithmic)! So that's interesting to consider and suggests that maybe the taper wouldn't fix it (?).

So obviously my gain needs are much lower than the average player (or I'm barking up the wrong trees...), but I'm still very interested in understanding the factors that go into controlling this parameter.

[by the way, I see antonis' reply while I was typing this out, but wanted to go ahead and post this first, even though he answers my question about the voltage divider network! Thanks!]

[antonis]

Just bear in mind that feedback resistance (pot + R6 in Der Breamb) without gain resistor (R7) turns op-amp into a unity gain voltage follower..
( no R7 -> infinite in denominator gain formula -> gain fraction equals zero -> gain = +1 )

[Mark Hammer]

The circuit is kind of a cross between an Xotic BB booster and a Boss SD-1.
Use a SPST toggle switch to add a 390k fixed resistor in parallel with the Gain pot.  That will drop the maximum gain down to about 60x from its current maximum of 213x.  Everything else will remain as is.

If you want, you could use a 2nd stompswitch, instead of the suggested toggle, to go back and forth between adding or not adding the parallel resistor, to go between lower and higher-gain settings.  While that might normally result in a big volume boost, because of the level limiting action of the clipping diodes, there won't be that big a change in volume; enough for a solo, but not a great deal more.

[PRR]

Maybe your guitar is too hot?

[ElectricDruid]

So then referring to the Breamb schematic linked above and comparing it to this description of a non-inverting op amp, which components make up the "Rf-R2 voltage divider network"?

Is it P1 and R6? Or am I totally off here...

Rf = P1+R6, they're in series so they can(should!) be treated as effectively a single resistance.
R2 = R7, because Vb is the equivalent of Ground for our single supply circuit.

What I'm gathering from the referenced description and your reply is that the amount of signal we "feedback" into the "inverting input terminal", controls gain, in this case. The higher the resistance, the more overdrive.. or I'm way off? But if so, why?

One of the "basic rules" of op-amps is that they like to have their two inputs at the same voltage. They'll do whatever they can (and produce whatever output they need to) to make that happen.
So consider the case when Rf=R2. That's a voltage divider that produces half the level at the centre point.

Now consider the op-amp. It's getting the input signal at it's +ve input. It wants the -ve input to be the same, but the -ve input is wired to that voltage divider that gives half the output level. So the only way it can get the *same* level as the +ve input at the -ve input is if *the output is twice as loud*, e.g. the gain is x2.

The same thing works for other division ratios for the two resistors. Basically, the less of the signal you feedback to the -ve input, the larger the gain has to be to keep the two inputs at the same level. Which is why increasing the Rf value (turning the pot up) increases the gain - because it *reduces* the feedback, forcing the op-amp to compensate with more gain.

Ok here's another newbie question... is the stated resistance of a potentiometer achieved when it's at "maximum" or at "minimum"? In other words, am I getting 1M ohms resistance with the knob turned down or the knob turned up?!

A pot has three legs, so you can do either of those depending on which one of the ends you use along with the wiper. The stated resistance is the resistance from one end to the other, and will be the maximum value you'll ever see between one end and the wiper, but whether that's turned down or turned up depends on how you wire it.

[Chugs]

Change the pot to a 1MA.

Log instead of linear.

That will give much better control over the lower end of the gain by spreading it out more evenly.