How can I lower a circuits maximum gain?

[heartben]

Hello! I am pretty sure this is going to be an incredibly easy thing to do - and I imagine there are at least a few ways to do it.

Essentially I just want to tame the gain down. This particular one uses a TL072 - I am guessing there will be something to do with a resistor? It also has 2 BS170s.

To save some time I will just upload a schematic of the circuit in question - although I am just generally curious for future builds as well. I am guessing the same parts could be swapped for more gain too if you wanted?

Hopefully someone on here can point me to the component(s?) to tinker with. Thanks a bunch!

[GibsonGM]

Hi Ben,

Welcome to the forum  

For the first opamp section, the 1M gain pot can be decreased in value (500k perhaps...even 100k, to taste).  This will work with R3 and R4 depending on switch position to lower the gain of that section.   The gain of a noninverting opamp is:  1+Rf/Ri    where Rf is the feedback resistor (the gain pot), and Ri is the input resistor (R3 and R4 depending on switch position).

R12 in the 2nd half of the opamp can be decreased to lower that section's gain.     You would figure out what is going on by using an audio probe to listen to the output of the 1st, and then 2nd, opamp stage as you go about this, to determine which change is giving you the sound you want. 

The BS170s are just there for clippers, they're not active in that role.

May I suggest some reading on how opamps work?  There are a LOT of tutorials on the subject.  Many are re. DC amplifiers, where here we want AC amplifier info, but the principles are very much the same re. feedback and gain.   


https://www.youtube.com/watch?v=7FYHt5XviKc
https://www.youtube.com/watch?v=kbVqTMy8HMg

[antonis]

You can raise R4 value..
(but it will work only in CUT mode..)

Better raise R3 value..
(it will work for both mods...)

Alternativelly, you can deal with R9 value to alter IC1B input level (not gain) and/or raise R10 value..
(it will deal with tonal responce to an extend..)

P.S.
My vote goes to R3..

edit: Sir Mike is by far faster..
(although, his Highness deals with expencive modifications like pots change..)

[antonis]

<off-topic ON>

Do I see a 10.26 DC gain ( 1 + [R11+R13]//R12 / R10 ) for IC1_B or I should have another cup of coffee..??

<off-topic OFF>

[ElectricDruid]

+1 agree with what others have said about the gain control. IC1.A is a non-inverting op-amp, so you can tweak either the pot value (lower=less gain) or R3 (bigger= less gain).

Changing the pot has one advantage in that it doesn't mess with all those tone-shaping caps around R3 and R4. Only C5 would be affected, and that would give you less gain, but more treble. Increase C5 by the same factor that reduce the pot by (so if you make the pot 100K, C5 gets x10 bigger, for example).

Do I see a 10.26 DC gain ( 1 + [R11+R13]//R12 / R10 ) for IC1_B or I should have another cup of coffee..??

It certainly looks like there's no DC blocking between that first gain stage (with its 4.5V bias on the output) and the next active Mid control stage. Another side-effect of is going to be that that volume control will be noisy, since it has 4.5V across it.

It needs a cap in series with R9. 100n would do it.

<edit>I was wrong! There's no bias! See further down.</edit>

[duck_arse]

Tom - where is that 4V5 being produced, please?

I'd think the simplest mod would be to add a resistor in parallel with the gain pot outer lugs - like 470k to reduce max gain by half. I've been wrong on these things before, tho.

[ElectricDruid]

Tom - where is that 4V5 being produced, please?

I'd think the simplest mod would be to add a resistor in parallel with the gain pot outer lugs - like 470k to reduce max gain by half. I've been wrong on these things before, tho.

Agh! Sorry! You're right - I'm looking at it as if it's running on 0-9V, but it's actually got a full bipolar supply.

Ok, move along. Nothing to see here. Ignore me. 

[antonis]

I'll insist for a cap between R10 & GND..
(unless someone can ensure for the absolute absence of GND DC offset on pin 5..)

[heartben]

Holy moly! Thanks for all the info everyone. I still have a ton to learn so I will do my best to take all this info in.

[John Lyons]

Heartben
Where did this schematic come from?
Can you post a link to any other info on it?

[heartben]

Heartben
Where did this schematic come from?
Can you post a link to any other info on it?

It's from Effects Layouts in a build doc for a Olde Dirty Bass clone.

[John Lyons]

Thanks!

[idy]

Interesting there are neither input or output caps. Any thoughts?

[GibsonGM]

Good point idy.  Since it's a dual polarity supply, there's no bias current to block, but I looked this up...there should be a coupling cap at in and out, with a return path to ground, because any small leakage can disturb the opamp's input and cause malfunction.

[antonis]

there should be a coupling cap at in and out, with a return path to ground,

Or else, a happy oscillator..
(till final burn-out..)

[GibsonGM]

Or, it can just degenerate and pass no signal at all, can't it?  I think the cap without path to ground can charge, and cause blocking...