general od pedal question. changing amount of gain

[pete74]

Hi.  Question not related to any specific circuit here...   If I have an OD pedal that has too much gain, even when the gain control is turned all the way down, how can I decrease it?   

Most circuits I've seen when looking into this, have a resistor in series feeding into the gain pot.  This makes me think that I could either increase the value of the resistor or just change the pot to a higher value.   

Assuming I'm correct here, is there a generally accepted sweep range of a gain pot?   I've seen a few different circuits with only 1k gain pots and others with up to 500k...

[GibsonGM]

What pedal is it, and do you have a schematic?   

There isn't really any "universal value" for gain pot. Depends on the circuit topology. It is a transitor-based OD?  Opamp?   Gotta have a look-see! 

[pete74]

Not really referring to any specific pedal.    I'm really just not a high('ish) gain kinda person and I've found there are many pedals out there from the purple plexi to the ocd that I have and like, but find the gain doesn't go low enough for my taste.   That being said, I never turn the gain up more than 1/4 of the way...

[GibsonGM]

If you look at the schematic, you can see how the pot works to set gain.  Then you'd take the appropriate step of adding, or removing, resistance.   

It may mean that like you thought, you remove a series R next to a pot...or ADD resistance to an opamp input...there are a couple of different setups...post a schematic to something you're interested in and the forum will tell you how! 

[Kipper4]

I guess it depends on your taste and rig.
If your using a tube amp. I like mine set so it breaks up nice when playing the strings hard.
Using a diode clipping distortion such as the Plexi. Well it's always gonna have that diode clipping sound.
However a simple booster might do the trick depending on your taste. If your only using the Plexi, OCD as a booster might as well go with that.
I don't have all the answers. I just know what I like.
Nor am I a tone expert.

[antonis]

I'm really just not a high('ish) gain kinda person and I've found there are many pedals out there from the purple plexi to the ocd that I have and like, but find the gain doesn't go low enough for my taste.   That being said, I never turn the gain up more than 1/4 of the way...

If we are absolutely sure that we are talking abour Gain (not Volume) just act as proposed by Sir Mike..

If Pot is placed on NFB loop you have to choose between 2 almost cost/trouble equall remedies:
Change Pot + Cap on NFB loop or change R_gain + Cap to gain part..

If Pot is placed on gain part, lowering R_FB (and raising C_FB) seems more convenient..

[GibsonGM]

Or MAKE a boost that doesn't break up, and you will have a CUSTOM pedal that makes YOU sound great!!

Mosfet boost is one idea.  I use one to boost for solos because my amp has a lot of headroom, and it keeps my sound just about the SAME, only somewhat louder.   

One must remember that you can overdrive the amp input, creating more 'dirt', and there really isn't any way to 'fix' that other than some amp surgery, a new amp, or don't overdrive it so much.

[Frank_NH]

I think this topic has appeared before, regarding the difference between "gain" and "volume".  This is of interest to me because I'm always wondering if there is any convention for the outlet voltage level of a typical effect like an overdrive or distortion.

To me, "gain" usually refers to the increase in amplitude of the signal as it passes through the effect.  That may result in distortion as the signal is limited by clipping, as well as shaped by tone filters.  At the output, you have a maximum level of signal increase from the input, which can usually be controlled by a volume knob arranged as a voltage divider. 

Now, the Tube Screamer manual says:

Maximum Output Level  0 dBm
Maximum Gain  + 30 dB

http://www.ibanez.co.jp/world/manual/effects/TS9.pdf

I believe 0 dBm = 0.775V, which appears to be (as far as I can tell) the max output voltage level from the overdrive.

However, your signal gain can be up to 30 dB, which means a quiet 20 mV can be amplified 32X but higher amplitude signals will be limited (clipped) to the 0.775V max output.

Other effects, like the SHO booster or the Colorsound Overdriver, can achieve much higher output voltage levels.  This presumably makes it easier to overdrive your tube amp's input.   But if the output voltage is, say 2 or 3 volts, based on a typical guitar signal of 100 - 300 mV, is that too much?  Most amps can take that, but what if you run that signal into a tube screamer?   The signal will be clipped (massively) and will never go beyond 0.775 V.  But, hey, maybe that sounds good...

[J0K3RX]

Not really referring to any specific pedal.    I'm really just not a high('ish) gain kinda person and I've found there are many pedals out there from the purple plexi to the ocd that I have and like, but find the gain doesn't go low enough for my taste.

I have a Ferrari but I am really just not a speedy('ish) kinda person... How can I make it go slower?

[digi2t]

Not really referring to any specific pedal.    I'm really just not a high('ish) gain kinda person and I've found there are many pedals out there from the purple plexi to the ocd that I have and like, but find the gain doesn't go low enough for my taste.

I have a Ferrari but I am really just not a speedy('ish) kinda person... How can I make it go slower?

[ashcat_lt]

Maximum Output Level  0 dBm
Maximum Gain  + 30 dB

http://www.ibanez.co.jp/world/manual/effects/TS9.pdf

I believe 0 dBm = 0.775V, which appears to be (as far as I can tell) the max output voltage level from the overdrive.

dbm is a power measurement and unfortunately meaningless here.  0dbm is 0.775V into a 600 ohm load, but into a 1M pedal or amp input, it's more like 32V.  We know that can't be right, of course, since the power supply is only 9V.  We know it won't get to 9V because the opamps won't swing all the way to either rail.  It's probably closer to 7V when all's said and done.

To the OP - you're just not going to get a definitive general answer because there are different ways to do it and every circuit does it differently.

But ultimately, the gain of non-inverting opamp is the inverse of the voltage divided between the output and the - in.  Less division means more gain.  To get that you need more resistance in the ground leg OR less in the feedback loop.  Of course it's only the ratio that matters, and sometimes it's best to keep the total the same...

Because that voltage divider isn't just resistors.  There are always caps involved too, which means the whole thing is frequency dependent but also that frequency response depends on the value of those resistors. 

So it leads to the question of whether you want the sound you get now but with a little less overdrive OR the sound this pedal would make if you could turn the pot down more.  The first is kind of tricky and the second is easier but not exactly trivial.

[Frank_NH]

I have an LTSpice model of a generic tube screamer and looked at the output versus input for an 800 Hz sine wave input signal.  With the volume fully up and the tone centered at 10K, the simulation produced the following:

input amplitude   output amplitude   Gain=20*Log10(Vo/Vi)     

1 mV      50 mV       34 dB
10 mV    200 mV     26 dB
100 mV  350 mV     11 dB
500 mV  700 mV       3 dB

Of course, a more complete analysis would have to consider a range of input frequencies and tone knob settings.  But it's clear that the tube screamer compresses the higher amplitude signals due to the diode clipping, the clipping range being limited by the forward voltage of the diodes.

The main point, though, is that the output of the tube screamer under normal playing conditions is not likely to get beyond 1V.  In fact, for an extreme 2V amplitude input signal, the max output amplitude becomes lower than the input!

To the original question from the OP, if you want the gain to go lower, simply reduce the series resistor to as low as you want to go.  In the tube screamer, this is a 51K resistor.  Reduce it to 4.7K.  Then with the gain pot all the way off you only get a mild boost with little (if any) distortion and some tone filtering, with output slightly above unit gain.

[ashcat_lt]

The main point, though, is that the output of the tube screamer under normal playing conditions is not likely to get beyond 1V.  In fact, for an extreme 2V amplitude input signal, the max output amplitude becomes lower than the input!

This kind of can't be true.  I guess I haven't ever paid much attention to the tone section and how much loss it has, though I didn't think it was much if any.  But the gain stage can't ever be less than unity.  Even if the diodes acted like straight wires, you'd still have the original input level at least.  But they don't act as straight wires, so once input * gain is greater than the diode voltage, the output will be Vin+Vf.  If there were no rail constraints (opamp clipping), the signal would actually get cleaner with higher inputs or greater gain levels.  IDK what's wrong with your model, but this is established fact.

[Frank_NH]

My gain calculation is for the entire effect (including tone conotrol and output buffer).  I was using a 2V amplitude input signal with the gain maxed out.  The output level depends a lot on the tone setting.  At a minimum tone pot setting, the output is around unity gain, and slightly lower as you turn it up, again with an assumed input frequency of 800 Hz.

Admittedly a 2V input signal is contrived and unrealistic, but I was just seeing what the simulation would produce under such a circumstance...