# Informant mod for wimps



## MightySmallElectrons (Jul 14, 2019)

Wondering if there’s an easy way to mod the informant circuit to weight the gain knob to lower gain settings?
My gig never calls for as much dirt as this thing can produce but I’m wondering if more subtle dirt can be mined from down in the rat hole?


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## phi1 (Jul 15, 2019)

1. Use a smaller gain pot
2. Put a smaller value for R5 (10k). 

If you already have it built, you could try soldering wires onto the legs of R5 and wire a resistor in parallel, which would reduce its total value. You could also put that on a toggle switch and basically have two ranges to choose from.


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## MightySmallElectrons (Jul 15, 2019)

I think I’ll try the smaller pot...thank you.


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## phi1 (Jul 15, 2019)

For quick info on why...

Check out the circuit under the Non-Inverting section








						Op Amp Gain Calculator
					

These inverting and non inverting op-amp gain calculator calculates the voltage gain based on the input resistance and feedback resistance.




					circuitdigest.com


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## MightySmallElectrons (Jul 15, 2019)

Thanks again..very helpful


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## Chuck D. Bones (Jul 20, 2019)

If you want to drop the gain 20dB, remove C2, R4 or both.  If you're looking for a smaller gain reduction, just increase R4.  You could also increase R8.
1st stage gain = 1+R5/R4.
2nd stage gain = 1+(DRIVE+R9)/R8 where DRIVE varies from 0 to 100K depending on the DRIVE pot position.
These calcs are for mid-band gain where C2, C3, C5 & C6 are not limiting the gain in the low bass or high treble regions.


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## MightySmallElectrons (Jul 20, 2019)

Chuck D. Bones said:


> If you want to drop the gain 20dB, remove C2, R4 or both.  If you're looking for a smaller gain reduction, just increase R4.  You could also increase R8.
> 1st stage gain = 1+R5/R4.
> 2nd stage gain = 1+(DRIVE+R9)/R8 where DRIVE varies from 0 to 100K depending on the DRIVE pot position.
> These calcs are for mid-band gain where C2, C3, C5 & C6 are not limiting the gain in the low bass or high treble regions.


This is after the low and high pass then?


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## phi1 (Jul 20, 2019)

The first op amp stage is a full range boost (no low cut or high cut). Technically R4-C2 form a high pass filter, and R5-C3 form a low pass, but both these filters are well outside the audible range, so R4 or R5 could be adjusted to change gain but not affect the filters. 

The 2nd op amp stage has notable high pass filter (low cut) by R8-C5 around 1.5kHz. So changing R8 would affect the eq. There is also a low pass filter by R9+drive and C6, which depends on the position of the drive pot. This is typical on op amp dirt pedals, so that as the gain is increased and there's more clipping, more highs get cut to smooth out the sound.


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## Chuck D. Bones (Jul 20, 2019)

Let me say it a different way...
C2 & R4 make a high-pass filter with a corner at 1.5Hz.  Making R4 bigger will lower the gain and the corner freq.  
C3 & R5 make a low-pass filter with a corner at 40KHz.
The 1st stage's low-pass & high-pass corner freqs are outside the range of hearing; it's basically a flat freq response.
C5 & R8 make a high-pass filter with a corner at 1.5KHz.  Making R8 bigger will make the corner freq lower. 
C6, the DRIVE pot and R9 make a low-pass filter with a corner at 477Hz when DRIVE is at 10 and 48KHz when DRIVE is at 0.
Because the 2nd stage high-pass and low-pass over lap at high DRIVE settings, the 2nd stage gain peaks out at 34dB around 800Hz.  
The tone control is after the clipping diodes.  It forms a low-pass filter whose corner freq varies from 480Hz up past 20KHz.
If you want to reduce the gain with one part and not change the tuning of the pedal, change R4.  
If you change R8, then you also have to change C5 the same % in the other direction to keep the same freq response.

Kinda long-winded, I know.  I hope it answered your question.


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## Chuck D. Bones (Jul 20, 2019)

Phi1 said it better...


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## phi1 (Jul 20, 2019)

Chuck D. Bones said:


> Phi1 said it better...


Haha I disagree, you nailed it!


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## stevel (Dec 28, 2019)

I tried a 3.3K resistor in place of the 2.2K (R4), and the difference wasn't huge. I calculate the gain to be 4 rather than 6 (per the formula provided above). The circuit overall seems to reduce the bass a fair amount (regardless of the flat response design). I may try strapping a 100K resistor across the gain pot (I don't have a smaller value pot on hand at the moment) to see how it responds.


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## Chuck D. Bones (Dec 28, 2019)

3.3K vs 2.2K for R4 reduces the gain from 5.5 to 4, less than 3dB, so you're not going to hear a big difference.  Putting 100K across the DRIVE pot will reduce the max gain by 6dB, which isn't huge, but will be noticeable.

The freq response is far from flat, see my 7/20 post above.  C5 & R8 make a 1.5KHz high-pass response.  The effect is more pronounced at higher DRIVE settings.  If you want more bass, you'll have to increase C5 or R8.  If you want less gain, increasing R8 will do that too.

You can calculate the max gain you desire from the where you set the DRIVE pot.  A-taper pots cover 15% of the resistance from zero to noon, the other 85% from noon to 10.  If you put a resistor in parallel with the DRIVE pot, then the effective taper is different.


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