OD pedal Gain mod

[stav92]

Hey, So I have this Barber Direct Drive pedal and I find the gain is too high even with the drive pot set to minimum ...

What happens if I swap the 1Meg Drive pot for a 25k pot and swap the accompanying 10k resistor to 2.7k ?

Would this help lower the gain & prevent a huge jump in gain at the start of the pot sweep WITHOUT affecting the core tone of the pedal too much ??



Let me know your thoughts please. I'm not entirely sure my
mod suggestions are correct

[Mark Hammer]

Just tack a 100k to 220k fixed resistor between the two outside lugs on the Drive pot.  That should drop the min and max feedback resistance down to something you'd make better use of.

[stav92]

Just tack a 100k to 220k fixed resistor between the two outside lugs on the Drive pot.  That should drop the min and max feedback resistance down to something you'd make better use of.

Thanks for the suggestion Mark . My only concern is whether the additional parallel R will alter the sweep . Do you have any comments on the 25k pot idea ? My friend suggested that to me but I'm curious why we would reduce the 10k resistor at the middle lug ...

[dschwartz]

If you want to keep the high gain, but reduce the minimum gain, replace the 10k resistor with a jumper..that will transform the first stage into a buffer at minimum.
If you still get too much gain, then something is clipping on the following stages..because it does not look like a super high gain circuit to me..

[Mark Hammer]

The pedal uses the same trick that the Rat employs to provide different gains for the bottom and mids/highs.

Let's set the unaltered pedal to minimum gain.  This will put the 10k in parallel with the entire 1M pot, which makes the effective feedback resistance roughly 9.9k.  With the gain switch set to B (lower drive setting), we have two paths to Vref.  The 15k/22nf pair provides a gain of 1.67x for the entire spectrum, rolling off the bass end around 480hz.  The 5k1/10nf pair provides a gain of 2.96 for content above 3.12khz.  Of course, this is insufficient to result in clipping in that first stage, and likely results in no clipping from the secondary diode pair either.  But it makes the mids and highs a little more obvious.

Leaving the drive pot alone, but flicking the gain switch to A, the 2k2/100nf pair gives a gain of 5.5x for content above 723hz, with a rolloff below that.  The 2k4/82nf pair gives a slightly different gain for content above 808hz (I'm thinking there may be a typo in the schematic, but I'll accept it at face value for now).

If you straddle the two outside lugs of the drive pot with a 100k fixed resistor, you'll get a max gain of roughly 42x in the high gain (A) switch position, and 18.8x for the mids and highs in the low drive setting.  That will likely yield fairly mild clipping in the high drive setting, and just a hint of clipping when you hit full chords in the low drive setting.  If that's not enough, consider straddling the pot with 220k.  That will get a max gain of about 83x in the high drive setting.  Consider that a Tube Screamer applies a maximum gain of 118x.