How to decrease OD pedal gain?!

[stav92]

Hey, I'm wanting to mod my barber direct drive pedal by decreasing the total gain as the pedal is way too gainy even at minimum / low settings and I'd like a more usable range from the drive pot.

The stock pedal has a 1meg drive pot with 10k resistor from outer to middle lug. (as shown)
I'm not really sure what this 10k resistor does... If anyone could tell me that'd be great.

I'm thinking of changing the 1M pot to 25k or 50k with a 2.7k resistor instead of the 10k ? Would this help?

[tubegeek]

It's probably making a linear-taper pot behave more like a log-taper pot. Read RG's "Secret life of pots" on the GEOFEX site for full details.

[Mark Hammer]

It's probably making a linear-taper pot behave more like a log-taper pot. Read RG's "Secret life of pots" on the GEOFEX site for full details.

Or possibly some other taper which suits the tonal qualities of the circuit.  Note that when the pot is turned up all the way, the 10k resistor effectively doesn't exist, and the feedback resistance is equal to the full value of the 1meg pot.  When the gain is turned down all the way, the 10k resistor is placed in parallel with the whole pot, giving a combined effective value of 9k9.  So I wouldn't change that resistor, since it sets the minimum feedback resistance.

You'll want some dirt, obviously, or you wouldn't be building a drive pedal, just not quite so much.  I would recommend swapping that 13k3 resistor/resistance for 18k.  The stock values provide a bass rolloff starting around 440hz, and a max gain for that stage of 76x.  With 18k, the max gain becomes roughly 56x, which is a big drop.  Although keeping more bass in will help that reduced gain provide a little more oomph.  The treble-boost provided by the Presence control will still be available, and capable of yielding whatever seering qualities you want on top of the more modest basic drive.

BTW, you say it's a Barber Direct Drive, but the schematics on my drive indicate what you show is part of the LTD Silver, rather than the Direct Drive.

[stav92]

Yes you're correct I had to use an Ltd schematic because I couldn't find a DD schematic. I just wanted to show the pot and resistor value I mentioned...

So you'd recommend raising the value of the resistor to ground and keeping the 1Meg pot rather than go for a lower pot value?

[Mark Hammer]

Both play a role in setting the gain.  Swapping the resistor is probably the easier of the two, and dropping the low-end rolloff a bit won't hurt.  I have what somebody posted a while back as a "preliminary" Direct Drive Drawing, and the ground leg of that first gain stage is not like what you show, so my suggestion of 18k may be irrelevant.  That said, increasing the value of the ground-leg resistance a bit - even a few kilohms - will drop the gain by a desirable amount.

[Steben]

Marks suggestion is the easiest way of geting a mild drop in gain. The presence is a kind of treble boost though as pointed out. And the circuit really counts on that boost for the gain. This means with presence all up not that much difference. If you want all settings reduced you will need to swap the 10k resistor.

[stav92]

Both play a role in setting the gain.  Swapping the resistor is probably the easier of the two, and dropping the low-end rolloff a bit won't hurt.  I have what somebody posted a while back as a "preliminary" Direct Drive Drawing, and the ground leg of that first gain stage is not like what you show, so my suggestion of 18k may be irrelevant.  That said, increasing the value of the ground-leg resistance a bit - even a few kilohms - will drop the gain by a desirable amount.

I've taken a look inside the pedal and it looks like a 2.2k... I could try raising it to 4.7k or 5.6k . In that case I'll also lower the 100nF cap to 56nF to maintain a roll off between 500 - 600 Hz

[pinkjimiphoton]

hmmm, well, ya could also take the sleazy way out and just decrease the value of the gain pot, tho it will change the tone somewhat... if i recall the barber dd is kinda peaky, that may help some to lose some of it.

[niektb]

Or possibly some other taper which suits the tonal qualities of the circuit.  Note that when the pot is turned up all the way, the 10k resistor effectively doesn't exist, and the feedback resistance is equal to the full value of the 1meg pot.  When the gain is turned down all the way, the 10k resistor is placed in parallel with the whole pot, giving a combined effective value of 9k9.  So I wouldn't change that resistor, since it sets the minimum feedback resistance.

I don't get that, if you would short the 10k resistor the minimum gain would be much lower right? (cause you have more feedback)


And if you want to reduce the range of your pot, you can just put a resistor in parallel to the outer lugs (so f.e. 1Meg parallel to the pot would give you a resistance of 500K when the drive pot is at max)

[antonis]

What Mark tells you is you can have Max gain of (1 + 1M/Rg) and Min gain of (1 + 9k9/Rg), where Rg = equivalent resistance of 13k3 // (10k presence +470R), negleting 27nF & 10nF HPF caps reactance..

By shorting 10k feedback resistor you'll get Min gain of unity..

P.S.
I can't see the point of argument..

[merlinb]

By shorting 10k feedback resistor you'll get Min gain of unity..

Exactly. Just short out that 10k resistor.

[Ben N]

Maybe have a look at the LTD schematic and compare to your DD.

[niektb]

What Mark tells you is you can have Max gain of (1 + 1M/Rg) and Min gain of (1 + 9k9/Rg), where Rg = equivalent resistance of 13k3 // (10k presence +470R), negleting 27nF & 10nF HPF caps reactance..

By shorting 10k feedback resistor you'll get Min gain of unity..

P.S.
I can't see the point of argument..

oh sorry, my 'dont get it' was mostly pointed at Mark's advice to not change the 10k resistor