BOSS DS-1 'Distortion pot'

[newfish]

Hi All,

I've recently modded a DS-1 (extra switch for Si / None / Ge clipping diodes) - and all is good.

However, the 'Distortion' seems bunched up at the 4 - 5 o'clock (maximum) end of the pot's sweep.

The 'stock' pot is Linear, a Log pot has opened it a *little*... my question is this - would a reverse log pot even this out any further?

I've been looking at this schematic for my modding info...

http://www.google.co.uk/imgres?imgurl=http://cdn.tonegeek.com/wp-content/uploads/Boss-DS-1-schematic.png&imgrefurl=http://www.tonegeek.com/musicgear/pedals/boss-ds-1-classic-distortion-effects-pedal-modded.php&h=784&w=1332&sz=183&tbnid=5OWrkbSPzTM3yM:&tbnh=85&tbnw=145&prev=/search%3Fq%3Dds-1%2Bschematic%26tbm%3Disch%26tbo%3Du&zoom=1&q=ds-1+schematic&usg=__IUYsU_iyMz2Q4RdVl5iW_VfKqSQ=&docid=vdKIsoudMOePGM&hl=en&sa=X&ei=Qw5BUf6jB6-X0AWK14HgBQ&sqi=2&ved=0CDgQ9QEwAw&dur=979

Thanks for your thoughts!

Ian.

[Mark Hammer]

Yes, changing to a reverse-log pot will improve dialability.

The reason is pretty simple.  When op-amp gain is varied via the "ground leg" resistance, gain increases as that resistance gets smaller.  Changes in resistance at the high end produce negligible changes in gain.  For example, the MXR Distortion+ also uses the ground leg, and a 500k pot.  With a 1M feedback resistor and the gain pot at max, you have a gain of 3x.  Reduce that pot resistance from 500k to 200k and you've only changed gain from 3x to 6x.  At the other end of the scale, dropping the pot resistance from 50k to 10k changes the gain from 21x to 101x.

So, as you can see, when using the ground leg, it is changes at the low end that matter more than changes at the high end.

A reverse log pot will get you "through" the highest resistances quickly so that more of the pot's rotation addresses changes at the low end of resistance.

[newfish]

Wow!

Thanks for such a complete answer, Mark.

Cheers, Ian.