How to improve the range of the distortion pot of a Boss HM-2?

[gigimarga]

Hello,

I've just bought a Boss HM-2 (schematic: http://www.aronnelson.com/gallery/main.php/v/diyuser/gigimarga/boss_hm_2.gif.html?g2_imageViewsIndex=1...I have no ideea from where I downloaded it, so my all my gratitude for the author!) in a good condition and I like very much his sound (unmodded...yet...).

Anyway, it has two little problems:
1. the mids gyrator share the same pot with the highs gyrator (easy to modify...I will add another little pot for the mids)
2. the distortion pot act very abruptly, as described on http://www.bossarea.com/loadpage.asp?file=boxes/hm2.xml: "An oddity with the HM-2 is that the Dist knob doesn't actually change the sound betweeen 3 and 8 (assuming 0-10 scale). When you start turning up the dist knob you will hear the amount of distortion increase until the button is around 3. Then the level of distortion remains the same until you get to 8 and then it starts increasing again." (thanks to all the people from BossArea forum, too!).

Looking at the schematic of the Boss HM-3 (schematic: http://www.aronnelson.com/gallery/main.php/v/diyuser/gigimarga/boss_hm_3.png.html?g2_imageViewsIndex=1...I have no ideea from where I downloaded it, so my all my gratitude for the author!) it seems that Boss modifiy this circuit exactly in this section (among the gyrators...a little).

Anyway, I hope that they made a serious improvment, because I hadn't the chance to play a Boss HM-3.

So, it's possible to improve the range of the distortion pot disconnecting the 47K resistor from the right side of the distortion pot (and rewiring the resistor to the ground, maybe halving it to 22k, as in the HM-3...I think that only will double the gain) and leaving the left side (150R resistor and the 10uF cap) as it is?

If it's necessary, I can add a 56K resistor in parellel with the distortion pot to lower it to 50K, as in the HM-3.

Thank you very much to all from this forum!

[gigimarga]

Nobody tried to health this pot?

[goulashnakov]

I like the idea of reducing the 47k resistor for more gain.  I had the opportunity to buy a used hm-2 at one point, but when I played through it, there just wasn't enough gain for my liking.  I'd be curious to see if you can mod it for more gain in the op-amp stage without mucking up the sound.

Also, I agree to leave the 150 ohm resistor as is.  If you were to short it out completely, then when the gain pot is turned all the way down, it would essentially short to ground all of your audio signal from Q6.

But I think you have the right idea with modifying the value of the gain pot.  You could try adding the resistor in parallel with the whole pot, or you could parallel a resistor between just the middle lug and one of the outer lugs.  That might help change the taper of the pot to get a more logical relationship between how much the pot is turned up and how much distortion is present.

Let us know what results you get?

[gigimarga]

Thank you very much goulashnakov for your advice!

I hope I will have time to try these changes as soon as possible