how to mod this circuit?

[blanik]

here's dist/OD circuit i like a lot (reminds me of the BD-2), i would like to know how to mod for more output level (if that is possible with the TL071) and also how to make the low frequencies a little tighter, (it's supposed to have part of the dry signal mixed with the driven signal, my guess is the dry signal messes a little too much with the affected one) i tried changing the input cap but doesn't do what i'm looking for, what part of the circuit is the dry signal going through?

[Pedal love]

You could increase gain by reducing R6 - more drive.

[earthtonesaudio]

The "dry" signal goes through c1 and r5 directly into the opamp.  This circuit does not have any dry signal mixed with the driven signal, it all goes through the opamp.  So I guess it doesn't do what it's "supposed" to, but if you like it, that's not a big deal...
I would look at changing c3 to a smaller value first.  That might help with the low end "tightness."

[petemoore]

  That appears to be an active driving a tone control and volume control.
  If more output is needed, consider putting another active after the tone control.
  i tried changing the input cap
  to...?
  Smaller values might 'tighten'...
  You could try changing the value of C3.
 

[blanik]

thanks guys,

i didn't think of C3 (was afraid it would mess-up the drive channel too much), i'll give it a shot...

i noticed that when the drive pot is at 0 the sound is exactly the same as bypassed (volume has to be around 2:00), meaning the low freq aren't flabby, they become slightly annoying as i turn up the drive, it's not that bad, i can live with it but i'd like to fine tune it to my taste... 


would changing the volume pot value give it more output? (or headroom)

[earthtonesaudio]

would changing the volume pot value give it more output? (or headroom)

No, but...

The volume pot and c7 form a highpass filter with a cutoff at 57Hz.  Changing the pot to 100k drops the cutoff to 28Hz, but the low E string is like 80Hz, so there's not much point to the mod for that reason.



Also, I would change r2 to 1M and move the bias point to the right of r5.  That just seems to make more sense to me.

[petemoore]

R2 is 1/2 of the voltage divider, if you make it 10x as large as the 'bottom' 1/2 of the divider [r4 = 100k] that will offset bias by a good amount I guess.

[George Giblet]

> R2 is 1/2 of the voltage divider,

It supposed to be like that because the opamp doesn't swing symmetrically.

When the presence pot is dialed to the R9/C5 end do you find it has too much flab? Try dropping R8 to say 4.7k.  Tweak R8 if this mod does what you want.

If you find it only has too much flab with the presence pot at the R8/C6 end then try adding a 47k resistor in parallel with about 22nF between the R8/C6 node and the presence pot.  Try tweaking the cap value if this mod does what you want.

You may need to do the C3 mod as well.

[Gus]

Are you playing open back speakers?  If so, you might be overdriving the speakers at the low end.

You might want to move the highpass at the input up in frequency so not to drive the speaker too much at the low end.

[petemoore]

Gus, It supposed to be like that because the opamp doesn't swing symmetrically.
  Also, I would change r2 to 1M and move the bias point to the right of r5.  That just seems to make more sense to me.
  This is the suggestion I was referring to.
  If R2 = 1meg and R4 = 100k, that would put the bias point near ground..

[Gus]

petemoore  I was posting about C1, R3 highpass decrease C1 or decrease R3 or both for clean you can drop R3 to about 470K without loading a guitar pickup(s) to much.  I sometimes like to reduce bass at the input of effects when reducing bass, less harmonic " mid high mess"

Now George gave a big hint about ICs and R2, R4.  If you scope things you can learn a bit more

  1/2 battery voltage were did that bad advice come from? it should be what works and gives the desired output.

[petemoore]

If you scope things you can learn a bit more
  I don't have a scope...wish I did.
 

[frank_p]

This might be an insolent question but, where does this circuit comes from ?

[WGTP]

Looks strangely familiar.    

I think the clean mixed in with the processed is a a reference to using a non-inverting op amps that have clean signal coming in all the time via the input that mixes with the distortion going on in the op amp.  This is different with inverting op amps where the distorting mixes back in with the clean signal via the FBL before it goes back into the op amp.

Would doubling the R7 mess up the tone control?

Reduce the input cap by half until you hear a slight low reduction.

For the Drive control, change R6 to 47 for max gain.  C3 is to large for any bass roll off and only impacts the lows in the signal at near max.  If you drop it to 4.7u or 2.2u which I recommend, it should help flabby bass at max.  As you turn it down, the bass will be uneffected once again.   

Increasing the Output pot to 100K might help. The op amp is pretty maxed out. 

[blanik]

thanks for all the "input" i start to get a sense of what's happening in there!

i should buy a breadboard to try all of this (i don't think the vero board would survive all those experiments...lol)

[kurtlives]

^Do you have a vero board or PCB layout for this effect?

Btw who is the author of this schem and where did you get it from?

[George Giblet]

>  If R2 = 1meg and R4 = 100k, that would put the bias point near ground..

Pete,  as Gus mentioned the divider is R2 (*82k*) and R4 (100k); R2 to + rail R4 to ground. The 1M is *R3* which connect the opamp input to the voltage tap off the divider.   The way the circuit is drawn might be throwing your eyes off.  With R2 slightly above R4 the bias point is *slightly* above Vcc/2.

[earthtonesaudio]

Gus, It supposed to be like that because the opamp doesn't swing symmetrically.
  Also, I would change r2 to 1M and move the bias point to the right of r5.  That just seems to make more sense to me.
  This is the suggestion I was referring to.
  If R2 = 1meg and R4 = 100k, that would put the bias point near ground..

After I saw this I realized I was looking at it wrong.  I thought it was biased real close to +V.

>  If R2 = 1meg and R4 = 100k, that would put the bias point near ground..

Pete,  as Gus mentioned the divider is R2 (*82k*) and R4 (100k); R2 to + rail R4 to ground. The 1M is *R3* which connect the opamp input to the voltage tap off the divider.   The way the circuit is drawn might be throwing your eyes off.  With R2 slightly above R4 the bias point is *slightly* above Vcc/2.

...Yep, that's exactly what I did.  I thought the bias was set by r2 and r3.    

I was thinking that if it was biased super close to the supply voltage that the headroom would be seriously restricted in one direction, limiting bass response... but it's nowhere near as bad as I thought.

That's what I get for trying to read schematics late at night!

[George Giblet]

> That's what I get for trying to read schematics late at night!

I've fallen for that trap many times too!

Check this out,

http://www.montagar.com/~patj/bclipper.gif

I always always suspected the 20k was 200k, which makes a lot more sense, but I have a vague recollection someone checked it and it was 20k!!!  Hmmm, looks like tonepad has gone with the 200k.

http://www.tonepad.com/project.asp?id=49

[gigimarga]

can you post some soundclips of this circuit, please?
i am a big fan of boss blues driver...