A slightly embarrasing admission or 2 and some help needed.

[holio.cornolio]

Over the past few years I've built and modified quite a few pedals, but I have to confess (and I'm pretty sure that I'm not alone in this) that I'm pretty much a build / mod by numbers kind of person. Give me a hot soldering iron and a crib sheet and I'm away... don't get me wrong, I've learnt some stuff along the way, but I'm really not all that clued up.... and so it was that yesterday I was modifying my SD-1 for use with Bass. A simple mod, removing 1 component and jumpering 2 more. Well the instructions call for the jumpering of R10 in the circuit, however the diagram for the mod shows the jumpering of R9. I looked at the diagram first and jumpered R9, and have since looked at the schematic to see what makes most sense, but I'm none the wiser. The schem is here.
So by jumpering R9 what have I done? Should I have jumpered R10 and left 9 alone? The instruction tells me this mod 'lets more bass through', and what I've done, in conjunction with the other 2 mods seems to have worked, but I don't know if what I've done is right.... The other 2 changes were to remove C6 and to jumper C3.
There's no problem with the pedal, it works fine and sounds great as a Bass Overdrive, but I'm curious as to what the mod REALLY should be.
Can anyone help???
Other confessions. Well I really like my wife's new shoes, whisky makes me puke, I can't stop dancing to Crazy In Love. Embarrasing enough for ya??

[Galego]

I don't know the answer to your doubts, but if you jumpered R9, there's no point in removing C6, it won't make any difference. My uneducated guess would be that you just reduced the gain of the active tone control by removing R9. I think that resistor is there to compensate the signal level lowered by the R7/C4 network. Lowering the value of C3 should result in less Bass...

I thought bass mods always started by changing the input cap, changing it to a higher value cap to let more bass in...

[holio.cornolio]

See that's where my lack of knowledge lets me down. I assumed that C3 was part of a pre tone control lowpass filter, and that jumpering would feed some extra uneffected signal to the output. Don't know why - that was just a guess. Having looked at the schem, post mod, I was confused about the R9 mod, but since C6 is removed, the R10 mod doesn't make much sense to me either.
The C6 mod is decribed as removing an unnecessary filter..... where - all i see is R9 in parallel with C6 which (with both parts combined) presumably IS the filter? So leaving in R 9 keeps gain high enough for the tone stack (which now you mention it, does have less obvious range on it now, but that's ok), so what would jumpering R10 do?
The mod also suggests (although I didn't do it) sticking a 220pf cap in place of one of the clipping diodes.

[Galego]

C3 i think affects the frequencies that are clipped. You could remove D5 or D6 if you want symmetrical clipping, and you could add a small cap between the ends of D4 to smooth the clipping, the Tubescreamer uses a 51pF cap. I don't know what C6 does, but i'm sure that having jumpered R9 it's not doing anything. What does a cap do when both ends are connected to each other? Don't know about R10.

[R.G.]

Jumpering R9 turns the tone control section into something with much less gain and a different tone shaping curve.

Jumpering R10 increases output level by removing it from the voltage divider of R10/volume control.

I think that jumpering R10 is much more like what an amateur mod developer would want to do.

See "The Technology of the Tube Screamer" at geofex for some pertinent info on a similar circuit.

[holio.cornolio]

Awesome, thanks chaps for the info. I'm not convinced that this mod needs more volume, the sweep of the tone is perfect for the context the pedal is being using in, so I think I'll leave this one for now.
I checked out the Tubescreamer article and I'm reading and rereading until it makes sense!

[holio.cornolio]

C3 i think affects the frequencies that are clipped. You could remove D5 or D6 if you want symmetrical clipping, and you could add a small cap between the ends of D4 to smooth the clipping, the Tubescreamer uses a 51pF cap. I don't know what C6 does, but i'm sure that having jumpered R9 it's not doing anything. What does a cap do when both ends are connected to each other? Don't know about R10.

So by taking C3 out of the circuit, it should have the opposite effect to the effect it actually has? There's definitely a load more bass coming through the pedal now, so why is that. I understood the same as you in as much as, by increasing the value of in and out caps, you allow more bass through the pedal. I've used this mod before on a Dod OD250 and it worked well, but it still sounded like a guitar pedal with some extra bass (if that makes sense). The SD-1 with this mod, definitely sounds more like it was intended for bass. Why does this particular mod work?

[holio.cornolio]

Is this right? C2 and R4 make a high pass filter, which in conjuction with the low pass filter that is made up by R6 and C3 - controls the bandwidth of the signal being clipped. (presumably, what is not clipped passes to the output buffer??) by removing C3, all of the low frequency stuff is pushed through the circuit, thus giving a bass oevrdrive?? Or is my tiny mind very very confused now??

[earthtonesaudio]

Is this right? C2 and R4 make a high pass filter, which in conjuction with the low pass filter that is made up by R6 and C3 - controls the bandwidth of the signal being clipped. (presumably, what is not clipped passes to the output buffer??) by removing C3, all of the low frequency stuff is pushed through the circuit, thus giving a bass oevrdrive?? Or is my tiny mind very very confused now??

You ready to get even more confused? 

C2/R4 do make a high pass filter, correct.  But the low pass made by R6/C3 is sensed by the INVERTING op-amp input, which causes this low passed signal to become SUBTRACTED from the op-amp output.  The net result is that R6/C3 (together with the feedback impedances) make another high pass filter.  By shorting C3 to the bias voltage, you're letting the op-amp output pass more lows.  The same could be accomplished by increasing the value of either C2 or R4.  There's a good reason not to short C3 too, because C3 keeps the DC gain of the op-amp down to unity.  With a DC path for both op-amp inputs, you have DC gain which causes the static DC level of the op-amp to shift away from the bias voltage, potentially reducing signal swing and leading to unintended clipping of the op-amp.

Like some have opined above, I would not choose to jumper R9.  That mod seems to cripple the tone control, in my opinion.


Finally, if the mods were supposed to increase "bass" or perceived relative bass content, it makes no sense to remove C6 (removing it increases treble which can sound similar to decreasing bass).  Also, if the mods called for jumpering R9 AND removing C6 I would strongly suspect the person who came up with the mods made a typo or else doesn't grasp some key electronics concepts.

[amptramp]

You can let the following site do the donkey work of calculating frequency response as 1/(2*pi*R*C) where R is in ohms and C in farads:

http://www.muzique.com/schem/filter.htm

Let's take the turnover points in turn:

1. C1-R2||R3*hfe 11.3Hz
2. C2-R4 88.5 Hz
3. C3-R6 720.8 Hz
4. C7-(R10+VR2) 10.8Hz
5. C8-R12||R15*hfe 6.8Hz
6. C10-(R16+R17)  1.6 Hz no load

As you can see, items 3 and 2 should be addressed while the others can be ignored.  Item 3 sets the gain.  At low frequencies, C3 appears to disappear and the gain is unity.  At high frequencies, it appears to be a short and the gain is high.  Increasing or shorting out this capacitor will reduce or eliminate the turnover frequency.  If you short out this capacitor, you will have a gain of (R5+R6+VR1)/R6 for DC offset or a gain of ~220.  If the offset is 10 mV, the output voltage of the stage will be 2.2 volts.  This may give you a problem with headroom going into the second stage.  If you enlarge C3 to, say, 1µF, the DC gain is still unity so the output for the stage would be the offset or (in this example) 10 mV but the turnover would be 33.9 Hz which would be acceptable.

Increasing C2 or R4 would have the same effect of reducing the turnover point.  I would double one (or both) device values or increase one by a factor of about 4 - season to taste.

Jumpering R9 changes that stage into a unity-gain buffer, making the tone control treble cut only rather than the treble boost/cut that it was originally.  It will keep you from enhancing the treble, but this does not have the same effect as the mods to items 3 and 2 above.

[holio.cornolio]

Thanks. Confused? Yes. But infinitely better informed.
So in summary, the C3 mod does achieve what it's supposed to, but is a bad mod because it introduces instability to the power supply of the IC. The same effect could be acheived by altering the values of R4 and C2 without introducing this issue? R9 is the wrong resistor to jumper because it messes with the tone control's mojo. R10 is the correct item to jumper, but all this does is give an output boost (which I'm not convinced it needs anyway).

So now I have a new dilemma. I like the sound this pedal makes with the bad mods in place, so should I correct them with better mods, or leave as is.... Time for some more iron time and some experimentation

[earthtonesaudio]

So now I have a new dilemma. I like the sound this pedal makes with the bad mods in place, so should I correct them with better mods, or leave as is.... Time for some more iron time and some experimentation

And now you reach a crossroads in your journey.  You like the sound, so if you keep it as-is, the musician side of you is happy.  But then what will become of the soldering iron-wielding, capacitor-substituting side that's been awakened within?  Can you strike a healthy balance? 

[holio.cornolio]

'healthy' and 'balanced' are word that are not often used to describe my psycological state.
I'm more of the obsessive, fickle and compulsive ilk.