Why is Volume increasing with Gain going up?

[ayayay!]

I've got a problem with the DS-1 mod I've been performing.  On most pedals I do it on, everything's fine.  However on two of them, I've got a serious problem:  Volume is increasing dramatically as the Dist knob is turned up. 

After many, many side-by-side comparisons (same OpAmp, same Mods, same clipping, same, same same) I am stumped.  So in the case of ANY pedal, what could the cause of this be?  I'm open to any suggestions.

Now for the side notes:  All my DS-1's use an LED (blue clear 3mm) on D4, and a switch between 1n4001 or Germanium on D5.  This may also be suspect because:  I can't perceive a difference when switching diodes, yet I've triple checked the wiring scheme and it should work.  On all but these two pedals I hear less bass and mids on the Germanium setting, with a little more fuzz.  The 1n4001 setting boosts the output overall slightly, and adds more bass; it does what I want.  Not these two guys...

I'm afraid I mis-biased something, but I just don't see how since I've performed this on so many other DS-1s. 

Like I said, maybe a more accurate question would be:  Why would ANY pedal do this? 

[zachomega]

Well gain is by definition output/input so as the gain increased the output is increasing. 

Usually the output voltage is controlled by the clipping circuitry...or rather limited by the clipping circuitry to whatever the clipping threshold of the diode is (germanium would be .23-.3 and silicon would be .6-.7 or something like that). 

My advice, double check your work. 

-Zach 

[squidsquad]

Someone correct me if I'm wrong. 
Different pedals use different means of achieving distortion (duh).  If the components are set-up to have plenty of headroom...and no restrictions...yes....more gain always = more volume.  However, if the circuit is something like a fuzz which has transistor clipping...or diodes on it's tail end...they act like a crude limiter.  Only SO much current can get thru...the clipping begins...and if you feed it more signal...it only clips more...the waves are chopped off on the tops...and become more square (so it sounds more distorted) but no louder.

[ayayay!]

There was a similar thread almost identical to this one a few days back by someone else (sorry can't find it.) and they all pretty much said the same thing, so let me phrase this another way:  Why is my Distortion knob acting like a Volume/Gain knob?  As gain goes up, so does volume. 

This is kind of what squidsquad below is getting at.  It should clip more, but not increase volume.  That's how it is in stock form and for most of my modified pedals. 

So let's throw this model out for a second and consider this:  What would make a pedal, ANY pedal, do this?  I'm sorry, but I've been playing for 18 years and never had a distortion/overdrive pedal increase volume on an exponential curve with the gain/distortion being turned up.   I'm looking for a cause here...

Thanks! 

[zachomega]

Check the clipping part of the circuitry...the diodes...Make sure they are installed correctly.  Make sure they are not burnt out as well. 

-Zach

[ayayay!]

This isn't the happy ending I was hoping for, but here's the status anyway:  I completely replaced all clipping diodes and used a DPDT switch instead of SPDT to switch between Ge & Si.  I checked the voltage coming through the clipping, and it matched my "good" pedals. 

Next, I put the original opamp back in.  I know that's a stretch, but I did it anyway.  Results are pretty much the same.  I will say I can slightly tell the difference now between clipping selection, but it's nominal.  Most ears wouldn't be able to tell the difference. 

I just don't get it, how can the switching between the types be so profound in every other modded pedal I've done, but not this one? 

The pedal still sounds great, but for some reason the difference when I switch between clipping types it's barely discernable.  Anyone else have any thoughts? 

[johngreene]

This isn't the happy ending I was hoping for, but here's the status anyway:  I completely replaced all clipping diodes and used a DPDT switch instead of SPDT to switch between Ge & Si.  I checked the voltage coming through the clipping, and it matched my "good" pedals. 

Next, I put the original opamp back in.  I know that's a stretch, but I did it anyway.  Results are pretty much the same.  I will say I can slightly tell the difference now between clipping selection, but it's nominal.  Most ears wouldn't be able to tell the difference. 

I just don't get it, how can the switching between the types be so profound in every other modded pedal I've done, but not this one? 

The pedal still sounds great, but for some reason the difference when I switch between clipping types it's barely discernable.  Anyone else have any thoughts? 

There is something wrong with your clipping diodes. The diodes should be hard-limiting the signal. So either they are not connected to the signal line or to Vref. Also make sure that the bypass cap for Vref is in good working order.

--john

[ayayay!]

It's not the clipping diodes.  I've triple checked and even completely replaced them.  They also worked perfectly in another DS-1.  I took the whole clipping assembly (switch & wire hookups) and replaced it in a known working pedal, so this eliminates the clipping. 

I will look into the bypass cap though.  In fact, that just might be it...

[ayayay!]

I got to work on this extensively today with some good news.  This is a brand new DS-1.  I have about 5 more in reserves.  So I grabbed a stock one (new) and plugged it in:  same thing!  As Gain goes up, so does volume!  So there's this current pedal, the one of the shelf, and my friends... they all do this.  Seems to be the newest ones. 

I grabbed another one I have that's older, but hasn't been modified yet.  Gain goes up and... that's it.  Gain goes up!  Volume is the same.  Works like it's supposed to.  (I KNEW I wasn't imagining this...) 

Sooooo... back to the schem.  Then I looked at the NEW schem that 5thumbs and Miqbal submitted.  Didn't see much difference in the areas I suspected (which were C5, C9, and that feedback loop from pin 1 to 2, all OK) but 1 thing intriqued me.  C7 is 100pF on the new pedals, and C6 no longer exists.  Wha?  Why 100pF across the input & wiper of the Dist pot?  (I'm being rhetorical, I know why.)  But I would just as soon remove it unless it has to be there. 

So I removed C7.  Sounded much, much more even as far as the volume not increasing, but didn't have enough picking attack & brightness unless maxxed out (duh).  So 10pF on C7 is what I settled on, after trying 68pF then 50pF.  The "gain sweep" is nearly perfect now, meaning from "0" to "10" the volume of the pedal hardly increases as the distortion goes up.   Sounds SOOoo much better.

Now that that part is solved, I still want to figure out why I can't tell much difference between the Si & Ge clipping when I switch them. 

But I don't think I have the energy to tackle that tonight. 

[johngreene]

I got to work on this extensively today with some good news.  This is a brand new DS-1.  I have about 5 more in reserves.  So I grabbed a stock one (new) and plugged it in:  same thing!  As Gain goes up, so does volume!  So there's this current pedal, the one of the shelf, and my friends... they all do this.  Seems to be the newest ones. 

I grabbed another one I have that's older, but hasn't been modified yet.  Gain goes up and... that's it.  Gain goes up!  Volume is the same.  Works like it's supposed to.  (I KNEW I wasn't imagining this...) 

Sooooo... back to the schem.  Then I looked at the NEW schem that 5thumbs and Miqbal submitted.  Didn't see much difference in the areas I suspected (which were C5, C9, and that feedback loop from pin 1 to 2, all OK) but 1 thing intriqued me.  C7 is 100pF on the new pedals, and C6 no longer exists.  Wha?  Why 100pF across the input & wiper of the Dist pot?  (I'm being rhetorical, I know why.)  But I would just as soon remove it unless it has to be there. 

So I removed C7.  Sounded much, much more even as far as the volume not increasing, but didn't have enough picking attack & brightness unless maxxed out (duh).  So 10pF on C7 is what I settled on, after trying 68pF then 50pF.  The "gain sweep" is nearly perfect now, meaning from "0" to "10" the volume of the pedal hardly increases as the distortion goes up.   Sounds SOOoo much better.

Now that that part is solved, I still want to figure out why I can't tell much difference between the Si & Ge clipping when I switch them. 

But I don't think I have the energy to tackle that tonight. 

I'm really sorry, but this doesn't make any sense.

--john

[slacker]

Like John said, that doesn't make any sense. Assuming that new schematic is correct removing C7 or making it smaller shouldn't make any difference to the gain/volume. It's only there to stop the opamp oscillating, and only affects frequencies above about 16kHz.
What was C6 doing that might explain some of it.

[gez]

Blue diodes - well, the ones in my suppliers' catalogues - have a forward threshold of around 4V- 4.5V.  From what I gather (link to schematic?), this is a back-to-back diode at output jobby.  So, the LED is, in effect, an ornament as you'd need 8V - 9V output swing in order for the blue LED to be factored into the equation, and most op-amps fall short of this by a few volts.

What you're most likely hearing is op-amp clipping during one half of the wave form's cycle and Si/Ge diode clipping during the other half.  As amp clipping is pretty harsh, this is probably the reason why you can't discern much of a difference when switching between the Si/Ge diode: the sound of the op-amp is swamping all else.  How much swamping occurs will depend on the op-amp, I suppose.

The increase in volume with distortion?  Perhaps a result of the op-amp's output swing increasing when pushed into saturation? 

Whatever, you're going to need more than 9V supply to actually hear what a blue LED is doing.  I bet if you pulled it from the circuit (as is), you wouldn't perceive any difference.

PS  I reserve the right to talk utter cr*p.