Blend/phase alignment academic question.

[karbomusic]

I don't have the schematic handy but possibly not needed... An OD I build/sell which is a take on the tube screamer circuit with a boosted blend of the clean signal + other features.... So imagine a TS with it's normal dual op amp (gain/tone) which feeds into another dual opamp (OP2134) on channel 1 with a copy of the original clean guitar signal on channel 2. These two outputs go to the blend knob who's output heads downstream to the master volume knob.

Both sides of the blending op amp have some gain (non-inverting) and that has been calculated so that the clean and the overdrive are the same relative output and in a sort of a sweet spot so that when I blend, they (clean/dirty) work well together. This also means I can crank the blend full left and crank the output knob and it's a very healthy clean boost (~3.5 volts) or anywhere in between.

Every time I complete a build and before I box it up, I run a small suite of tests with my meter and my oscilloscope. If those tests pass, it pretty much has to work as advertised and I also use the data gathered for a spec sheet that goes with the product. My final test is a control signal (original sine wave) on one scope trace, with the pedal output below it on another trace. I then test the blend control because it's imperative these two signals maintain their phase relationship since it's a blend.

Last night I saw an unwanted shift and volume drop when blending, the distorted signal was moving 10-20 degrees out of phase. I had experienced this 18 months ago when designing/breadboarding and found it is due to the gain cap that goes to ground being backwards - Sure enough, it's backwards in this build and fixing that will fix the problem. I also think it 'may' occur if the cap isn't polarized or if a polarized one backwards.

I explained all that only ask how come this affects the phase of the signal.

[R.G.]

it is due to the gain cap that goes to ground being backwards - Sure enough, it's backwards in this build and fixing that will fix the problem. I also think it 'may' occur if the cap isn't polarized or if a polarized one backwards.

I explained all that only ask how come this affects the phase of the signal.

(1) a non-polar cap will not cause this issue
(2) polarized caps cause it because the impedance of an electro put in backwards is very different from an electro put in correctly.

[karbomusic]

Hmm... well, when I found this on the breadboard wayyyy back, I was using a NP film cap. That's where I originally found it so I believe you but now I have to solder in a NP to see. Is there no EE/math reason it does this? Meaning academically since it seems fairly repeatable. Last night was just a reminder (luckily).

[garcho]

Is there no EE/math reason it does this?

the impedance of an electro put in backwards is very different from an electro put in correctly

http://www.allaboutcircuits.com/textbook/direct-current/chpt-13/practical-considerations-capacitors/

maybe i misunderstood your question

[karbomusic]

Thanks garcho, I'll check the link since I was hoping for more than the fact it is backwards - I already knew that and was looking for a conversation on the phase shift in the event it becomes something I exploit in some other design down the road. 

[karbomusic]

Checked the link... I may have missed the fun stuff but that link is just a basic "what is a capacitor" article (on that page anyway). I have some other avenues where I can ask and study deeper on my own where phase shift is concerned, just thought I'd try here first thinking phase might be an interesting discussion. Thanks guys.

[PRR]

What is the circuit impedance? (A handy schematic would avoid asking.)

The To-World capacitance is different inside foil or outside foil. In audio this rarely matters. If you must phase-check, in hi-Z circuit, the different to-world capacitances will give different results.

[karbomusic]

Thanks Paul - I'm home now and testing a few things and will return in a bit. This was really intended as academic since the circuit works fine (when I put the parts in right that is).

[karbomusic]

OK.... It looks like it's really easy to touch the tone control while testing and that is the cause of the shift which I'm going to assume is perfectly normal. By unknowingly moving it slightly between changing caps it made it appear the cap was the reason but it's unrelated.  Fixing the polarity fixed another anomaly that is in line with a backwards cap. Just to top it off in the video below the control signal is top/pink, circuit output bottom/blue.

First Video: Changing blend with tone @ full CW - no shift:

Second Video: Blend first then, just changing the tone which affects the phase a few seconds in:

[karbomusic]

Previous build I had lying around... Doesn't this one look more coherent phase wise as I try various combinations of tone and blend controls? Still some there but not as much or am I imagining things? I may have a mis-valued part somewhere or silly mistake I overlooked in the build above, its almost always something silly so you guys can consider me a specimen on display as I troubleshoot. 

[samhay]

If you put a non-sine wave (i.e. a distorted/clipped signal) through a (e.g. RC) filter with a corner frequency low enough to filter out some/most of the high-frequency harmonic content, you get the behaviour shown in your 2nd and 3rd scope traces (more so in the 2nd).
The more harmonic content you have pre-filtering (i.e. more distortion), the more extreme this effect is likely to be, so my guess is that the tone control and/or gain/clipping controls were set differently in each case.

[karbomusic]

Thanks Sam, great info and that matches some of my testing.

Also, in the last video I'm changing tone and blend to most of their extremes. As long as I can keep the phase differences as small as that final video above I'm good because that is within my design goal. I'll find why this other build is different sometime this week and make the necessary adjustment. I've always been conscientious about keeping the blend knob from sounding like a smiley face looks (aka minimal volume drop due to phase cancellation when blending vs full ccw and cw).

Thanks again

[PRR]

> the control signal is top/pink, circuit output bottom/blue.

But you are triggering from the output, which is modified.

Trigger from the reference, the INput.

Now you will see the input steady, the output "lag" *and* "round-off" as the knob (or finger) is moved. This is just as expected: a high-cut will round-off *and* lag.

I don't see any phase lag which, by itself, gives me any concern for the audio.

Two "identical" builds which differ in this test is "interesting", shows something is not really identical, but is probably a minor stray capacitance such as choice of Outside Foil direction (or stray finger).

[karbomusic]

> the control signal is top/pink, circuit output bottom/blue.

But you are triggering from the output, which is modified.

Trigger from the reference, the INput.

Now you will see the input steady, the output "lag" *and* "round-off" as the knob (or finger) is moved. This is just as expected: a high-cut will round-off *and* lag.

I don't see any phase lag which, by itself, gives me any concern for the audio.

That makes perfect sense after a quick test, thank you!

Two "identical" builds which differ in this test is "interesting", shows something is not really identical, but is probably a minor stray capacitance such as choice of Outside Foil direction (or stray finger).

Very well could be. They have been consistent enough in the past that I may be able to find the reason once I get a chance to look closely; if I can't I'm not worried about it as long as it passes the 'play' test which I have no reason to doubt it will. Thanks again, good stuff.