Mixing Opposite Polarity Signals

[JFace]

Is it possible to mix two signals that are out of phase using one op amp?

I am mixing a clean sound with a basic distortion that has a JFET amplifier at the end of the path. I was hoping to set the level of each signal with a separate potentiometer, and have these signals summed using a comparator. The distortion would go into the non-inverting input and the clean going into the inverting input. I chose this implementation because it is easier to have a high impedance input on the (+) terminal without introducing excess noise due to a large resistors in series with the signal.

Is the basic idea correct? I've done this before with success, but I've breadboarded a similar circuit, and now I'm having unexplainable crosstalk between the signals. Now I am questioning if this is a valid way to mix the signals, and I just got lucky the first time. Do I need to flip the phase of one of the signals and use a standard inverting op amp to mix the signals?

[blackieNYC]

Not really optimal.  But see the controfuzz schematic.  It does a little more than mixing, taking advantage of the inverted polarity.  (The note will transition from half clean to full fuzz over time. See Mark Hammers explanation.
I think you really want the amz simple mixer. Two series 100k Rs prevent leakage, inverting input to the op amp. If you have a polarity issue with any pedals you might mix, the other half I the op amp can be The geofex polarity reverser. I think the buff n blend mixer might incorporate that. Each input can have a 10k pot to ground, or, if you have complete assembled pedals to mix, which is cool, you can just use their individual output level controls

[JFace]

Just to clarify a bit more, this is a prototype design, not a blender pedal. I would want to minimize the number of op amps and transistors. In addition to the distortion circuit, I am using one TL072; one half buffers the clean signal, the other half would ideally do the summing of the signals. The signals are definitely out of phase, and need to be in phase on the output.

[Gus]

Have you looked at
"Electronic Projects for Guitar"

[drolo]

Have a look at the Mutron Phasor II schematic. It has a similar setup, where the clean signal in this case goes to the non-inverting and the wet signal goes to the inverting input. But it should work the other way around as well.

I did the same in an adaptation of that circuit here:

http://www.diystompboxes.com/smfforum/index.php?topic=107010

Just that I added a mix pot using the values found in the Geofex article panning for fun

It worked well in this situation.

[blackieNYC]

I think I see what you mean.  But feeding inputs into the + and the - will give you some cancellation, no?  Correcting the phase coherency and then summing those two signals would be a very different sound. Louder, more robust I would think.

[drolo]

I think I see what you mean.  But feeding inputs into the + and the - will give you some cancellation, no?  Correcting the phase coherency and then summing those two signals would be a very different sound. Louder, more robust I would think.

It would only cancel if the 2 signals had the same phase polarity.
If I understood the original post right, one signal is out of phase to the other.
So let's say signal 1 is - and goes to the - input : - - = +
The signal 2 is + and goes to the + input : ++ = +

So they are added.

(at least it's how I understood it)

[JFace]

I think I see what you mean.  But feeding inputs into the + and the - will give you some cancellation, no?  Correcting the phase coherency and then summing those two signals would be a very different sound. Louder, more robust I would think.

It would only cancel if the 2 signals had the same phase polarity.
If I understood the original post right, one signal is out of phase to the other.
So let's say signal 1 is - and goes to the - input : - - = +
The signal 2 is + and goes to the + input : ++ = +

So they are added.

(at least it's how I understood it)

Yes, that's what I mean, and how I understand it. Now I just need to figure out why there is crosstalk. I'm wondering if having a high impedance source is at fault (the JFET amplifier). It isn't too difficult to replace the jfet with a non-inverting opamp that accomplishes the same gain as the JFET, and then sum the two phase coherent signals.

[electrosonic]

Cant you just use a differential amplifier.

http://www.electronics-tutorials.ws/opamp/opamp_5.html

Look at the schematic for the rocktave and see how the octave down and the clean signal are mixed.


Andrew.

Hope that makes sense I am posting via my phone

[JFace]

Cant you just use a differential amplifier.

http://www.electronics-tutorials.ws/opamp/opamp_5.html

Look at the schematic for the rocktave and see how the octave down and the clean signal are mixed.


Andrew.

Hope that makes sense I am posting via my phone

Yep that's what I'm using (I called it a comparator). I'm just trying to figure out why the signals are crosstalking when I set their volume level to 0.

[drolo]

When you say to 0, is it to ground or to Vref ?

[JFace]

When you say to 0, is it to ground or to Vref ?

Ground. Lug 2 of the pots goes through a 10u electro to block the DC from the op amp.

[antonis]

Yep that's what I'm using (I called it a comparator). I'm just trying to figure out why the signals are crosstalking when I set their volume level to 0.

Maybe because of the comparator's input offset voltage...
(try to put some hysteresis - Schmitt trigger type..)


or maybe I need some rest..

[Bill Mountain]

If one half of the 072 buffers the clean signal then can't that be a simple inverting buffer?

[JFace]

If one half of the 072 buffers the clean signal then can't that be a simple inverting buffer?

Yes, there are multiple fixes. I am more curious as to why there is crosstalk. Also I prefer non-inverting buffers on the input of an effect. To get high input impedance out of an inverting buffer, one has to use high value resistors in series with the signal, and in the op amp feed back path, and this could increase the noise.