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Op amp mixer circuits - to invert or not to invert

 

I've been looking at circuits for audio mixers using op amps.? Typically, they sum several signals to the op amp's inverting input, and mention that one can also use the non-inverting input.? One, in particular, went on to say that if you are planning to then mix several of these together you probably want to run them through an amplifier that inverts them again, so that you know that all your inputs will be in phase.

Or you could just stand on your head!

My question is:? Why not just use the non-inverting input in all cases, and avoid the whole question of whether the inputs/outputs are in phase or not?
 

Filter circuits do not always non-invert. There is a trick to use an opamp in an inverting mode if only a positive voltage is available. The method involves establishing a reference voltage that is 1/2 of the rail voltage as a virtual circuit return. Then work off that level as if it is the circuit zero-level. Then reference back to proper circuit return. I have performed this trick several times. Key is to have a good reference to work with.

DerekK
 

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The advantage of driving the inverting input input of the op amp is that the source always sees the resistive input to ground, provided the amp is operating within its limits. Driving several sources into an op amp in the non-inverting configuration will cause each source to see some of the other sources' signals. So inversion makes mixing much easier. BUT - now the signal is inverted and if it is to be otherwise mixed further downstream, re-inverting so that all signals coming together are of similar phase would be wise.

HTH,
Donald.

On 2021-11-29 11:05 a.m., KI7MWA via groups.io wrote:

I've been looking at circuits for audio mixers using op amps.? Typically, they sum several signals to the op amp's inverting input, and mention that one can also use the non-inverting input.? One, in particular, went on to say that if you are planning to then mix several of these together you probably want to run them through an amplifier that inverts them again, so that you know that all your inputs will be in phase.

Or you could just stand on your head!

My question is:? Why not just use the non-inverting input in all cases, and avoid the whole question of whether the inputs/outputs are in phase or not?

 

Isn't an analog mixer a linear time invariant system? If a source seeing other sources mattered, we couldn't use superposition to solve circuits - we'd have to account for second order effects (linearity would be violated).?

Could you elaborate on what you mean? Sorry my analog is...extremely rusty.?


On Mon, Nov 29, 2021, 9:40 AM Donald H Locker via <dhlocker=[email protected]> wrote:

The advantage of driving the inverting input input of the op amp is that the source always sees the resistive input to ground, provided the amp is operating within its limits. Driving several sources into an op amp in the non-inverting configuration will cause each source to see some of the other sources' signals. So inversion makes mixing much easier. BUT - now the signal is inverted and if it is to be otherwise mixed further downstream, re-inverting so that all signals coming together are of similar phase would be wise.

HTH,
Donald.

On 2021-11-29 11:05 a.m., KI7MWA via wrote:
I've been looking at circuits for audio mixers using op amps.? Typically, they sum several signals to the op amp's inverting input, and mention that one can also use the non-inverting input.? One, in particular, went on to say that if you are planning to then mix several of these together you probably want to run them through an amplifier that inverts them again, so that you know that all your inputs will be in phase.

Or you could just stand on your head!

My question is:? Why not just use the non-inverting input in all cases, and avoid the whole question of whether the inputs/outputs are in phase or not?

 

Of course, there may be some confusion because an "analog mixer" for audio signals is really using a summing amplifier, not mixing as in modulating one signal with another. So not the same thing as what we call a mixer in a radio, for instance.

I will stick with calling it an analog mixer, unless I am speaking of the actual op amp circuitry.

Whether or not you need to preserve phase polarity depends very much on what you are feeding into this. Two microphones in the same room, perhaps on a lead and backup singer, you may want to preserve the phase polarity, or you may even want to be able to invert one.

If it is two entirely separated signals, perhaps from lead, rhythm, and bass guitars where none can pick up anything from the other, phase polarity may not matter and in fact can't be guaranteed due to FX pedals inline.

--
Steven Greenfield AE7HD
 

re: "Why not just use the non-inverting input in all cases, and avoid the whole question of whether the inputs/outputs are in phase or not?"

That's a good question.

If you are mixing signals and then further downstream comparing them in some way with other signals, then you'd want to preserve the phase.? ("Phase" is not the right term, it's really polarity, but you get the idea.)? As an example, if you mix 4 signals in mixer stage? A, and then mix 4 signals in mixer stage B where one of them is the output of stage A, then you would probably want to preserve the phase so that the inputs to stages A and B don't end up out-of-phase with each other.? That's important if the signals come from microphones, because each mic can pick up some sounds that the other mics get, and having out-of-phase mics on the same stage can be a disaster.

In professional audio, they take care to make sure that polarity (or "phase") is preserved, between input and output.? That way you know that if you add a device into your audio signal chain, it won't mess it up.? It is really bad if you invert the Left channel signal but not the Right channel too.

But the other question is, does absolute phase matter?? If I invert the polarity of a single audio signal, can I hear the difference?? The answer appears to be, yes, to a limited degree.? Many experiments have been done over the years, which show that our ears can hear the difference, in SOME cases.? When you hit a drum, that initial burst is a positive increase in air pressure, and that's how we should hear it.

Having crappo speakers probably just scrambles the phase and then it might not matter so much.? But with a good audio system ("high fidelity"), and good clear sound sources to begin with, and sounds that are familiar to us, it's been shown in blind A-B tests that we can hear the difference.

The advantage of using an inverting amp as a mixer is that there is (ideally) no interaction between the inputs.? In this case, interaction could mean that some of input #1 feeds back into the other input signals, which might be a bad thing for those other signal?sources (e.g., unwanted crosstalk).? Also, if I have a volume control on input #2, as I change that volume control (typically a potentiometer), it changes the impedance feeding input #2 which also affects the levels from input #1 slightly.? That could be a little annoying.? If it's an inverting mixer with a summing point that is a virtual ground, none of that happens.

Andy

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