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2x inverting amps vs non-inverting amp
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author 2x inverting amps vs non-inverting amp
nurbivore
I'm looking at various mixer schematics, and a common pattern I see is using 2 inverting opamps in series to sum and "uninvert" the signal.

Also, in most "intro to opamp" texts that I've seen, the inverting config seems to be treated as the default. The non-inverting is sort of an afterthought, without any discussion of tradeoffs other than just the phase of the output.

Why would you do this instead of just using the opamp in a non-inverting configuration?
guest
the inverting amplifier is a better summing amplifier than the non-inverting. it has less cross-talk, noise, offset issues, etc., but it does invert, so you typically follow it with another inverter to get the phase right. you can use non-inverting amplifiers as a mixer, but you also need normalled jacks in this case or the gain will change with the number of inputs connected.
mskala
In addition to the points guest mentioned, with non-inverting configurations the DC voltage at the op amp inputs can vary, and you have to pay attention to the range over which it can vary and make sure that's not going to be a problem. For instance, with the common TL074, the inputs have to stay at least 4V higher than the negative rail, which would be -8V in Eurorack. With unity gain buffers and some other non-inverting configurations, you feed in a voltage that's below the negative limit and it misbehaves. In an inverting configuration, the inputs stay at 0V and this isn't an issue.
gooooob
Hmm where did you get the 4V above the negative rail number from?

I tried to investigate this, because I ran into problems with my breadboard circuit. But my final result was 2V above -12V worked fine for a non-inverting buffer (no feedback resistor). At -10.4 the output went to the positive rail.

Was I lucky with my TL074s? Should I change my design?
mskala
gooooob wrote:
Hmm where did you get the 4V above the negative rail number from?


"Recommended operating conditions" for common-mode voltage in the TL074 datasheet, page 10: http://www.ti.com/lit/ds/symlink/tl072.pdf

It's quite possible that any given individual chip will work over a wider range... Texas Instruments just doesn't promise that it will.
CLee
The inverting op amp can do actual summing at its inverting input. It's at a virtual ground so the mixer is summing all currents connected there. A non-inverting input is a high impedance connection, so if you use that pin as a mixer you're really creating a buffered passive mixer. The behavior will change based on what's connected to it.
stringsthings
In addition to the above points, a typical 8-pin DIP package has 2 op amps. 14-pin = 4 op amps. So pairing up op amps in a circuit is logical.
nurbivore
Cool, thanks everyone.
clorax hurd
Also non-inverting configuration cannot attenuate voltage. Can be used only as buffer stage or gain stage.
But normally not an disadvantage (either 2 resistors to voltage divide signal before non-inverting stage, or 2 resistors used for creating inverting one).
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