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OPamp "summing" question

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5/18/2000 5:28 AM

OPamp "summing" question

Vetrans,
which is most preferable when "summing" or
"mixing" with an opamp- The (-)inverting, or
(+)noninverting input?
Every mixing schem. I've seen uses the (-)
input, (but I think this is because the circut
adapts to "gain" adjusting easier, via pots or
resistors)
If you just wish to "re-combine" equal
amplitude, and phase, signals- Does it matter?
I hope this is clear enough.
Thanks in advance, Ed R.

5/18/2000 7:33 AM

Ed,

The "Best Way" of doing this depends on the circuitry before and after the summing junction.

There are at important things to consider when mixing signals:

1. Do you want the final phase to be in-phase (use + input) with the input or out-or-phase (use - input) with the input signals?

2. Does the input impedance need to be high (use + input) or is lower OK (using - input)?

3. Do you want to control the volume of each input individually, or just a master volume? This will affect your circuit topology.

4. Will you be using dual supplies or a single supply for the opamp (using the - terminal will save you a capacitor in this case)?

I guess my overall point is that either topology will work, as long as your requirements are not too critical. If you reply to the questions above, I may be able to guide you better. Keep in mind that a number of amps (Music Man, Peavey, etc.) use opamps all over the place in various configurations, but your case may be different than their design considerations.

I have read and heard that using (-) terminal introduces less noise than using the (+) terminal.
This makes sense, since input resistances are usually higher when using the (+) terminal, which makes for a noisier output from the opamp. My personal experience has proven this to be true, but in many cases the added noise is acceptable.

I would like to help if I can. Feel free to post a reply to this if you think I am being of any help to you.

Best of Luck,

Reverb

5/18/2000 1:27 PM
paul perry	This is treated well in Boscorelli's book, THe Stompbox Cookbook. Really, 'it depends on everything', what compromises you make. There is no simple answer to this.

5/18/2000 2:54 PM
peter	there is a reason why the inverting input is used: the negative feedback connection works to put the (-) input at "virtual ground", so there is no leakage between input signals, and the impedance to ground for each signal is constant. If you sum at the (+) input the voltage at the input is driven up and down by the signals so the impedance seen by the signal sources varies.

5/18/2000 4:23 PM
Joe Fuzz	Ed: the inverting and non-inverting op-amp amplifier circuits are 2 completely different animals. Stick with the inverting circuit for summing.

5/19/2000 12:35 AM
Ed Rembold	Re: OPamp "summing" question (Thanks) Thanks Guys. I know there is no "simple" answer that applies to All cases. I was just looking for "rule of thumb" or as my father-in-law used to say- "According to Hoyle!" Thanks to each one of you, Ed R.

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