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previous: Ed Rembold Vetrans,which is most prefe... -- 5/18/2000 5:28 AM

Re: OPamp "summing" question

5/18/2000 7:33 AM
Reverb	Re: OPamp "summing" question 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 7:33 AM

Reverb

Re: OPamp "summing" question

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