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Rather simple cv-summing question
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zipzap



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PostPosted: Wed Feb 01, 2006 8:01 am    Post subject: Rather simple cv-summing question Reply with quote  Mark this post and the followings unread

Hi. I was just wondering about different resistor sizes in op-amp summing circuits. Ive attached one of my favorites, think i maybe even found it in this forum. Another example would be the standard mixing input section of, say an vco.
Does it matter at all which resistor size i use? If i build the attenuverter with 100k resistors, 100k pot and one 50k r, wouldn´t it do just the same thing? Is the handling different?
For audio mixing, what size should be used (i guess noise might be a concern with larger rs)?
cheers


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mosc
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PostPosted: Wed Feb 01, 2006 11:20 am    Post subject: Reply with quote  Mark this post and the followings unread

The attenuator network you are using is a fancy example. If you change the values of any resistor, you should change all others by the same factor or percentage.

In audio mixer applications, it is OK to use anthing from 10K to about 100K. Much lower and you start using too much current, and much higher and you start getting noise problems. What's important is the ratios. Mixers generally have unity gain at the summing amplifiers.

In order to get the unity gain, make sure the summing resistors are the same size as the feedback resistor that goes from the negative op-amp node to the output.

The IC Op-amp Cookbook by Walter Jung is a great source of information about this topic. Highly recommended.
http://www.amazon.com/gp/product/0138896011/104-6772110-1387166?v=glance&n=283155
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zipzap



Joined: Nov 22, 2005
Posts: 559
Location: germany
Audio files: 24

PostPosted: Wed Feb 01, 2006 7:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

shure, i didn´t make myself clear enough. I want to change all resistors and keep the amplification the same. I have a big little pile of 100k resistors here and i´m not sure if i want to use them for all my summing apps.(and order a pile of 100k pots) thats why i´m askimg. If you look at different shematics you see 10 to 100k, just as you said. For cv its probably no big deal, but with audio it might be... Does 100k resistance give 2x the noise of 50k? It´s to late right now, but i think i´ll do some mesurering tomorrow.
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mosc
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PostPosted: Wed Feb 01, 2006 8:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

zipzap wrote:
shure, i didn´t make myself clear enough. I want to change all resistors and keep the amplification the same. I have a big little pile of 100k resistors here and i´m not sure if i want to use them for all my summing apps.(and order a pile of 100k pots) thats why i´m askimg. If you look at different shematics you see 10 to 100k, just as you said. For cv its probably no big deal, but with audio it might be... Does 100k resistance give 2x the noise of 50k? It´s to late right now, but i think i´ll do some mesurering tomorrow.


The 100K resistors will do just fine. There will be no where near 2X the noise of 50K. The noise calculation is quite complex. I would not worry about it at all. If you were using 1 M or 10 M then maybe there might be some concern, but the newer low power op amps perform much better in this respect that the older ones. Really, it's not a thing to be concerned about.

If you use 100K summing resistors, just be sure to use 100K feedback resistors too. I'm sure that is your intension. Then the gain will be the same. Put a 10 pf or 20 pf capacitor across the feedback resistor. This is sometimes called a compensation capactor. It will reduce the chance of your circuits from picking up noise from close radio transmitters or other sources of RF interference.

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