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Old 6th November 2013, 02:06 AM   #11
Johno is offline Johno  Australia
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Top circuit- R5 = 3k9 would be fine, or you might consider simply using 2 resistors in parallel (same values as R1 and R2 of course) which will be very close to 3k63 and therefore give the lowest error.

Bottom image/circuit is not something I would recommend.

Recommend you avoid using trimpots - they can be unreliable in very low current signal paths like this.

Resistor noise is unlikely to be measurable with these value resistors and op amp gains. Obviously there are occasions with very high gain circuits that all noise sources need to be considered with usually high complexity solutions.
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Old 6th November 2013, 03:01 AM   #12
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i see
one last question just out of curiosity, assume that i am actually building the amplifier and decided to use 2 dual op amp chips (such as say opa2134/2227 instead of 4 opa134/227s)

lets call the 2 dual op amps X and Y, X1 meaning the first channel of X and X2 meaning the second channel of X and etc

for my left and right input channels, should i have it such that:
each channel for the op amp is used for the same stage for both input channel as shown
Click the image to open in full size.

or should it be that each op amp is used for a separated input channel with each channel for the op amp being used for two different stages of a single channel as shown?
Click the image to open in full size.

it seems to me the method on top would allow me to use different op amps for X and Y for a unique mixed sound (such as using AD8620 for X and OPA2227 for Y) whilst the method on the bottom would reduce crossfeed issues sometimes related to dual op amp chips

Last edited by thelegendaryblah; 6th November 2013 at 03:05 AM.
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Old 6th November 2013, 03:44 AM   #13
Johno is offline Johno  Australia
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We tend to design so that the op amps are not mixed across channels. Also makes physical separation of the layout easier if it was a large amp.
That said, I am not sure that I could pick the difference, either by measurement or listening, between mixed and not-mixed. Intuitively I guess cross talk noise and distortions reduce.
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Old 6th November 2013, 04:24 AM   #14
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i see
i will probably prototype the above circuit during the coming weekend on a breadboard with some jellybean tl072s and see what happens
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Old 6th November 2013, 05:16 AM   #15
Johno is offline Johno  Australia
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Good luck and let us know how it goes.
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Old 6th November 2013, 06:25 AM   #16
Mooly is online now Mooly  United Kingdom
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I'm happy to use one IC for both channels. You will get far more interaction from wiring and PCB print issues than from the opamps. You don't need the resistors connected to the non inverting inputs, certainly not with FET devices. All they do is raise the impedance and create a possible point for noise pickup. Link them out.
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Old 6th November 2013, 06:37 AM   #17
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Quote:
Originally Posted by Mooly View Post
I'm happy to use one IC for both channels. You will get far more interaction from wiring and PCB print issues than from the opamps. You don't need the resistors connected to the non inverting inputs, certainly not with FET devices. All they do is raise the impedance and create a possible point for noise pickup. Link them out.
i thought the resistors connected to the non inverting inputs are supposed to cancel out potential noise generated by the input resistors and feedback resistors at the inverting inputs

now im confused
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Old 6th November 2013, 08:20 AM   #18
Mooly is online now Mooly  United Kingdom
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Here's how it works... simplified

for bjt (transistor input) opamps there is a small but significant current that flows into or out of the two input pins. Its the opamp input bias current. That current develops a small volt drop across any resistors that connect to those pins. If the resistance seen by the two inputs is unequal then the input pins then have an unequal voltage between them. So in order for the complete opamp (with its feedback network) to work the output of the opamp must assume a voltage that will push or pull current through the feedback network to cancel that "input offset" out. The result of that is that the opamp out has a small DC offset voltage. For these kind of opamps we can minimise that by ensuring each input sees a similar resistance. Similar resistance... the error is the same for both inputs and cancels out.

FET opamps (such as the OPAxxxx) have zero bias current and so there is no possibility of DC offset occurring due to this input offset current that affects bjts. So there is no reason to have that extra resistance. All the resistor does is raise the impedance (how it looks at AC) of the pin and makes it more prone to noise pickup.
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Old 6th November 2013, 02:34 PM   #19
jcx is offline jcx  United States
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inverting op amp connection only "fixes" one of the least important distortion mechanisms

simply using low, matching Z for both input and feedback already reduces most op amp common mode input nonlinearities in noninverting circuits

and sometimes you really want the high input Z of noninverting circuits


there is still i_b with fets - just very low, even CMOS op amps need DC path to both inputs to make up leakage currents

Last edited by jcx; 6th November 2013 at 02:37 PM.
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Old 7th November 2013, 04:45 AM   #20
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i see, so would it be safe to assume that most modern op amps would not have problems with DC offsets since most modern op amps are mosfets?

might be a noob question, but what is Z? and i thought currents flowing through the base of a fet should be low enough to be negligible
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