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Old 14th April 2010, 06:19 PM   #11
AndrewT is offline AndrewT  Scotland
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the two summing resistors go to the -IN pin, the inverting input.
The feedback also goes to this -IN pin.
The +IN pin should see the same impedance as the -IN pin to minimise output offset and offset drift with opamp temperature.
If you use three 10k resistors on your inverting summer, the -IN pin resistance is 10/3 k and the resistor load between +IN pin and Audio Ground should be ~3k3 for minimum offset and drift.

Why are you using opamps on the inputs to the pre-amp?

If the outputs of the pre-amp need buffering then the opamps should all be on the outputs not the inputs.
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Old 14th April 2010, 06:40 PM   #12
Mooly is offline Mooly  United Kingdom
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Good point Andrew makes about the offset problem, particularly as you are using electronic switching. Any difference in DC levels as you switch them will cause clicks and thumps in the audio.
If you use FET opamps (which I did...OP2604) there is no problem as the input currents are virtually zero (thus not developing a DC offset across the input resistances) . If you use bipolar types such as NE5532 etc then yes, it's a real issue.
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Old 14th April 2010, 06:43 PM   #13
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Hows this:

Click the image to open in full size.

Can pretty much use exactly the resistors u said as well. What would an inverting 0 gain amp circuit look like? Because I will have to invet the signal again wont I?

As for the first set of op amps, I'm just following this design - High Quality Audio Preamp

Those first op amps actually provide a gain of 6dB =]

Dan

Last edited by dannybeckett; 14th April 2010 at 06:58 PM.
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Old 14th April 2010, 07:05 PM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
the two summing resistors go to the -IN pin, the inverting input.
The feedback also goes to this -IN pin.
read this again.
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Old 14th April 2010, 07:06 PM   #15
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my bad
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Old 14th April 2010, 10:39 PM   #16
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Will the circuit not work the way I have illustrated?
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Old 14th April 2010, 11:08 PM   #17
Xoc1 is offline Xoc1  United Kingdom
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The point is when used as an inverting op amp, the inverting input that is connected to the input signal resistors, and the feedback loop resistor is always kept at a very low signal level (practically zero) by the feedback applied to the op amp. This VIRTUAL EARTH is what stops the input signal from one channel bleeding across to the other.
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Old 14th April 2010, 11:21 PM   #18
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I understand. How's this -

Click the image to open in full size.
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Old 15th April 2010, 07:21 AM   #19
Mooly is offline Mooly  United Kingdom
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Yes... and if it's a FET opamp the 3K3 isn't needed and can be linked out.
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Old 15th April 2010, 09:53 AM   #20
AndrewT is offline AndrewT  Scotland
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Beware: this is a summing amplifier.
If R=R=R then Vout = V1+V2.
The mono signal is nearly twice the voltage of each of the stereo signals.
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