Balanced line driver (DRV134) and noise - Page 4 - diyAudio
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Old 16th October 2011, 05:49 PM   #31
gootee is offline gootee  United States
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Originally Posted by Esperado View Post
Can-you explain the difference at normal line levels ? They are in serial. Currents are the same in both topologies. Or did-you believe in some magic fighting against the physical laws ? Or Op Amps sympathies to his nearest neighbor ? If it is about noise at very low level can-you provide difference in noise measurement in a well shielded housing and real situation?

The best sounding way to manage variable resistances is ... to not use them: tune, measure their values and replace them with a metallic resistance of the same value.
I believe that GK's point was that one should minimize the lead length (and thus the stray capacitance) seen by an opamp's inverting input, to prevent oscillation (and ditto for the instrumentation amp's gain-setting pins).

And if one desires to use a potentiometer or switched resistors, connected to an opamp's inverting input or to an inamp's gain-setting pins, then the pot or switch would typically have to be located some distance away from the chip's pin(s) and in that case a fixed resistor should be placed in series with and very close to the pin(s), to isolate the pin(s) from the stray capacitance of the relatively long wires or traces.

I do agree that it might be best to avoid configuring an opamp for variable gain, using an upstream attenuator, instead.

In the case of the inamp circuit shown in post 27, the switched R values shown would limit the series resistance at each pin (1 and 8) to 4.5 Ohms (while replacing the 9R with zero Ohms and lowing the others by 9 Ohms), which might not be enough but would probably be better than nothing.

Perhaps the inamp's gain could be fixed and an upstream balanced switched attenuator could be devised. But that seems like it might be difficult to implement.
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Old 16th October 2011, 06:17 PM   #32
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Thank-you for this explanation, i agree.
In my mind, it is more about layout design of the circuit board than witch of two serial elements should be placed first, as some kind of law ;-)
Well, i use exclusively since years current feedback op amps for audio, they are very stable with a good phase margin, even at unity gain, and i was never in concern with stray capacitances issues, despite there >10Mhz bandwidth. So i never asked myself the question, looking at square waves at 100 Khz. of course, when you're about to design a first stage mic preamp, you enter in a quite different world of thoughts and precautions.
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Old 16th October 2011, 06:51 PM   #33
AndrewT is offline AndrewT  Scotland
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Most opamps will not tolerate capacitance on the -IN pin.
Isolate that pin from parasitic capacitances by mounting resistor/s to the PIN or very close to the PIN.
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Old 16th October 2011, 07:00 PM   #34
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Most opamps will not tolerate capacitance on the -IN pin.
Isolate that pin from parasitic capacitances by mounting resistor/s to the PIN or very close to the PIN.
What about the mixing bus of a desk ?
;-)))
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Old 16th October 2011, 11:20 PM   #35
AndrewT is offline AndrewT  Scotland
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Show us how they do a mixing desk if you think that helps our understanding.
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