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Old 9th December 2012, 11:02 AM   #11
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by madtecchy View Post
I have tried the t network on my inverting lm3886. The extra connection to ground seems to inject ground noise into the input... Any comments?
Looks like layout/wiring issues, not a conceptual problem. T-network has a slight noise penalty in general, though. And higher than unity gain at DC with either only input coupling capacitor or T-network cap (in the GND leg).

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Ps can get 10k input impedance without t network in inverted mode
Yes, that means 100k++ feedback resistor which is a bit too high for my tastes. More important is that any inverting confic should have a capacitive feedback divider (to GND rather than -Input), too, to keep it stable with open or high-Z input termination.
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Old 9th December 2012, 11:55 AM   #12
AndrewT is offline AndrewT  Scotland
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Could either of you post typical schematic to show what your are referring to?
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Old 9th December 2012, 05:11 PM   #13
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Thanks for the help guys. Thats answered my question...
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Old 9th December 2012, 05:11 PM   #14
KSTR is offline KSTR  Germany
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Inverting amp with T feedback network
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File Type: gif inv_with_T.gif (2.4 KB, 121 views)
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Old 9th December 2012, 05:30 PM   #15
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KSTR..

I use a computer and an external dac.. I tried the T network on an already built and in service inverting lm3886. I had no issues with it at all. I read about the t net feedback idea and i gave it a try as it is simple to implement . and with the t network computer noise is audiable.. I have no stability issues with just a single feedback resistor . 220k
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Old 9th December 2012, 05:41 PM   #16
KSTR is offline KSTR  Germany
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If it's runng OK (also in terms of offset) with 220k that's fine. But what happens if you happen leave the input open circuit? Unless you've taken measures against it you'd have unity gain then and the circuit would most certainly oscillate.
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Old 9th December 2012, 05:53 PM   #17
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In my case leaving the input open circuit is not an issue as it is hard wired to the dac. The dac is buffered after the volume pot so the offset stays the same with diffrent volume settings too. The dac is not a diy unit.. i have had the lid off and done a little looking about i couldnt resist... KSTR you have raised some very good points and also given the solutions .. If i where to build a standalone amplifier i would implement them.
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Old 9th December 2012, 06:09 PM   #18
KSTR is offline KSTR  Germany
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That's simply because I'm in the (slow) process of designing an inverting nested/composite chipamp where stability at all working conditions is the paramount issue.
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