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-   -   Input capacitor requires atleast 20uf? seen in TI spice sim on LME49811 (http://www.diyaudio.com/forums/chip-amps/225037-input-capacitor-requires-atleast-20uf-seen-ti-spice-sim-lme49811.html)

rhythmsandy 5th December 2012 01:05 PM

Input capacitor requires atleast 20uf? seen in TI spice sim on LME49811
 
I was just simulating the TINA TI for LME49811 and found that smaller values like 2.2uf with series resistance of 247ohm is limiting the lower -3db response to around 300Hz?

http://imageshack.us/f/543/inputn.jpg/
Imageshack - inputn.jpg

using 20uf input cap its giving -3db around 30Hz but I see most of the circuits dont have beyond 4.7uf... is there any other way to get 30Hz -3db?
I tried using 47k by connecting the output of the cap and ground but still it doesnt get such lower values...

Robert Kesh 5th December 2012 01:25 PM

is that an inverting configuration?

anyway, 2.2uF and 240 ohms gives about 300Hz, so whadya expect.

your input impedance is also very low.

If you want to use an inverting config you are going to have to up those values a lot. I imagine the other schematics you looked at were non-inverting.

rhythmsandy 5th December 2012 03:36 PM

the following once posted by Andrew..
----------------

Cin>=1.4 * Ci * Ri+Rs / Rin >= 1.4 * 10 * 57k8 / 1k8 >=450uF

Ci could be reduced to 1u5F or 2u2F and still pass sufficient bass.
Cin @ 10uF will kill the bass response.

_____________________________________________

in this thread. http://www.diyaudio.com/forums/solid...lmore-amp.html

but how come 10uf will kill the bass response?

Robert Kesh 5th December 2012 04:31 PM

OK I see the schematic in the other thread.

Could you post all of your simulated schematic, as they don't seem to be the same. At the moment it is covered up by the response curve.

rhythmsandy 5th December 2012 04:42 PM

Imageshack - snam120.jpg

Robert Kesh 5th December 2012 04:58 PM

Your simulation seems to be getting the frequency response of the inverted input Vin, which now passes through 10uF and 243 ohms, not Vcm. The original in the other thread has a non-inverted input, and Cin and Rin, though labelled as "in" are actually part of the NFB voltage divider.

I wouldn't apply something about one configuration to a different configuration.


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