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Old 25th March 2014, 01:45 PM   #21
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OK, I found some site which let to calculate a LC -filter by few given paramaters. I picked a Chebyshev 8th order filter (ripple 0.01, corner 48kHz).

1. I placed the LC -filter just before final output and made some simulations, then
2. moved it to the input part just after the load components (i.e. before the input to LT1028) and made same simulations. Results attached.

Which is the right place for LC -filter, 1 or 2 or is it better to be somewhere else?
Attached Images
File Type: png phono_pre_v0.25_#1.png (27.7 KB, 80 views)
File Type: png phono_pre_v0.25_#2.png (25.8 KB, 78 views)
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Old 26th March 2014, 08:16 AM   #22
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NOTE: FFT windowing type isn't equal in above two pictures --> transient analysis result for #2 (shown in picture 2) is equal with the other one --> placing the LC-filter effects in noise only
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Old 26th March 2014, 09:37 AM   #23
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The LC filter is supposed to be on the supply rails of the LT1028. Therefore there will be two, one for each polarity of supply. I'm not sure any difference will show up in simulation as few opamp macromodels handle supply rail currents correctly.

I remembered that LC filtering was used on the USB supply rail to the ODAC but when I went to link to the schematic, I got an error as if that pdf had been moved. Hence no example to show you.
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Old 26th March 2014, 10:02 AM   #24
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Quote:
Originally Posted by abraxalito View Post
The LC filter is supposed to be on the supply rails of the LT1028. Therefore there will be two, one for each polarity of supply. I'm not sure any difference will show up in simulation as few opamp macromodels handle supply rail currents correctly.

I remembered that LC filtering was used on the USB supply rail to the ODAC but when I went to link to the schematic, I got an error as if that pdf had been moved. Hence no example to show you.
Thanks!
Now that I know where to implement the filter I'll keep searching if there are some examples available.
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Old 26th March 2014, 10:21 AM   #25
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About those noise results I got ... are those OK (different units between LTSpice and LT datasheet) ? Got ~221 nV/Hz for 1kHz (measured between 999.99Hz-1.01kHz).

Couple noobie questions about voltages and currents:
- should I use cartridge output voltage (2.0mv 1kHz 5cm/s (or 5.6mV 1kHz 10cm/s)) or measured turntable output voltage as a parameter in SINE() command (SINE(0 0.002, {freq})?
- which would be suitable output voltage (now it is 200mV (LT1010 outputs the same))
- which would be suitable output resistance when my intention is to connect the pre-amp to USB audio interface which offers three different type inputs ... 1,5k mic-preamp inputs, 10k line level inputs and 1M HiZ inputs?
-- how this final output resistance can be measured in LTSpice and if needed, how it is constructed/adjusted
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