low pass filter for DAC output

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#42
To avoid opamps completely you'll need either a discrete voltage gain stage, or alternatively use a common-base stage at the DAC's output which allows a much greater voltage swing. For a bipolar output DAC such as the PCM1704 the latter option is a little tricky. I suggest that you do use an opamp - AD815 is my choice as its wide bandwidth and very low noise and its output can be biassed into classA with CCSs.

<edit> Just to clarify - this AD815 opamp is not being used for I/V duty.. Rather its amplifying the band-limited audio signal which comes from the passive filter.
 
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#51
I can't figure out what you're trying to achieve here. Those inductors are bulkier than the TDK ones (10mm dia vs 7mm) but they're also lower DCR. You're wanting to go for a higher I/V resistor and hence need to put two inductors in series? I'm confused....
 
#55
Selection is because the original circuit has different values for the input and output inductors vs the inductors 'in the middle'. 20% tolerance means the actual value can vary from 540uH to 820uH.

Shielded is strongly recommended if you don't like to listen to hum in your audio....
 
#56
Does that mean you are not concerned with the change in Inductance due to tolerance? Or you want to get few more than needed to get some closer to required value, say 660uH and 600uH? Or is it because practical circuit has different inductor values in each pole?
 
#57
With a few spare inductors and an inductance meter its probably possible to get close enough values to the original circuit - if all inductors turn out to be on the low side (for example) we can adapt to that by lowering the I/V resistor a bit. If you just buy inductors and build with randomly selected ones there's the distinct possibility that the FR won't be very flat.

To me it seems a waste to spend a small fortune on the DAC chips and then leave the FR to the chance selection of a few much cheaper inductors. 'Ships and ha'peth of tar' springs to mind.
 
#59
I've built a few active filters in my time - the most recent was an active elliptic (shown on my blog). I found with that one that the power supply was the thing that I had to work on to get the best (i.e. most dynamic) sound. Originally I had thought that a passive filter was going to be more bulky than an active one, but the realization that the active filter's power supply would need lots and lots of capacitors in it caused a major rethink.

So in a nutshell - the problem with active filters is getting a low impedance, low noise power supply. Passive filters don't need power supplies and the best power supply is no power supply at all 😀
 
#60
abraxalito said:
So in a nutshell - the problem with active filters is getting a low impedance, low noise power supply. Passive filters don't need power supplies and the best power supply is no power supply at all 😀
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That is true. That reminds me of something I know you are working on my DAC's IV and line stage ,is it possible that the line stage uses 5volt (+/-). I can avoid 8volt from my board altogether if that happens. One less power supply headache.
 
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