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Old 16th August 2014, 08:06 PM   #1
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Default Low pass filter for Bitstream DAC.

Hi.

I have created DAC using UDA1320, it works normally, but I feel that it sounds a bit better (softer) with LPF.

I already have passive 1st order LPF (300R+10nF), because this IC gives relatively high output level.

Is it worth to make sth more sophisticated (I have at the disposal NJM2068 or NJM4580 for active filter), or in this case that easy solution (passive 1st order) will be the best?

Also I can make output stage on triodes (ECC88, E88CC), but does it makes sense to do it with this "low cost" DAC?
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Old 17th August 2014, 01:26 AM   #2
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I'd suggest a passive (LC) low pass filter. Having 300R in the series arm makes the RC LPF a bit difficult to drive at HF for a normal opamp, bitstream DACs do put out a fair amount of ultrasonic energy. An inductor's easier to drive.
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Old 17th August 2014, 10:24 AM   #3
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Could You recommend any well-functioning schematic?
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Old 17th August 2014, 01:15 PM   #4
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Here's a schematic and plot - the 33mH inductor is by Fastron, available from Mouser.
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Old 17th August 2014, 03:21 PM   #5
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Thanks, can it be something like this?
COIL0810-33 FERROCORE - Inductor: wire | TME - Electronic components


Meanwhile I run this stage using ECC88 (other heaters), because I had almost all needed parts.
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I'm testing it, but mains hum is a little too big, maybe 22uF filtering capacitor Isn't enough?
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Old 18th August 2014, 01:18 AM   #6
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You could certainly try with that inductor - it is different from the one I recommend in two respects. First its unshielded so may well pick up hum and noise from adjacent circuits. Second the DC resistance is considerably higher (90ohm vs 26). The Fastron details are here :

07MFG-333J-50 Fastron | Mouser
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