What would be a sensible set of filter characteristics for DAC - Page 2 - diyAudio
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Old 9th July 2009, 08:22 PM   #11
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Hi Martin.
Yes, I must visit and show you the latest installment of my comedy DAC

Well, I've managed to get it to work with faster opamps, but I'm concerned by the solution.
There was a nasty crackling noise in both channels with faster opamps (JRC 2114 was fine, but the LT1361 or LM4562 had this noise).
I added 100nF caps underneath the sockets as further decoupling. That didn't actually help.
Then, by accident, I discovered that if I put 100nF between gnd and just one of the inputs to the opamp IV stage, the problem is solved! So, essentially the same as a cap across AOR and GND, but at the other end of the cable. I didn't need a cap for each channel, just the one.
Any idea what could be going on there? I wonder if the problem is at the DAC end, rather than the IV stage.
Anyway, the noise is fixed. I have LM4562 for IV conversion, 2x OPA2107 for the filter, and a further LM4562 for the output stage. It sounds truly awesome (to me anyway). It is so much better than it was. I had no idea how bad my output was previously (in my defence, I had just knocked it up simply just to get it working).
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Old 9th July 2009, 08:32 PM   #12
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Well, if that cap works, it works: my bet would be it shortens some kind of 'long-way-round' at HF, ho ho. Need a scope, or a long stare at the reality, to check though. Do let me know when you are free, the calendar is sadly empty this end!

To everyone else reading - with the phraase 'comedy dac' Phil is rather understating his talents
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Old 10th July 2009, 06:31 AM   #13
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Indeed it works, so, umm, leaving it for now. Not my most professional moment interestingly, one for each channel allows the noise back! Nice.

(I am a professional, honest, but not in DIY DACs).

You're right, it's more comedic these days. I'm actually very pleased with it now, but it does sort of have its own enclosure I built for it - that it has now outgrown. Veroboard modules overflowing the front, like some kind of surreal flood. It does ooze with audiophile look and feel.

I have a new box for it all to fit in, although, I'm tempted to stack the boards into a cuboid - like that 4xTDA1541A thread elsewhere. But, before that, I still have to get my relay switching going for HDCD gain.
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Old 10th July 2009, 07:52 AM   #14
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No, you need AN-207:
http://www.analog.com/static/importe...60163AN207.pdf
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Old 10th July 2009, 10:06 AM   #15
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Thanks for the tip Quirck, but its already designed for me, where's the fun in that ?!
I'll draft up and post my schematic later on for comparison.
It's essentially similar, but with extra buffering. Four opamps instead of two.
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Old 10th July 2009, 10:07 AM   #16
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Sorry, four opamps instead of one - that DAC has a voltage output.
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Old 10th July 2009, 10:55 AM   #17
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Quote:
Originally posted by philpoole
Sorry, four opamps instead of one - that DAC has a voltage output.

All I wanted to say is that a third order Bessel filter is more than sufficient if you are using an 8x oversampling digital filter.
I think even one passive low pass filter will do.
I am looking forward to your schematic.
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Old 10th July 2009, 09:05 PM   #18
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I did have a passive filter, and my opamp was struggling to drive it. It worked okay, but this is much better.

Anyway, here's the schematic. There's no attention to bypass caps, and grounding, and I'm not using LT1361s, and the multiple caps in parallel is just because I was using bits I had to hand.
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File Type: jpg phils_bessel_filter.jpg (46.0 KB, 112 views)
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