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Old 14th April 2012, 07:00 PM   #1
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Default My NOS AD1865 DAC

I have just finished my NOS AD1865 DAC with AudioNote like Digital Stage, 7 Salas shunt regulators, and Pass D1 IV Stage. It seems to be working and sounding fine I'm just worried exposing my amp to the HF noise from digital stair steps I don't think that the 1st order low pass filter is enough. I also can hear some fuzz from the DAC when the volume in iTunes down a lot I think that is the images from the sampling frequency. the fuzz changes on different songs. I was thinking of implementing a notch filter to remove the 44.1Khz frequency and a LC resonant filter to compensate for the sin(x)/x rolloff from the DAC. Do you think that this amount of filtering is necessary or will the phase shift it causes degrade the sound quality? I simulated it in ltspice and it seems to work in the sim. I uploaded pic's of the sim and the DAC. I could use some help getting the values of the filters right, i'm not an expert on filters.
(In the circuit the 2N7002 should be an ZVN2106A in real life.)
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Old 14th April 2012, 11:43 PM   #2
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One problem with LTSpice sims with inductors is that real world components have frequency dependent losses (proximity effect mostly, skin effect sometimes) which the software doesn't model. I see you have fairly high values (4.7mH and 10mH) which will almost certainly be multi-layer in construction. Multi-layer coils are very difficult to model accurately.

If you look at my blog I have another way to implement sin(x)/x correction - in your case you'd need to piggy back another two AD1865s and add some shift registers. It requires a DAC with fairly high output compliance to work though because the AD1965 has no reference input - do you know the output compliance? By output compliance I mean what voltage swing will the current outputs tolerate?
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Old 14th April 2012, 11:50 PM   #3
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I got the fuzziness in the signal to go away by removing the 150uH inductor in series with the AD1865 output, it also sounds much better now
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Old 15th April 2012, 01:28 AM   #4
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I actually think I don't need a sin filter as the HF loss is very small. I don't even think it needs anymore filtering. I love the sound of it just as it is. I just need to know if the digital stair steps could upset my amp. (cause any oscillation) its a DX Precision I from the diyaudio forum.
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Old 15th April 2012, 01:32 AM   #5
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I loved the sound of my NOS DAC without the response flattening. But I have to say that with it, its even better - more dynamic sounding than before. And it was amazingly dynamic compared to S-D DACs prior to that.
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Old 15th April 2012, 03:24 PM   #6
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I have been messing around with spice and i found putting cap's in the spots in the the picture bellow make the frequency response peak at 20khz like the inductor circuit I soldered the parts on my pcb and i'm listening to it as a write this and it gives the highs that extra "sparkle" it was missing. but on the oscilloscope any square waves i generate with audacity have a lot of overshoot and undershoot. they didn't look like this before i added this filter. I just want to know if this type of filter is "proper" I don't even understand how its making a peak from putting them in that spot.
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Old 15th April 2012, 04:07 PM   #7
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I just removed the 10nf from source to drain and it does not need it and it sounds so much better with out it.
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Old 17th April 2012, 12:57 PM   #8
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Quote:
Originally Posted by abraxalito View Post
I loved the sound of my NOS DAC without the response flattening. But I have to say that with it, its even better - more dynamic sounding than before. And it was amazingly dynamic compared to S-D DACs prior to that.
My subjective reaction to NOS response EQ is essentially the same as abraxilito's, although I use an LC tank EQ circuit constructed of film & foil (not metalized) capacitors and amorphous core inductors. The reason I believe the benefits of such EQ are readily audible is because NOS response droop broadly affects the upper audio band, bout two octaves worth, from 5kHz-20kHz. Inverse sinx/x EQ is just as broad in correcting those two octaves, not just the famous -3dB@20kHz.

Regarding, the output compliance of the AD1865. I've utilized passive I/V resistor values as high as 330 ohms on the AD1865 without perceptible distortion. Which equates to an output pin voltage compliance of at least 330mV peak.
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Last edited by Ken Newton; 17th April 2012 at 01:15 PM.
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Old 17th April 2012, 01:02 PM   #9
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Quote:
Originally Posted by Ken Newton View Post
The reason I believe the benefits of such EQ are readily audible is because it broadly corrects the upper audio band, over about two octaves from 5kHz-20kHz, not merely 3dB@20kHz.
Yep, concur - I'm of the view its more the making up of the energy loss between 5k - 15k which is achieving the difference for me. My own correction doesn't manage to add 3.2dB @ 20kHz anyway.
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Old 17th April 2012, 11:53 PM   #10
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I'm thinking of making a new digital PCB so I can try the SM5842 digital filter. I need to know how to interface the CS8414 to the SM5842 to the AD1865. I do not know what pins to pull high and low to get the chips to work together, could someone please help me with this step. Also I see the SM5842 needs 60ma of current my current post-regulators on the PCB can only source 30ma (TL431).
I have 3 Salas shunt regs that make 6.2V for Digital and +/- 6.2V for Analog, they connect to 6 TL431 regulators on the digital board to drop the 6.2V to 5.00V for the CS8414, etc. since I cannot use these shunt regulators anymore with the newly required high current. what would be the best method of dropping the 6.2V to 5.0V could I use one of the Linear Tech's LDO reg's or would that degrade the SQ, etc.?

I have uploaded the circuit diagram that I hooked up, I just need someone to look it over, Thanks.
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Last edited by benproiii; 17th April 2012 at 11:59 PM.
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