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benproiii 14th April 2012 08:00 PM

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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.)

abraxalito 15th April 2012 12:43 AM

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?

benproiii 15th April 2012 12:50 AM

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 :)

benproiii 15th April 2012 02:28 AM

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.

abraxalito 15th April 2012 02:32 AM

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.

benproiii 15th April 2012 04:24 PM

2 Attachment(s)
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. :confused:

benproiii 15th April 2012 05:07 PM

I just removed the 10nf from source to drain and it does not need it and it sounds so much better with out it. :)

Ken Newton 17th April 2012 01:57 PM


Originally Posted by abraxalito (Post 2985724)
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.

abraxalito 17th April 2012 02:02 PM


Originally Posted by Ken Newton (Post 2988809)
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.

benproiii 18th April 2012 12:53 AM

2 Attachment(s)
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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