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View Full Discussion Hi All,
I thought I would provide some technical details about the SDAC design that are not evident by reading the specifications.
When Massdrop approached us to design a USB DAC that could provide equal or better performance than the ODAC Rev B or Schitt Modi 2, but be more affordable than our m9XX, we jumped at the challenge. The SDAC contains mostly new circuitry for us and there was not much we could reuse from previous products we have built in the past.
Successful DAC design begins with the understanding that a DAC chip is in essence a mixer. The output from a DAC is a mix of four primary inputs: 1) Digital audio data 2) Clock Jitter 3) Power supply noise 4) Analog circuit distortion.
Modern delta-sigma DACs have vanishingly low noise and distortion so the "sound" of a DAC is really less controlled by the actual IC selected for the design and more by the quality of the clock, power supply, and analog amplifiers. Also, to a lesser but significant degree, passive components like capacitors and resistors in the analog signal path.
The SDAC employs asynchronous data transfer from the USB host, which allows for the use of fixed crystal oscillators rather than phase locked loops (PLLs) or digital clock synthesis. A low noise fixed crystal oscillator is the lowest jitter sample clock and there are two in the SDAC: One 22.5792MHz crystal for 44.1kHz and 88.2kHz sample rates, and one 24.576MHz crystal for 48kHz and 96kHz sample rates. These are located very close to the DAC.
I should note that asynchronous mode allows for the use of a high quality clock but it is not intrinsically better. It is certainly possible that there asynchronous designs out there that use low grade clocks and therefore are not benefiting from asynchronous mode.
The power supply for the DAC is designed for very low noise and good noise rejection. This is critical to the overall performance and is not easy to do given the constraints of USB bus power operation. There are separate voltage regulators for the DAC, processor, and analog circuits, which minimizes interaction between these circuit areas.
The analog output circuits are based on very low noise and low distortion amplifiers. With harmonic distortion below -120dB, they are well below the distortion floor of the DAC.
The SDAC is direct coupled with no capacitors in the signal path, which gives low frequency response that is perfectly flat and free of phase shift.
Following is a list of some of the technical features of the SDAC:
I will attempt to upload a fft plot showing the SDAC passing a 10kHz sine wave. Notice that there are no visible jitter sidebands (spurs that appear evenly spaced on either side of the fundamental tone). The line at 20kHz is the first harmonic at -92dB. The level of the noise floor suggests that the upper limit of random jitter in the SDAC clock is around 1.5ps at any given frequency.
If anyone has questions let me know.
I will try to check in here as time permits...
- XMOS X200 USB streaming controller operating in asynchronous transfer mode.
- IDT XLH series oscillators provide and exceptionally stable and low noise sample clock for DAC. 12kHz to 20MHz phase jitter is 750fs. Note that the jitter below 12kHz is generally more important in audio applications but the 12kHz-20MHz specification is commonly used for specifying oscillators so it is useful in comparing to other clocks. In the SDAC the clock jitter artifacts are well below the DAC noise floor so they are essentially irrelevant.
- AK4452 DAC provides 32 bit internal processing , 115dB dynamic range, and the same architecture as the AK4490.
- Output amplifiers are Burr Brown OPA1652. These are extremely low noise low distortion FET input amplifiers with excellent output cable drive capability. Note that the output impedance is set at 150 Ohms so the SDAC is not intended to drive headphones. While it will not harm anything to do so, the fidelity will not approach what is possible with a good headphone amplifier.
- The negative audio power supply is generated with a switched capacitor circuit that operates synchronously to the audio sample rate to eliminate any possibility of power supply related spurs in the output spectrum.
- DAC power supply is regulated with the TPS793 low noise linear regulator to reject any contamination from noisy USB power.
- 4 layer FR-4 circuit board with careful attention to high speed digital layout, power supply grounding, and low noise analog signal routing.
Can you provide FR graphs？
Thanks a lot!, It looks definitely better in every ways than ODAC rev.b(It would be not audible lol)
So I'll go with this drop!
The AKM Dac chips have selectable digital filters. I guess it is the case for the Ak4452. I understand that the SDAC is a lower cost DAC (compared to your 9xx DAC/Amp) and therefore does not provide a switch to select such digital filters. But one of the available filters on the AK4452 had to be selected as the standard choice for all SDACs. May you tell us more on this choice? Which of the filters has been selected and why? Did you do a sound test comparison of all the available filters provided on the chip before stoping your choice on such filter? Thanks in advance and best regards,
Will this Dac support an Android phone via a USB OTG cable?
would like to know this too
Is there any issue with using the 3.5mm and RCA outputs simultaneously?
The SCAC is a USB Audio Class I compliant device so it should work on anything that supports that standard.
We have tested the SDAC with a Samsung Galaxy S5 and HTC One M9. Both phones are on Android 6.0.
Both phones will power the SDAC directly and both phones work with a powered hub. Note that the SDAC draws 130mA so powering it directly from your phone will likely drain the battery fairly quickly.
The SDAC will work with iOS devices too but a powered hub and the Camera Connection Kit is required.
Hi Mr Grace, I am using a "Belkin Rockstar Lightning Audio + Charge" with Apple "Lightning to USB Camera Adapter" on iPhone 7 (iOS 11.2.5). The phone insist SDAC is drawing too much power. Any suggestion how we can overcome this? Thanks!
I can suggests you to Use with Powered USB hubs
Thanks for responding. Do you have a recommended way of connecting? I tried a Logitech 4 port USB 2.0 powered HUB. The SDAC is not detected. Thanks again!
Ummm...I am sorry that the way I suggested does not work. 😭
I hope you to find the way!
One user in the discussion thread says:
"the 3.5mm jack is a pass through connection so when you plug another set of headphones into the DAC the RCA connection looses voltage and cuts it's output in half to make the 3.5mm work - I'm sure I butchered that explanation."
What does this mean? I want to use both outputs simultaneously at the same volume that they would have if I only used one of them, does this mean that isn't possible?
Both the 3.5mm and RCA jacks are wired in parallel. That means they both have the exact same signal. The output impedance of the DAC is about 150 Ohms. If you plug each output in to an amplifier there will be virtually no loss in signal level. However, if you plug headphones in to the 3.5mm jack there will be signal loss on the RCA. For example, if you plugged 150 Ohm headphones in to the 3.5mm jack that would make a voltage divider of .5 so you would loose 6dB of signal. This will not hurt the SDAC but the 3.5mm output was never intended to be a headphone amplifier. It is best to use an external amp.
Hope that makes sense,
r1:headphone (32 Ohms)
1/r1 + 1/r2 = 1/R = (r1+r2)/(r1*r2)
2V=2000mV=62.5mA * 32 Ohms
1/r1 + 1/r2 = 1/R
R = (r1*r2)/(r1+r2)
2V=2000mV=0.4mA * 5kOhms
62.5 mA = 0.4mA *(almost)156
Hey I just racived your dac it's grate. I'm using 3.5mm ouput with addapter to 1/4 jack, headphones Fostex th610 (25 ohm). The volume enough. Will I hear any difference after connecting some amplifier (THX AAA)? I'm think th610 are easy to drive and I wondering if amp have any sense.
Hi 500 Plus. Glad you like the SDAC! I am not surprised that you get enough volume driving the TH610 directly. However, you will get better performance with an external headphone amplifier. This is primarily due to the fact that an amplifier will have a much lower output impedance than the SDAC line outputs.
The output impedance of the DAC is about 150 Ohms. The output impedance of a good headphone amplifiers should be under 1 Ohm. This will give you a much higher damping factor which is a measure of how well the amplifier can damp resonances and back EMF from the headphone transducer. Basically you will get better bass definitions with less overhang and better impact.
plz make optical and coax version of it....
Just got my SDAC - Outstanding value! Quick question: I'm using Tidal's MQA "Master" quality for streaming thru my Macbook Pro - Is this codec supported? Grace was not listed on MQA's list of supported DACs.
Glad to hear you like your SDAC.
As for MQA we do not support that format.
Is it possible for my SDAC to change to longer USB cable? I have standard charging 24A USB to micro B cable.
can I use it and it does not affect anything?
The longest cable we have tested SDACs with is 10 ft (3M).
It is possible you could go longer that that if the cable impedance is correct. You would have to try it. I do not think you can do any damage to the SDAC with a longer cable. If it does not work you will probably get no sound or error messages from your computer operating system.
Hope this helps,
Yes, the cable is exactly 3 meter length. I changed it now and it works. Nothing is weird going on. So thanks for the reply.
Thanks for your reply Michael.
May I ask why? Is it to keep cost down ( I understand that there’s a license fee to use it) or that you don´t believe its a good codec, being lossy, etc.?
when will be available the updated SDAC (SPDIF input and XRL) ?
I want to use this for two seperate amps 3.5mm going to one and RCA going to another. So this should be ok ?
I won't have both turned on at the same time just plugged in and switch one off and the other one on when i want to swap.
Great little DAC!
This should be no problem at all! Keep in mind though that some headphone amplifiers, depending on their input circuitry, can actually cause a little distortion in the output of the SDAC if being driven with the power off. So you might just do a listen test with the unused amp on and off to see if you hear a difference.
I will have the Little Dot Mkii on the RCA output and the JDS Labs Atom amp on the 3.5mm output of the DAC. I will do a listen test as you said.