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easyDAC = NOS R-2R AD1862 DAC + Headphone Amp
easyDAC = NOS R-2R AD1862 DAC + Headphone Amp
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Old 19th September 2017, 01:21 AM   #1
miro1360 is offline miro1360  Europe
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Default easyDAC = NOS R-2R AD1862 DAC + Headphone Amp

I like DACs powered directly from USB in small case/design and I like sound based on R-2R. This was reason why this toy was created.

Here in gallery are pictures: DAC gallery

Once again small description:
- NOS R-2R AD1862 DAC
- LINE-out direct from I/V
- headphone out based on TPA6120A2 with potentiometer, easily drives big headphones with a lot of power (AKG K701)
- no additional filters (only RC)
- no caps in series with signal line
- switch between line or headphones (no need unplug something when I choose between headphones or amp), with relays
- for USB/I2S conversion is used PCM2706 (on board are also pins for another I2S), simplicity, no additional drivers, I like Plug&Play and I am listening mostly in 16bits
- power from usb is converted with isolated DC-DC DCP0205xx, regulated to +-5V for digital and AD1862 digital, +-12V for AD1862 analog and I/V, +-5V for headphones amp TPA6120A2
- it can handle custom I/V boards in small design, +-12V
- 4 layers board

I am not sure if it is DIY project, becase it is full of small 0805 SMD parts and few SMD ICs, board is 4-layers

Some build hints:
- first mount USB/i2s part and test with scope if some data signals are going out from i2s
- continue with power supply part and test, if all voltages are right
- dont forget for thermal pad holes
- mount DAC portion with relay, switch and I/V
- connect it to PC and measure with scope if sine is right (or test sound directly), because in this state it must working
- if all is right, mount headphones amp part, better if you buy 3 potentetiometers and measure them for resistance offset (left and right) in few places and choose best one (with lowest offset), dont forget carefully solder TPA6120 through thermal pad hole
- connect it to PC and measure dc offset on headphones connector, it must be small offset, about few mV
- if you wish, you can clean board from solder resin using isopropyl alcohol
- I used SMD thick resistors
- as I/V opamp choose something good, like LME49990, ADA4898-1, and many others ...
- if you use external USB/I2S (for example asynchronous XMOS) resistors R6, R7, R8 can be disconnected and also PCM2706 power jumper opened ... on board are pins for I2S https://ctrlv.cz/shots/2017/09/19/DGlu.png
- output voltages can be easily set with change in feedback resistors R9,R10, or for headphones in gain R17+R18, R21+R22
- when you use fast I/V opamp, like AD844, only to be sure - check output for oscillations, I have no oscillations
- only to mention, here is no headphones protection circuit, so after every change in I/V measure headphones output if there is not big DC offset, must be few mV
- in normal condition, there is no clunking in headphones when switch position is changed or device is disconnected


I need thanks to pavouk.org site from where I got I2S/PCM logic and thanks to Dohny for hint about DCP0205xx DC-DC.
Attached Images
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Attached Files
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File Type: zip easyDAC_1.2_PDF.zip (800.8 KB, 131 views)

Last edited by miro1360; 21st October 2017 at 06:24 PM.
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Old 19th September 2017, 10:09 AM   #2
x3060 is offline x3060
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thanks a ton, can i replace PCM2706 , with PCM1794 for 24-192.
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Old 19th September 2017, 04:54 PM   #3
miro1360 is offline miro1360  Europe
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you can not replace PCM2706 with PCM1794
... but if you need higher resolution 24/192, instead of PCM2706 use XMOS USB/I2S board (here are many, from diyinhk, jlsounds)
on my DAC are pads where you can connect I2S pins from XMOS board

Last edited by miro1360; 19th September 2017 at 04:59 PM.
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Old 22nd September 2017, 06:52 PM   #4
analog_sa is offline analog_sa  Europe
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easyDAC = NOS R-2R AD1862 DAC + Headphone Amp
It's not just the resolution: the async solutions all have much better clocks and galvanic isolation. The Jlsounds also has the ability to output pcm data rendering the additional logic unnecessary.
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Old 22nd September 2017, 08:12 PM   #5
miro1360 is offline miro1360  Europe
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asynchronous mode decrease jitter in sound and it is in theory good for sound result ... it is why I added these pins for connecting advanced I2S like xmos boards ... this advantage is measurable with super high end devices and was done many times, with devices ...

now back in human ear reality ... know somebody from how much pS/nS/uS is jitter listenable for human ear? ... personally, when I was switching between these xmos devices and PCM2706 in 16b/44khz, I have not heard difference with my ears ... I did these listenable tests, because I am going to do DAC with PCM1704 and I need be sure ...
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Old 23rd September 2017, 08:28 AM   #6
analog_sa is offline analog_sa  Europe
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easyDAC = NOS R-2R AD1862 DAC + Headphone Amp
Perceived sound is not affected by the async mode per se, what matters is the quality of the oscillators and their PS. Without getting into these specifics it is hard to make meaningful comparisons.
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Old 30th January 2019, 06:16 PM   #7
TNT is offline TNT  Sweden
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Quote:
Originally Posted by miro1360 View Post
asynchronous mode decrease jitter
It might - done right. But what is certain with asynchronous sampling-rate conversion is that your original signal is recalculated so bit-perfectness goes down the drain.

//
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Old 30th January 2019, 07:20 PM   #8
Ken Newton is offline Ken Newton  United States
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Originally Posted by TNT View Post
...But what is certain with asynchronous sampling-rate conversion is that your original signal is recalculated so bit-perfectness goes down the drain.

//
Synchronous sample rate conversion (oversampling) also alters the original data. That’s what digital filters do, alter the data fed them. Analog filters also alter the original analog signal. Filtering in general alters the original signal. That’s it’s purpose.

I’m aware that Schitt talks about ‘closed-form’ digital filtering which retains the original samples. I know of no technical reason why this would produce more accurate reconstruction filtering. I suspect this is more playing on audiophile intuitions, which typically don’t apply to digital signal processing, as it tends to be non-intuitive.
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Old 30th January 2019, 07:29 PM   #9
TNT is offline TNT  Sweden
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Originally Posted by Ken Newton View Post
Synchronous sample rate conversion (oversampling) also alters the original data. Thatís what digital filters do, alter the data fed them. Analog filters also alter the original analog signal. Filtering in general alters the original signal. Thatís itís purpose.
Jepp!

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Old 30th January 2019, 07:47 PM   #10
analog_sa is offline analog_sa  Europe
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easyDAC = NOS R-2R AD1862 DAC + Headphone Amp
Quote:
Originally Posted by TNT View Post
It might - done right. But what is certain with asynchronous sampling-rate conversion is that your original signal is recalculated so bit-perfectness goes down the drain.

//



There is no trace of ASRC here, where did you get this from?
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