making USB DAC -- CM102-A+ --

Japanese/English

Japanese version here.

listmarkintroduction


I had something on my chest.
That was audio environment of my girlfriend's house.
I am really a troublesome person.
But this is the nature of people who love audio.
You may understand this sad nature.

I tried to make a device which bridges between PC and MD stereo component this time.




listmarkparts


I searched a IC which would work as digital analog converter connected by USB.
Then I found CM102-A+ for $5.

CM102-A+ is often found in cheap USB speakers.
CM102-A+ is used in a speaker which is sold for 980yen.
I hesitated to purchase a IC for 500yen which was found in a speaker which was sold for 980yen.
But I heard that AUDIOJACK-USB uses CM102-A+ and it sounds good, so I decided to use this IC.

datasheet of CM102-A+ (C-Media) --- I can't read circuit because of its rough pdf.

a evaluation circuit of CM102-A --- I can read it.

For other parts, I used that were in my stock.




listmarkplanning


I imitated a evaluation circuit of CM102-A.



(If you click, the figure will expand.)


I aimed good sound smoothing USB bus power.
In addition to this, I selected audio grade parts as the places which seemed to be important.


I was particular about S/P DIF in vain.
I used 74HCU04 as a buffer and RF core as a pulse transformer.
And I consider about impedance matching of coaxial cable (75ohm).
I am satisfied with this digital output.
But I don't have any plan to use digital output.


June 20, 2007 added
I recieved some e-mail which contained questions about my circuit.
So I'd like to supplement about those points.

1: The resistor between OSC and CM102-A+ counts about 10-300ohm.
It is called as "damping resistor" and it works to remove reflection of high frequency waves.
The 22ohm resistances of USBDP and USBDN are also damping ones.

2: We can use crystal unit in exchange of crystal oscillator between XTL_OUT and XTL_IN.
In general crystal oscillator is higher accuracy than unit.

3: I inserted 0.022uF capacitor and 22ohm resistor in output of CM102-A+ because I found this circuit in the evaluation circuit of PCM2704.
I answered that it was aimed to work LPF.
But now I reached second thought that it was for impedance matching.
Please give me the answer.




I set for check.
Each parts fitted closely.


Crystal oscillator is very important for USB DAC.
I used a oscillator which is under plus minus 30ppm of temperature equation.
It was $1, but it looked nice and I couldn't help expecting it would work well.

MUSE KZ is so huge.

If I chose small parts, I could get small USB DAC.



listmarkmaking


I built the circuit actually.



The sector of digital output has a large space.


The case is TD-7-10-3 which is made with aluminium.



Red, White: analog output:
Yellow: digital output which waits its turn forever




I'm already good at processing aluminium.
Chassis reamer and file help me a lot.




listmarkimpression


It souds nice against my expectation.
It was a bit cloud at fast, but now it got a bit clearer.

Its bass is so powerful.
This may be a characteristic of CM102-A.

I paid approximately only $30 for this, but would be enough for listening musics connecting to MD stereo component.
However, if you want very nice USB DAC, I won't recommend this IC.
It will good for the use of digital output(S/P DIF).



July 13, 2007 Added

I made a NOS DAC (TDA1543 + CS8414), so I got a chance to measure the power of CM102-A+ at last.

It is very nice.
I feel each sounds got more dense than optical output of Modified SoundBlaster Digital Music LX.

I heard that CM102-A+ can output only 48kHz, but I feel this coaxial output is better than optical.


discussion:
Digital signal is transmitted as block pulse, but it is very hard to make complete block pulse.
Leading edge of the clock pulse often becomes dull and it makes over shoot.
So when the timings of leading edge of the clock and reciever clock are overlapped, it'll be hard to discriminate 0 from 1.
If converted into optical signals and noises are joined, the dullness will be more bad.

Because of these reasons, I think it has differences between coaxial and optical transmission methods.




Testing coaxial output of CM102-A+ connecting to NOS DAC.


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