I have been hoping to DIY a DAC but I don't have the expertise to wire up a DAC on my own. Fortunately, one day while was browsing the web, I found Audio Workshop. Audio Workshop has a numbers of DAC PCB for sale, I picked the newest one : AW-D2. This DAC is based on the AD1955 DAC chip and the DIR1703 digital receiver. The AD1955 is being used in the Philips 963SA DVD player which was getting lots of good review.
我一直想DIY一部CD解碼器﹐但由於我沒有造DAC的經驗 ,全部從頭開始會比較困難。幸好有一天我在瀏覽網站時發現了AudioWorkshop 。AW有數款CD解碼器的PCB板售賣。我便選擇了最新的一款 : AW-D2 。這款DAC是用AD1955的解碼晶片及DIR1703數碼接收器芯片。Philips的963SA DVD機也是用AD1955 的﹐ 而963SA也獲得了很好的評價。
After I ordered the PCB and the ICs from AW, they arrived a few days later. Audioworkshop also give me some free components like capacitors and some inductors. My next step is to start ordring components, which is the most time consuming part of building this DAC. BTW, AW sells components at a very cheap price, much cheaper than I could find anywhere else. For example, BlackGate 3300uF 16V capacitor, SSP sells at over HK$100, AW sells them for only HK$35.
於是我便在AW訂購了PCB及芯片。訂購後的數天便收到了他們寄來的郵包。AW還免費送了一些電容及電感給我。之後便是最費時的部份﹐就是要到處找零件 了。有一點要提一提的是 AW 所賣的零件價錢都比外面平。例如BlackGate 3300uF 16V capacitor, 在鴨寮街要 HK$100 以上,在 AW 只要 HK$35 便可。
First thing I do is to go to Sham Shui Po to buy all the general components like case, transformers, cables ,connectors, and LEDs. I also tried to buy all the resistors there but some values are hard to obtain. Then I ordered the other components from Farnell. Farnell carries most non-hifi components and they do have free shipping for HK$200 order. For hi-fi specific components, I ordered them from audioworkshop. I originally used a pulse transformer that I bought from Farnell, but I find out later that its bandwidth is only up to 1MHz, so I bought the Pulse PE-65612 from DIYZone.net to replace it.
買零件我會先到鴨寮街買一些比較普遍及易找的零件如機箱﹐變壓器﹐線材﹐插頭﹐ LED 等。我也會盡量在鴨寮街買齊所有電阻﹐但是有的電阻比較難找。其餘在鴨寮街買不到的我會在Farnell online買。Farnell大部份 零件都有。 HiFi 零件除外。要買 HiFi 零件的話可在 AW 看看有沒有﹐或到旺角的 HiFi 零件鋪買。我原本是用一個從 Farnell 買回來的pulse transformer, 但後來發現它的頻寬只有 1MHz, 所以便從DIYZone.net 買了Pulse 的PE-65612來代替。
After I got all the required components for the DAC, I start doing the soldering. The DIR1703 and the AD1955 is pre-soldered by AW. That make my life a lot easier as these 2 chips are quite hard to solder. I solder all the low profile components first, it would be easier this way. The first problem I met is I got the SMD capacitor size wrong. I ordered 2220 from Farnell but I should use 0605 type. The 2220 is a lot bigger so I have to solder them at the back of the 0.1uF pads.
買齊零件後便可以開始焊零件了。幸好AW已把 DIR1703 及 AD1955 預先焊上﹐否則的話我可能焊不來﹐這兩粒 IC 的腳位很密﹐非常難焊。焊接由比較婑的零件開始, 這樣比較容易。我第一個碰到的問題是我買錯了大幾個碼的 SMD 電容。原來的 0605 我郤買了 2220。 結果要把它們焊在 PCB 背後的 0.1uF 電容的腳位。
I used IC sockets for all the ICs on the board. It is safer for the ICs and also I can change OP-Amps easier for sound tuning. The OP-Amps used are 4x OPA627AP for the I/V conversion and 2x OPA637AP for the low pass filter. I used mainly 3300uF Panasonic FC, Philips 0.1uF, and ELNA Starget 47uF for de-coupling. Only one blackgate 100uF is used for the AD1955’s power decoupling. So most of the capacitors I used is not really exotic.
所有的芯片包括 OP-Amp, 74HC04, Oscillator 我都用了 IC 座﹐原因是焊接時會比較安全﹐萬一焊壞了都只是毀了 IC 座﹐不會把寶貴的 OP-Amp 弄壞。另外將來要換 OP-Amp 來調聲也方便。我用的 OP-Amps 是四顆 OPA627AP 用作電流電壓轉換及兩顆 OPA637AP 作為低通濾波。濾波電容主要是用 3300uF Panasonic FC, Philips 0.1uF, 及 ELNA Starget 47uF 。 Blackgate 我只用了一個。是 100uF 的用在 AD1955 的電源上。我所用的電容不算高級﹐所抱的心態是先開聲再說。
For transformers, I used 2 R-style transformers. One for analog circuit and one for digital circuit. I use R transformers because they are smaller in size, efficient and low noise.
The aluminum case is connected to the earth of the power socket。The GND of PCB is connected to the case via a ferrite bead and a 10 ohms resistor in series.
變壓器用了兩個 R 型變壓器﹐一個比數碼﹐一個比模擬。用 R 型是因為它們有體積小,效率高及低噪音的好處。機箱是全鋁的,和電源的地線連接。PCB的GND經過一個ferrite bead和一個10 Ohm電阻再接落機箱。
The power regulator board I used is AW-R1. I selected using LT317/337 as the regulator ICs. The input capacitors I used is Rubycon 4700uF for its cheap price. Output capacitors are Panasonic FC 2200uF.
電源穩壓是用了 Audioworkshop 的 AW-R1. 穩壓晶片是用 LT317/337 。輸入電容是 Rubycon 4700uF 。用它是因為它價錢平。輸出電容是 Panasonic FC 2200uF 。
After everything is soldered and put into the case. I first adjusted the voltage output of AW-R1 before connecting it to AW-D2. After everything is connected, I do a double check to make sure nothing is shorted or connected wrong before powering it on. It was fortunate that I do this step, because I found the +/- 15V was connected in reverse! The coaxial cable used to connect the RCA socket and the D2 PCB need to have a good shielding if the cable length is long. Otherwise some digital noise will be coupled to the analog side. The one I am using has 100% aluminium foil plus 93% copper braid.
所有零件焊好後便把變壓器及 PCB 固定在機箱內。先不把 R1 及 D2 連接﹐然後調整 R1 的輸出電壓。調整後把所有元件連接﹐然後再次把所有連接從新檢查一遍﹐也用萬用錶檢查有沒有短路。幸好有做這一步。我發現了我竟然把 +15/-15 倒轉接了 ! 如果沒有檢查的話﹐相信便要付出沉重的代價。機箱內的同軸線如果是比較長的話, 最好用Shielding較佳的, 用來防止數碼噪音走入模擬的部份。我用的是有100%鋁箔加93%銅網的一種。
After power on, the LOCK led is off, which is normal because my CD player is not on yet. Then I truned on my CD player, one second later the LOCK led was on. That’s a good sign since that means the DIR1703 can lock the SPDIF signal. I play some music and then I measure the AC voltage of the audio output. I got some reading. Then I measure the DC offset of the DAC output to make sure there is no DC offset. Everything is fine then I connect the DAC to the pre-amp and turn on the power amp at the same time. I heard music from the speakers!
把電源打開後 LOCK LED 是沒有亮的。屬於正常,因為還沒有接 CD 機。把 CD 機接上後﹐並把你 CD 機電源開啟, LED 隨即亮起。那是好現象, 因它代表 DIR1703 可以接收到 SPDIF 數碼訊號。然後播放 CD 及量度 DAC 的模擬輸出,可度出它有交流訊號。再度它有沒有 DC 電壓, 結果是沒有的,令人滿意。最後把 AW-D2 接到前級上,再把音量打開,聽到的就是悅耳的音樂 !