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Non Over Sampling USB DAC

*** update ***
I am considerng having a quantity of professional boards made for sale. See here for a few more details.

The standard arrangement for a non oversampling DAC has been to use a CS8412 or a CS8414 as a receiving chip to convert S/PDIF into I2S which is fed into a DAC chip like a TDA1543. However, to use this setup with USB audio, one would need to use something like a PCM2902 to convert USB to S/PDIF to then be fed into a CS8412 to convert the signal to I2S. While digital conversions should be lossless, this clearly offers too many opportunities for jitter and noise to creap into the signal. A better solution, it would seem, would be to use a USB audio chip that could output I2S directly. And indeed TI offers a pair of chips, the PCM2706 and PCM2707, that can do just that.

A quick search of the DIYAudio.com forums dug up a thread with a basic schematic of how to implement the PCM2707.

I decided to simply couple this to a non-filtered passive I/V stage as well as a small power supply and cram it onto a ExpressPCB sized circuit board.


passive I/V

For the power supply, I copied the AC filtration from Tangent's STEPS, but I left off some of the filtering stages to save space. One of the transformer's secondaries is dedicated to the PCM2707 and is regulated by a LM7805 and reregulated by a higher quality 3.3V regulator. The other secondary is for the DAC chip, and is first regulated by a LM7809 and reregulated by a high quality 8V regulator. Additionally, every power connection to the PCM2707 is run through a ferrite bead and decoupled with a 0.1uF ceramic cap. The TDA1543 is decoupled with a 0.1uF film cap.

Digital connections from the PCM2707 to the TDA1543 go through a small resistor and a small inductor. Last, the USB host connection is filtered with a small inductor.

Below is a picture of the circuit board file (click it for a larger version.) The actual size is 3.5"x2.8".

Below are several images of completed and partially completed boards. These are an earlier version of the PCB design and are thus slightly different.

Okay, here are the rules for the use of these board files...
The board files are licensed by me to you under a creative commons license. The gist of the license is that you may only use these files and/or boards produced from these files for non-commercial use, and any use or derivative use must give attribution. You may not make boards from these files, or from derivative files, and sell them for any more than you paid nor may you sell a completed board for any more than the cost of materials (i.e., the cost of a board plus the cost of the parts to populate it.) Further and in addition to the creative commons license, these boards may not be produced in quantities greater than 3 at a time without contacting me for permission. This means no large "group buys" are allowed. Any boards produced with these files, or with derivative files, must contain "http://diyaudio.ecp.cc" and a copyright notice. This must be etched on the board such that it is readable on a completed board (i.e., in copper on the ExpressPCB boards if there is no silkscreen and not covered by parts.) Finally, these files are distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

To download the board files, click here

Assembly Tips
There are a number of tips for assembly that might prove helpful.

  • There are a couple of components (the USB jack and the crystal in particular) with metal bodies that can short traces on the board due to there not being a soldermask on the boards. To prevent a problem, I suggest finding some sort of insulating material (insulating pads for mounting components to heatsinks work well) and placing them between the component and the board. The leads will easily pierce the pads.
  • There are two suggested 8v regulators. The AN8008-ND seems to sound better, but is only rated at 50mA. The TDA1543 draws on the order of 60mA. I have not had a problem, but if you are worried, use the other one.
  • The parts listed for C1, C2, and C3 are .082uF caps. The .1uF cap in the same series is what you really want, but DigiKey was out of them when I built this.
  • C17-24 are a pain. They are also optional. I tried soldering them on both before and after D1 and D2, and both methods presented some difficulty. If you do the caps first, be careful to not let solder fill in the rectifier pads. If you do them second, cut the leads of D1 and D2 first.
  • Many of the parts were chosen because they are low profile. In particular, C4 and C5 are the short versions of these caps. If you use other caps, be sure that the lead spacing is correct as the boards are pretty tight, and a fatter cap will not fit.
  • To add the fuse clips, put the fuse into two clips, then solder the clips to the board. If you simply solder the clips, the fuse may not later fit.
  • While the bulk of the board is low voltage, do not let this lull you into a sense of safety. There are places on it that are not. They are at wall voltage and could kill you if you touch them. Be very careful with this project, and only build it if you are experienced with high voltage. Electricity is dangerous.
  • Metal standoffs (such as shown in the pictures) may bridge the case to ground or live as the standoffs are wide enough to extend beyond the mounting holes.. The standoffs shown are insulated with plastic washers. Similarly, this project will not slide into a case. It needs to be mounted via the mounting holes.
  • I have found that there is some noise somewhere, most likely from the onboard transformer. At low listening levels, this is not really a problem. However, as you turn things up really loud, the noise does creap in. Using metal film I/V resistors instead of Allen Bradleys helped reduce it a bit, but not as much as might be hoped. Also, the small Blackgate output caps are quieter than large film caps hanging underneith. My sense is that, unless space is a big concern, that one might to better using an Amveco 62001 transformer. This allows you to mount it off the board by soldering the leads to the pads provided for the 70001. This preserves the Baby Steps (tm) circuitry while reducing the noise induced by the transformer (just don't forbget to twist those leads.) You might even move up to a 62011 or 62021 as the larger VA rating might help out the sound a little.
  • For informatrion on how to calculate the resistor values in the I/V, take a look at this post at DIYAudio.com.
  • Other tips and tweaks for TDA1543 based DACs can be found here.

Parts List
Here is the parts list. I believe it to be complete and correct, but you should check it carefully. If you are in doubt which part goes where on the board, use the board file to check.
Position Part Type Part # Quantity Supplier
U1 USB Receiver PCM2707 (296-15276-5-ND) 1 DigiKey
U2 DAC Chip TDA1543 1 You can usually find these on ebay. Note that the TDA1543A does not work.
T1 Transformer 70001 (you might also want to try a 62001, 62011, or 62021 -- see the note above.) 1 DigiKey
L1 AC Filter Line Choke PLK1156-ND 1 DigiKey
D1, D2 Bridge Rectifier W01GDI-ND 2 DigiKey
C4, C5 Capacitor Electrolytic (1800uF/25V) P12382-ND 2 DigiKey
C6, C7 Capacitor Electrolytic (680uF/10V) P12365-ND 2 DigiKey
C8, C9, C10 Capacitor Film - MKP (0.1uF/63V) BC2054-ND 3 DigiKey
C11 Capacitor Film (0.1uF/63V) This is a polyester cap. The MKP used in C8-10 arguably sounds better (though probably not noticably,) but does not fit the board. You can use it instead, but it has to be mounted underneith. 495-1103-ND 1 DigiKey
C1, C2, C3 Capacitor XY Type (.082uF/275VAC) P11005-ND 3 DigiKey
C17, C18, C19, C20, C21, C22, C23, C24 Capacitor Ceramic (SMD 0805, 100pF/50V) BC1268CT-ND 8 DigiKey
C15, C16 Capacitor Ceramic (SMD 0805, 33pF/50V) BC1262CT-ND 2 DigiKey
C12, C13, 14, C27 Capacitor Ceramic (SMD 0805, 0.1uF/16V) BC1300CT-ND 4 DigiKey
C25, C26 Capacitor (Output, 2.2uF-4.7uF) The board is spaced for Blackgate 4.7uF/50V N series, but see my capacitor page for some other options. 2 PartsConnexion, Michael Percy Audio
U3 Voltage Regulator (9V) 497-1448-5-ND 1 DigiKey
U4 Voltage Regulator (5V) LM78M05CTNS-ND 1 DigiKey
U5 Voltage Regulator (8V) AN8008-ND (or AN77L08-ND) 1 DigiKey
U6 Voltage Regulator (3.3V) LP2950CZ-3.3-ND 1 DigiKey
X1 Crystal 12.000MHz CTX058-ND 1 DigiKey
X1a (optional) Crystal Insulator. If you don't use one of these, you'll need to find some way to keep the crystal from shorting traces on the board. HC49INS-ND 1 DigiKey
L2, L3, L4 Inductor (1uH, SMD 0805) PCD1184CT-ND 3 DigiKey
L5, L6, L7, L8, L9, L10 Ferrite Bead (SMD 0805) 445-1567-1-ND 6 DigiKey
R1 AC Filter Resistor 1M, 1/4W . 1 .
R2, R3 AC Filter Resistor 100R, 1/4W . 2 .
R4 LED Resistor 680R 1/8W (or Panasonic 1/4W) . 1 .
R5 LED Resistor 480R 1/8W (or Panasonic 1/4W) . 1 .
R6, R7 Output Resistor 47K, 1/4W . 2 .
R8, R9 I/V Resistors. (configuration 1 = 1K, 1/4W) (configuration 2 = 2.7K, 1/4W) (Different values and types will change the sound, sometimes for better, sometimes for worse. Be sure you know what you are doing if you change these.) . 2 You can get these from DigiKey, but high quality Metal Films (PRP, Draloric) or Carbon Comps (AB, Riken, Kiwame) are recommended and sound better.
R10 Bias Resistor (configuration 1 = 1K, 1/4W) (configuration 2 = 1.4K, 1/4W) (Different values and types will change the sound, sometimes for better, sometimes for worse. Be sure you know what you are doing if you change this.) . 1 DigiKey
R11, R12, R13 100R (0805) . 3 .
R14, R15 1M (0805) . 2 .
R16, R17 22R (0805) . 2 .
R18, R19 1K5 (0805) . 2 .
J1 USB Jack 154-2442 1 Mouser (There do not seem to be any available at DigiKey that will work)
J2 Terminal Block ED1623-ND 1 DigiKey
F1 Fuse Clip F058-ND 2 DigiKey
F1a Fuse 5x20mm (100mA/250V) WK5027-ND 1 DigiKey
J3 IEC 486-1003-ND 1 DigiKey
J4 RCA Jacks . pair PartsConnecion, Michael Percy Audio