Friday, 8 February 2013

Improving the Zero DAC

The Zero DAC is a low cost Asian designed and manufactured digital to analog converter pre-amplifier. Can be bought on ebay for under USD130 plus shipping for the OPA627 version plus USB, less for the OPA2604 op-amp version.








The Zero DAC uses the Analog Devices AD1852 24bit, 192kHz max as its DAC IC. The unit is most useful interfacing to digital sources such as:
  • CD player
  • PC

This fits the bill exactly on what I want. The PC connectivity is very handy. The Zero DAC uses the TI PCM2704 USB to S/PDIF integrated circuit (IC) which makes the Zero DAC look like a soundcard. Unfortunately this IC is limited to 16bit resolution and 48kHz sampling rates. However for digital music files that have been ripped from CD's this is sufficient. CD quality is typically 16bit, 44kHz.
I intend on using this unit for both as a digital sources pre-amp interfacing to a class d amplifier.

The unit also has a headphone amplifier however this is the weak link of the package see Headphone Amplifier schematic .  Do NOT buy a Zero DAC if you intend on using it solely for headphone output.

Pros

  • Digital coaxial input
  • Digital optical input
  • Generic USB soundcard, supported by Win7&8, and latest versions of LinuxMint (Ubuntu?)
  • AD1852 DAC IC
  • Volume control (both a pro and a con!)
  • Line voltage output
  • Toroid transformer
  • Op-Amp upgradable via DIP socket
  • The use of ferrite suppression on PCB layout
  • Able to be modified

Cons

  • Headphone circuit sub-standard (refer to headphone circuit upgrade)
  • No unbalanced line outputs. Personally I don't see the need for this unless you need a long run cable between the pre-amp and the amplifier. I plan on using at most a 1m RCA cable! 
  • High quality volume control will need a new headphone amplifier circuit. Since I'm not planning on using headphones I'm going to simplify the existing circuit to use just a pot (stepped attenuator 10k) and an op-amp.  This can be done by utilising the existing headphone PCA. You will need local decoupling capacitors hung off the supply pins of the op-amp say 47nF, I had oscillation at the output. Also a sizable low ESR capacitor should also be hung off the supply pins. The below schematic is of 1 channel. Make sure you use an audio op-amp, the NE5532 is cheap and cheerful that is unity gain stable.


Modifications
  1. Remove(snip) 2 x 22pF ceramic capacitors  from line input.
  2. Increase capacitance on digital supply add small bypass capacitor on rear of PCB (33-100nF)
  3. Increase capacitance on analog supply add small bypass capacitor on rear of PCB (33-100nF)
  4. Change AC rectification diodes 1N4007 to UF4007.
  5. Upgrade supply capacitors on AD1852, bypass with low value capacitor on rear of PCB (33-100nF).
  6. Upgrade supply decoupling capacitor on TCXO
  7. Modify Amplitude / phase response of Op-amp (refer to Bode Plots)
  8. Remove 4 x AC coupling capacitors from inputs of op-amp. These are not required as any DC is common to both inputs and thus rejected (refer to AD1852 datasheet).
  9. Add local low ESR capacitors to supply rails directly off Op-Amp supply pins.
  10. Change to stepped attenuator if potentiometer begins to cause issues. 


Et voila. If you are handy / know what you are doing when it comes to electronics and want a low cost digital source DAC pre-amplifier with potential this is it. The modifications are tedious without a solder sucker desoldering station. However it can be done with patience desolder wick, and using long component leads to help clear the through holes of solder.

I suggest buying the OPA627 version with USB. For around $20 less the OPA2604 version is probably okay too.
Interestingly the 2012 version of the Zero DAC includes the opa637 as an op-amp and more expensive?! The problem is this op-amp requires voltage gains of at least 5 times to be stable. The op-amp circuit in the Zero DAC is at unity gain!

Photo of Toroid transformer