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

 


18 comments:

  1. Larry,

    I own the 09 OPA2604 version of this DAC, but unfortunately the toroid transformer is shorting out. Do you happen to know where I could find a replacement transformer for this unit? The original part is not labeled in any way whatsoever...output voltages, etc. Thanks for the article. I hope to try some of these modifications as soon as I get my unit running again.

    Craig

    ReplyDelete
    Replies
    1. Hi Craig,
      The toroid transformer on my unit has no part number or load rating. I don't know where you would find a replacement.I do however have a label on mine which states transformer wire colours and ouptut voltages.
      I have added the photo to the blog.

      Cheers

      Delete
  2. Hey Larry,
    I want to change the volume pot of my DAC Zero cause there's a lot of noise/lost channel when using my headphones.
    Witch part can I replace it with?
    Thx.
    Samy

    ReplyDelete
    Replies
    1. Hey Sam,
      I changed mine with a part like this
      http://www.ebay.com/itm/DACT-Type-SMD-Stepped-Attenuator-21-step-/190844814249

      make sure its 10k.
      You will have to solder links / leads from the through holes on the attenuator to the main board.
      Also if its anything like mine the shaft will be a little long.

      Cheers

      Delete
    2. Hey again Sam,
      If you are just replacing the original just check / measure the full scale resistance of your existing potentiometer. I can't recall what the original value was.

      Delete
  3. Thanks Lawrence yep I'll measure it to be sure.

    ReplyDelete
    Replies
    1. Hey Lawrence I ended up ordering this http://nz.element14.com/bourns/pdb182-k430k-104a/rotary-potentiometer-100kohm-17mm/dp/1822848 and apart from the shaft that I had to shorten it works!!!
      Really happy to have it working!!!

      Delete
    2. Hi Sam, Great I see you bought a pot that has terminals on it, makes it easier. Truth be said I haven't even gotten around to shortening the shaft, mine is only about 10mm too long.
      Cheers

      Delete
  4. Is there anyway to get a replacement headphone amp? I keep losing audio on my right channel. Not sure if it's the volume pot or the headphone jack. I think it might be the headphone jack because once i move the jack around I can get solid sound for a few minutes.

    ReplyDelete
    Replies
    1. Not as far as I know. Have you tried re-soldering the headphone jack?

      Delete
  5. Larry,

    On a whim, I purchased a modified 2009 Zero DAC for $69 on eBay. The modifications are:
    - Opamp upgrade to the OPA627AU (DAC side)
    - 2 LT1364 upgraded opamps (AMP side)
    - HDAM upgrade

    I have a couple newbie questions. My only experience with DACs is when I setup my current desktop system with a cheapo Orei Da21x. Then I bought a AudioQuest Dragonfly Black 1.5 for my phone and I was blown away and decided I needed to change my deskop. I'm streaming the lossless Tidal to a pair of Emotiva 5s powered monitors and a Def Tech 10" Supercube sub.
    1) Is the Zero DAC as modified a good DAC? My current budget is about $200.
    2) Should I make any additional modifications? IE, the above blog you wrote.

    Thank you for the help.

    ReplyDelete
    Replies
    1. Hi Scott,

      Sounds like you have a nicely modified Zero DAC unit.
      I'm unable to comment how my modified Zero DAC unit performs, as I have nothing to compare it with. However the changes I have made should make it better than the stock standard unit.

      If you have access to a solder sucker station, I would make all of the changes I have proposed (otherwise it is a bit tedious). Although I haven't mentioned it in this blog, I have also changed the TCXO to a tighter specification part (this was the most expensive part of the upgrade). Personally I would stick to a unmodified DAC unit as you can compare reviews of cost vs sound quality.

      Hope this help.

      Delete
  6. Please could you explain how to install Bypass capacitors? As described 2 and 3 point?
    many thanks!

    ReplyDelete
    Replies
    1. Quite simple. Find those large electrolytic capacitors, turn board over to the solder side find the leads of these capacitors, and solder a small bypass capacitor across the leads of these electrolytic capacitors.

      Delete
    2. Great, already done!
      Please could you clarify capacistance of supply decoupling capacitor on TCXO and other 4 (pink one on your board),
      and clarify about this: Add local low ESR capacitors to supply rails directly off Op-Amp supply pins - capacistance and which pins to solder?

      Many thanks!

      Delete
    3. The large electrolytic capacitors are DC storage capacitors that want to be of low ESR type and as big as you can physically fit in the available space, paying attention to the voltage rating also, and the polarity.
      For example in the above circuit schematic electrolytic capacitors C2 & C4 want to be more than 15VDC rated. I would select a 220uF 25V. These capacitors need to be as close as physically possible to the op-amp supply pins. I have bypassed all electrolytic capacitors. Suitable value is 33nF to 100nF.

      Delete
    4. Thank you!
      Sorry for stupid question - op-amp supply pins how to find?

      Delete
    5. The best way is to download the datasheet for the device. All specifications including package information and pin outs are in the datasheet.

      Delete