How to Perform the MCM DAC Mod October 26, 2010 07:10 13 Comments

Jim Williams of Audio Upgrades first tipped recording folks off to the MCM 50-7774, a "hidden treasure" D/A converter with a stellar DAC chip and a very low price tag. The design is an early version of the audiophile-endorsed Beresford TC-7510 that was licensed to OEM manufacturers. And, while it is marketed as a home hi-fi DAC, the 50-7774 has gained popularity among engineers as a professional mixing and mastering converter. The stock unit is impressive in itself (various Gearslutz members have said they prefer it to RME ADI8-Pro, Presonus Central Station, and MOTU 828 mkIII converters), but a couple of mods proposed by Williams and others can put it on par with really high-end DACs. The mods can be done in a couple of hours, and the whole project costs about $135 including the stock unit and premium replacement components.

Tech info: Up to 96kHz/24-bit operation. 2x Toslink and 2x AES/EBU coax S/PDIF digital inputs.

Williams' basic mod consists of three steps:

  1. Replacing the output chip with two opamps from Analog Devices
  2. Swapping the .1uF ceramic capacitors for a high-quality alternative
  3. Replacing the output caps with Panasonic FM series


Sourcing Parts

1. The stock unit

The 50-7774 is available here from MCM Electronics for $118.99. From time to time MCM runs a special in their catalogs, which can save you $15 or so. If the 50-7774 goes on sale again, I will post the discount code here.

2. Opamps and adapter

Update 8/26/11: Commenter CB has pointed out that AD now offers a dual version of the 4898-1 chip which may be a direct replacement for the stock output amp. Evaluation units are available from the Analog Devices website.

a. The ADA4898-1 opamps are available from Analog Devices as free samples. On the ADA4898-1 product page add the model 1YRDZ to your sample cart. Make sure you order two!

b. The new opamps will not fit directly on the converter's PCB, so you need to buy a Brown Dog component adapter. The correct model number, 031101, is available here from the Cimarron Technology website for $6.95.

3. Capacitors

a. 2x .1uF: Williams replaced the .1uF ceramic capcitors with very high-quality polyprops from MultiCap. These can be hard to find, but I have tracked down two suppliers who seem to have it in stock:

  • Canada: Parts Connexion
    • Follow the link and scroll down to: MULCAP-036-51402. Price: $4.48 CAD/piece
  • US: Percy Audio
    • Percy Audio takes orders by email. You want the MultiCap PPMFX seires, .1uf 200VDC. Price:$3.70/piece

If you want a cheaper part that you can combine with your existing Mouser or Digikey order, something like the Panasonic ECW-F .1uF will also do the job.

  • Digikey #P12092-ND
  • Mouser #667-ECW-F4104JL

b. 2x 470uF: The output caps can be replaced with Panasonic FM 470uF 25v, thin style electrolytics. It's important to make sure you get the thin style so they will fit in the unit! Double check that it has the 0.315"(8mm) diameter.

  • Digikey #P12389-ND
  • Farnell #1219470
  • Mouser #667-EEU-FM1E471L

For those keeping an eye on the price, let's add it all up:

  • $119 for the stock unit
  • $6.95 for the Brown Dog adapter
  • $7.40-$9 for the high-end .1uF caps
  • $2-or-so for the Panasonic FM caps
  • Total: about $135-140

Modding It

The only unique challenged posed by the mod is replacing the SMD opamp. Working with surface-mount components is no more difficult than the through-hole stuff you are used to if you've got the right tools. A small soldering tip and a magnifying glass help a lot. Also, a solder sucker is probably a bit heavy handed for this project, so I would advise picking up some desoldering wick.

When removing the stock amp, the first thing to do is to remove the two tall 470uF capacitors on either side of it for better access. Since we're junking the old chip, you can make it easier to remove by clipping the leads off at the body. Then remove the leftover legs by heating them and pulling them out with tweezers. After this, clean up the pads with your iron and desoldering wick, but make sure not to linger too long on each or you might lift the pad from the PCB.

Solder the two ADA4898-1 chips to either side of the Brown Dog adapter and then solder that to the old chip's footprint on the PCB.DIY DAC Converter Mod

All that's left is replacing the the standard, through-hole capacitors and putting everything back together. And that's it! If you've got any questions about the mod, or ideas for additional mods to the 50-7774 please post them in the comments section.

Disclaimer: This post represents my research on how to do this DIY project. Modding electronics can be dangerous and will void your warranty. I do not recommend anyone attempt this who does not have professional knowledge of electronics. I cannot not be held responsible for injuries or damaged equipment as a result of doing this mod.

Additional info:
You can see Jim Williams' original post on Gearslutz here.
Ortiz Audio has a great blog post about doing the mod.