I am really enjoying my Maverick Audio Tubemagic D2 DAC and thought I would take the time to post my impressions.
I elected to purchase a "open box" D2 DAC with the upgraded GE5670W tubes which was $20 off of the list price. The unit was shipped to me from a Florida address instead of China. And it arrived quickly and in perfect shape.
I plugged in the D2 to my 2 channel listening setup which includes: Parasound 2100 preamp, two Parasound HCA-1000A amps biamping a pair of Magnepan 1.6QR speakers. I have a music PC which drives via USB a Firestone Bravo SPDIF reclocker which is spec'd at 50ps jitter which in turn (via a RCA to BNC cable) drives the D2's BNC input.
I listened to the D2 and was very happy with the sound. Clear, 3D-like, and distortion free. I had previously been using a ebay DAC called the SMSL SD793. I compared the tube outputs as well as the normal solid state outputs of the D2. The difference is really very subtle. Over time I came to realize that the tube output just sounds more pleasant and enjoyable over a long listening session. Not really a dramatic difference in sound, but rather one of "comfort". Hard to describe really.
I then decided to see what some simple MODS could do.
First up, I notice that the D2 came with a really low end looking power cord. I am not one to go for a super expensive super duper high end power cord. However, I used my Greenlee GT-16 electrical field tester (http://www.amazon.com/Greenlee-GT-16-Adjustable-Non-Contact-Detector/dp/B001QIJOW0) and noticed that the standard D2 power cable was spitting out significant power line noise indicating very poor shielding. I swapped out the standard power cord for a Volex 17660 ($10) power cord. The 17660 is a very well made and more importantly well shielded power cord. Retesting with the Greenlee GT-16 was now dead silent.
Second, I took a hard look at the BNC connector on the back of the D2. BNC connectors come in several flavors including 50 ohm and 75 ohm. For SPDIF you want 75 ohm to minimize "reflections" of the signal which can, if large enough, introduce sound quality robbing jitter errors. It is important to maintain a 75 ohm connection all the way thru the signal path which includes the jacks, plugs and cable. Now RCA jacks/plugs are not truly 75 ohm, so there is not much you can do there. But the BNC connector is supposed to be 75 ohms. Looking closely at 75 ohm female connectors you should see that the insulation ring around the center female pin should be smaller then what is commonly found on a 50 ohm BNC connector. Without expensive test equipment you cannot absolutely measure for sure what type of connector you have. Also, at the signal frequency of SPDIF (7MHz or so), the signal impact of a mismatched less then 1 inch connector should be small (but not zero). However, the D2 BNC connector appears to be a 50 ohm connector and seemed like it would be easy and inexpensive to replace. So I replaced it. You can see the pictures below. Note in the picture where I am holding the original (on left) and replacement (on right) connector you can clearly see the difference in the width of the center pin insulation material.
Original D2 50ohm? BNC (Left), Replacement 75 ohm BNC (Right)
To replace the BNC connector, simply loosen the nut on the inside of the D2 chassis holding the BNC connector inplace. Use a wire clipper to cut the solid wire going between the BNC signal pin and the D2 main board. Remove the old BNC connector but leave the shield/ground solder tab in place. Solder a short copper solid core wire to your signal solder cup of a new 75 ohm BNC chassis/bulkhead connector (sourced from digikey, mouser or newark).
I used this one from digikey: http://www.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&keywords=A33815-ND&x=0&y=0&cur=USD
Cut the solid core wire so that it is long enough to extend down to the D2 main board. Melt some of the existing solder from the old connection to the new solid core wire. See the picture below for details.
The power cord and the BNC connector swap did make a difference in the sound quality. A sharpening of the sound.
Next up, I tried some OPAMP rolling. Stock the D2 has the fairly good OPA2604. Maverick Audio sells and recommends the OPA627 OPAMP as an upgrade. Both OPAMPS are FET style OPAMPS. Generally, if one does not know the specifics of the rest of the circuit design, it is safest to stick to the same technology (FET for FET, BIPOLAR for BIPOLAR). I had OPA827, LME49880, and ADA4627-1 FET OPAMPS on hand to try out. I exchanged email with the ever helpful Ryan@Maverick Audio and he told me that the D2 supplies +/- 15VDC to the OPAMP socket which is also a needed piece of information for selecting compatible OPAMPs. The OPAMP candidates above all are ok with 15VDC.
In general the newer higher performance OPAMPS require power supply capacitor bypassing and decoupling to operate at spec and deal with signal noise coming in from the power supply lines. Since I was not sure what was in place in the standard D2 circuit design, I added the bypassing capacitors between pin 4 and 8 of the SOIC to DIP adapter boards I was using for the replacement OPAMPS. Pin 4 and 8 carry the +/- 15VDC. You can see the orange capacitors in the close up figure below. The D2 requires 3 dual OPAMPS to operate. All 3 must be swapped together per Ryan@Maverick Audio.
Maverick Audio D2 Dac Cover off with OPAMP rolling candidates shown right.
Close up of Maverick Audio D2 with replacement OPAMPs on SOIC to DIP adapter sockets. Note orange bypass ceramic capacitors.
I listened closely to the sound before and after each OPAMP rolling experiment. I noticed a significant jump in sound quality moving from the stock OPA2604 to the first OPAMP, the OPA827 I tried. The sound was clearer and the instruments in the music sounded like they had better separation. The higher pitched female vocals sound much smoother especially in the "S" sounds. However, when I swapped in the ADA4627-1 everything seemed to really come together with a very smooth yet detailed sound. I have yet to try the LME49880, but I will at a later opportunity. For now I am happy with my upgraded power cord, BNC connector and ADA4627-1 OPAMPS.