[dcbingaman]

I finally finished my Pass Pearl phono preamp, and since my Liteaudio DAC-60 worked so well with it, I couldn't leave well enough alone and decided to change the output coupling caps.  I put in two huge 4.0 mfd/400V Marsh MultiCaps, but there is little room to get them very far away from the tubes.  I don't want to blister these caps, (the wife would not appreciate the smell), so I'm wondering if I can insulate them with little squares of aluminum foil bonded to the cap body with enough RTV to insulate the polypropylene tape.  Anyone tried this ?  (Also, I'm replacing the Electro-Harmonic 6922's with RAM Labs Grade A 6DJ8's - should improve the sound, and the glass envelopes of these tubes are smaller around.)  How close is too close ??  The only other alternative is to move the board closer to the front of the case, so the caps don't interfere with the output jacks, and can be moved rearward.

Also, I don't think the electrolytic control caps around the B-B PCM1704's make a lot of difference, but I would guess that replacing B+ BR6 with a "Hexfred"  soft recovery diode bridge, and replacing the non-descript 120 mfd / 400V and 45 mfd / 400V electrolytics with Blackgates or Nichicons and film bypass caps with in the B+ for the output stage might make a difference.  The working voltage is only 150 x 1.414 = 220 VDC, so it should be easy to bump the capacitance up quite a bit.  (I think you could even get a couple 600 mfd photoflash cap.s on the side of the board, but my ears are still ringing from the last time one of those bastards fragged on my workbench.)  How much capacitance can you put in here without straining the x-former ?  Does this have any appreciable impact on the sound ? 

Any other changes you crazy modkateers have tried ?  See my Gallery for pictures......

[dcbingaman]

Ok guys, I've been looking closer at this thing, and it appears that the board can easily be slide forward ~1/2" to accomodate the larger caps by drilling 3 new holes in the chassis floor.  (I think the other three posts are redundant - this thing looks like it was designed to withstand 60 G's !!   )  I just need to replace the Toslink input 3-wire tie-in to the motherboard, (which I inadvertently broke anyway.)  Anyone try to convert this input to another co-axial input ?  I can't figure out where the relay is that switches this little bugger......

[JoshK]

Anyone try to convert this input to another co-axial input ?  I can't figure out where the relay is that switches this little bugger......

I've pondered this as well but I haven't tried to sketch/trace it out yet.  I'd rather have two coax (BNC actually) connections.  But do you even have to know where the relay is?  I mean, I have very little clue here but I'd have thought that the optical connection would be tied into a converter, rip that out and wire in the coax in its place.

Come to think of it, I think the switching is done by the microcontroller, not a discrete relay.  But I don't think the optical convertor is in the MC, so it isn't an issue (if I'm right).

[dcbingaman]

I took a closer look at the DAC-50 schematic last nite, and the three wires that come off the Toslink socket, (which is mounted to the backplate), are: Digital In+ which is routed to the relay, ground, and a power connection for the electro-optical converter.  I think if one were to connect a female BNC or Canare 75 ohm RCA to the board in the same spots for the digital in and ground, and just leave the power eyelet unconnected, it would work just dandy - two co-axial digital inputs.  (You just need to "roundize" the square hole to fit the co-axial connector).  When I move the board forward, I will make this change also, and see if it works.  Stay tuned.

[dcbingaman]

Well, modketeers, I moved the board 1/2" closer to the front, put her all back together and plugged it in, and sure enough, it still works !  Haven't done any long-term listening trials, BUT, the initial listen with the MultiCaps is good - a little more focus and sparkle on the top, but still as sweet as white corn-on-the-cob in the summertime.  I still can't get over how well built this thing is out of the box.....

Future mod.s: HV supply - IXYS Hexfred bridge, Nichicons and film bypass caps.....maybe also a series choke between the filter caps.  Replace Toslink input with 75 ohm BNC connector.

[JoshK]

Well, modketeers, I moved the board 1/2" closer to the front, put her all back together and plugged it in, and sure enough, it still works !  Haven't done any long-term listening trials, BUT, the initial listen with the MultiCaps is good - a little more focus and sparkle on the top, but still as sweet as white corn-on-the-cob in the summertime.  I still can't get over how well built this thing is out of the box.....

Future mod.s: HV supply - IXYS Hexfred bridge, Nichicons and film bypass caps.....maybe also a series choke between the filter caps.  Replace Toslink input with 75 ohm BNC connector.

Thanks for sharing.  Your idea of moving the board forward is a good one!  I'm going to have to do that too, as it will also give me more room for tube stage mods. 

I still haven't done any mods to mine yet, as I am trying to finish up the box I built for my SB power supply and trying to finish my Arvo Part baffles.  I just got my Hexfreds & Series choke in and I have some decent coupling caps, so that is where i intend to start.  I have a ton of Rubycon ZA/ZL and Nichicon Gold caps from a diy'er parts buyout I did a while back, so I'll probably replace some in key places. 

After cleaning up the B+ PS and coupling caps, I was thinking of tinkering with some regulator tricks for all the many 317 regulators in the box.  Even before tinkering though I'll probably replace the two digital inputs with BNC's, cause that is an easy one.  Then I'd like to mess about with alternative tube stages, since this is my real hobby. I have some ideas that I've learned over the last year that I'd like to try out.   Then after that, if I still have ambition, there is a great circuit for i2s transmition that I'd love to play with.    This DAC is just an amazing test board for a lot of ideas I wanted to play with, which is why I bought the DAC.  Its base circuit is precisely the kind of base for that makes testing worthwhile.

[dcbingaman]

Josh - sounds like some great ideas.  While you're at it, you might also consider upgrading the regulator IC's.  Micheal Percy, (the parts king), and Digikey both have Linear Technology LT317AT regulators that are form, fit and function replacements for the ancient National Semiconductor LM317 5V regulators that populate the digital and analog power supplies.  Not sure how they would work here, but they did make a noticable difference on an electronic X-over I built a few years back.  Supposedly, they feature much lower output impedance that the LM317's.  Good luck, and let us know how it turns out !!