It is hard to imagine how I can improve on the excellent sound I am getting from my RTZ DACs (both of them), however, I am embarking on building one more DAC board so that I can test out some more of the ideas put forward in this discussions so far. Plus I really enjoy building! (That's probably the real reason!)
This time I've ordered the stencil too so that'll be a new experience for me and hopefully make the application of the paste more uniform and have more reliable joints.
I didn't want to play about with my two currently working and completed DAC boards so having a 3rd board is great to be able to make changes and slot in to one of my DACs and A/B compare the results (since all other factors are the same) Plus once I'm finished I will have a complete spare DAC/Filter that someone else could benefit from. Seems a win-win.
This time I've ordered the stencil too so that'll be a new experience for me and hopefully make the application of the paste more uniform and have more reliable joints.
I didn't want to play about with my two currently working and completed DAC boards so having a 3rd board is great to be able to make changes and slot in to one of my DACs and A/B compare the results (since all other factors are the same) Plus once I'm finished I will have a complete spare DAC/Filter that someone else could benefit from. Seems a win-win.
This is what I am also attempting with my latest build, once the pcbs arrive. I bought 150 of these and have whittled them down to 32 with better than 0.1% spread. I have no idea yet how I will stick them down to the board to solder but I'll cross that bridge when I get there!
Thanks! I used the stencil this time. In fact I didn't scrape off quite as much as I should've on this side of the board but managed to recover the bridges with solder wick. The 2nd side turned out a little better with the benefit of experience.
Apparently the soldering wasn't quite perfect, lol. 
I've had a nice diagnostic session lasting quite a few hours. Ironically, the part I had expected to be trouble was the transistor stack in the reference but that worked perfectly first time. No, this time I got a nice 4.96v on the output of U1A but then only 1.6v on the clock supply. Ultimately traced (after assuming it would be the excess solder causing a short somewhere) to R19 having a bad joint. (As can be "clearly" seen in the above photo, if you know where to look. D'oh!
So, #3 DAC up and running! Hurrah!
Next step is to replace the DAC board in my 2nd built unit with this one and do some comparisons with the "stock" build. The only alterations to this board is the use of the Beyschlag MELF resistors and the change in resistors R124, R127, R129 & R131 to 270ohm.
I will report back in due course whether I can detect any difference by listening tests along.
I've had a nice diagnostic session lasting quite a few hours. Ironically, the part I had expected to be trouble was the transistor stack in the reference but that worked perfectly first time. No, this time I got a nice 4.96v on the output of U1A but then only 1.6v on the clock supply. Ultimately traced (after assuming it would be the excess solder causing a short somewhere) to R19 having a bad joint. (As can be "clearly" seen in the above photo, if you know where to look. D'oh!
So, #3 DAC up and running! Hurrah!
Next step is to replace the DAC board in my 2nd built unit with this one and do some comparisons with the "stock" build. The only alterations to this board is the use of the Beyschlag MELF resistors and the change in resistors R124, R127, R129 & R131 to 270ohm.
I will report back in due course whether I can detect any difference by listening tests along.
Seems to me you are likely going about this backwards. If the dac sound is muffled and smeared to some small-ish extent as I found it to be, why would you expect to hear a change that might very well be masked by other, bigger problems? if you don't hear any difference, it might not mean much.
Anyway, I would definitely suggest other experiments to perform first, ones that may have more dramatic effect on SQ.
So far you are the only one having reported issues with the sound. At this level perceived differences are small and quite likely subjective. Your subjective opinion does not carry any more weight than other opinions especially since it is clear that you have an ulterior motive.
I'm not really "going about" anything really Mark, I am not that organised.
My intention was just to a) build another board as I am a sucker for torturing myself with tiny SMD components ;-) and b) check out Beyschlag MELF resistors effect over the Susumo thin films. Since I didn't want to rip off all the Susumos from one of my existing DAC boards then building another board as a test-bed was the easiest. Since it'll be a "spare" so to speak, it can be used to test other alterations. However, one thing at a time with plenty of time for listening tests in between. I admit that it will be purely subjective and so probably not of much interest to others but ultimately I just want it to sound the best it can to my ears (which are nearly 62 years old so suspect at best!). As for muffled and smeared - I am not hearing that at all. I do prefer the sound using the positive only outputs and I will get around to trying your other suggestions Mark.Great. That's very helpful already. May I ask you to clarify/remind me as to how you are processing the positive outputs? It is through the Filter Board? Is it through transformers at some point? Also, are you loading the inverting outputs at all?
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Both my DACs are configured the same way at the moment. Which is:
1. "Simplified" filter board using OPA1678s
2. Positive side of each output channel taken through Lundahl LL1588 transformers
3. Negative outputs tied to ground (simply because this was the easiest way to make a switched test)
4. Separate diyAudio super-regs for the filter boards.
5. PCM2DSD with the latest version of bit file.
6. PCM via I2S from Logitech Transporter and Duet.
1. "Simplified" filter board using OPA1678s
2. Positive side of each output channel taken through Lundahl LL1588 transformers
3. Negative outputs tied to ground (simply because this was the easiest way to make a switched test)
4. Separate diyAudio super-regs for the filter boards.
5. PCM2DSD with the latest version of bit file.
6. PCM via I2S from Logitech Transporter and Duet.
Have you ever tried the transformers to see if they can be used to replace the Filter Board (with pin-8 maybe tied to the receiver ground)?
Yes, I tried the transformers with blocking capacitors. It was very sweet sounding but I think lost out somewhat with transients and pace. A bit too "tube like" for my taste these days. That's not to say I don't like tube amps and the sound of SE ones especially, I've built a few from kits. However, I've moved more to liking the sound of SS amps, if done well. Tastes change. It's all subjective.