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I'd like to share something with users of BII and BIII dacs who operate them in dual mono configuration.
The idea of dual mono is to keep both left and right channels of a dac completely separate. In the case of the BII specifically (and likely too with the BIII), the use of Volumite creates an unfavourable common connection between the two dac sides.
The BII in dual mono will also have a second common connection between the two dac channels and this is at the input signal connection between the dacs (TTL from right to left channel).
Our measurements and listening tests have shown that there is something to be gained from eliminating this unwanted interaction between the "grounds" of the two dacs. Not having a second Volumite on hand, we worked around the problem by reverting to the individual on board controller chips. I would say with some confidence that if you must use Volumite, use two - one for each dac
In addition, for BII dual mono users, use a 1:1:1 input transformer to feed each dac with the SPDIF signal. You can source a very good amorphous core transformer for exactly such purpose from Lundhal (LL1573). You will need to change the SPDIF 75ohm loading resistor on each of the BII dac boards to 150ohms to ensure proper loading. Also keep the lengths of the wires connecting the two transformer secondaries to the dacs exactly the same length. Connecting the dacs in this manner eliminates another common ground connection between the two dacs. You will also gain from the input isolation that the transformer provides.
Cheers
David
PS. Thanks John
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Care to share your measurements?
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A couple things.... Are you using one set of power supplies or two? In other words will the two DACs be sharing a common GND? If they will the solution is extremely simple. wire the volumite GND to the 5V supply ground directly. Do not run any GND wire to either DAC. If you are using two completely isolated supplies (no GND connection at all) then you will need an I2C isolator. You can use two supplies but you will need to tie their GNDs together as a single point and then home-run the volumite GND to that point. Edited by user Tuesday, July 10, 2012 3:58:50 PM(UTC)
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Hi Russ,
I use two placid power supplies supplied by individual transformers without any real ground reference. We added common mode chokes pre and post Placids.
To be clearer about my set up, I am not using an active output stage for the I/V, opting instead for 1:1 transformers. These are connected via a modified version of Joe Rasmussen's resistors to ground scheme.
Brian,
Apart from our scope picking up very low level 50hz noise running around the boards, our real world test was to play CD test tones. Prior to doing what we mentioned, the dac could not resolve the -60db and -70db tones.
It now plays the -60db tone cleanly. -70db hasn't been possible at this stage. More important than all this is that the playback has now become so much more lifelike with much improved micro dynamics.
For the past several days we have been listening to my system for hours on end. The small mods that we did to the set up of my BII have made a very good dac into a very very good one. And with high rez material played via the Japanese SDTrans unit built by Chiaki and Bunpei, the play back can be staggering!
Cheers
David
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Ok David, sounds like this is a simple case of wiring for the situation. :)
I sounds like your situation/scheme is far from typical - even for dual mono.
If you want to keep things completely separate you just can't share a common component directly. Honestly I can't think of any good reason to do that, but hey its your project. A great solution for anyone trying such a thing would be to use an I2C isolator at one or both of the DACs.
Better yet, use the thing as designed in a common chassis with a common ground, and home run you volumite ground as I suggested. :)
I am glad you are enjoying it. :)
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Hi Russ, The point of my post was to share the fact that connecting two BII boards via a single Volumite and the via the recommended SPDIF/TTL input connections creates unwanted loops that have a negative effect on the sound of the DAC. Your suggestion to use I2C isolators is undoubtedly a more practical solution than using 2 Volumites. I really would not say that my scheme is far from typical. Both dacs and their respective power supplies are on a single "chassis". Transformer coupling the dac outputs is also not unconventional. To our ears, transformer coupling sounds more natural than other active options that we have purchased, built and listened to. We have always favoured floating the "ground" of our dacs where possible as referencing them to ground in the conventional sense does have an unfavourable effect on the sonics. Grounds typically are electrically very noisy and in our systems, we can hear this. The performance of my SDTrans and BII combo continues to impress me and others. This is even with 16bit 44.1khz material and via SPDIF connection between source and dac. It is indeed a tribute to the DIY community that both these products are the creations of people who are "amateurs"  . I am glad that I own both products!! A BIII will soon be attached to the SDTrans via the favoured I2S manner. Cheers Edited by user Friday, July 13, 2012 3:52:09 AM(UTC)
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What I am saying is that using teleporter in a dual mono situation on its own predicates a shared ground. It is a common point. :) So using it in some other way is not typical. By diverging there you strayed from the norm. :) That is not bad. It just is. In any case there are simple solutions for just about any scenario you could dream up. I am happy you are pleased with your set up. Have fun! :) Edited by user Friday, July 13, 2012 4:01:50 AM(UTC)
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