First, I want to thank you for providing this kit - it has provided me with countless hours of audio bliss.

But now to my tale of woe: After about 6 months of enjoying the BIII in stereo, I decided to try the dual mono configuration. Once I put the whole thing together - not the spectacular sound I was expecting - something is wrong.

So here is what I know:

-- old BIII (we'll call master v1.0.2) works great still all by itself with Legato
-- new BIII (which oddly is older v1.0.1) works by itself as well, but has high pitch noise at high volume

I only mention version number so you can distinguish the two in the pictures. My understanding is that there is no substantial difference between the two.

Here is what I tried:

-- swapping power supplies - they are both powered by placid HDs. No difference.
-- swapping AVCC modules - master is still dead silent, noise on slave is reduced but still present.

I tried "slave" by itself (firmware controller installed) - it worked but high pitch noise remained.

Then I tried looking at the noise with an oscilloscope. Attached are pics of of output of both master and slave for comparison. I'm taking signal directly from the BIII board (03, 04) with no Legato attached.

Any help on where to go next with this would be appreciated.

Edited by user Monday, January 28, 2013 4:05:40 AM(UTC)  | Reason: typo

Are you controlling the volume digitally? Or with a preamp.

I notice you have the older AVCC modules. It may help to add some compensation to them. It is pretty easy.

If you have a couple 100-220pf caps try putting one across each -IN and OUT of the dual opamp on the AVCC. This makes the module very stable and more immune to instability that can be induced by RF/EMI etc.

Since you are going dual mono ultimate quality is likely very important to you. I would strongly suggest you upgrade the AVCC to the current version if possible. It's just a better circuit. The old version will work fine, but you may need the compensation as suggested above.

Some other possibilities:

1) A ground loop (high speed)
2) EMI pickup from some other source?

It can be tough to diagnose from afar. :)

I will try to help any way I can.

Could be several factors. I think the most likely is a difference in the accumulative value of bypass caps on the AVCC pins of the DAC between the two buffalo revisions - just by normal part variation, and they may actually be slightly different parts.

I think the comp caps will do the trick.

Please let us know.

Cheers!
RUss

Excellent! Glad to help.

I highly suggest running dual mono into a single Legato if you wish to run dual mono.

I use the Legato with stock values and then add a 392-680R across + and - at the balanced output. This will lower the output back down to 2VRMs. It also reduces THD+N. This configuration sounds superb. 392R will get you a bit less than 2VRMs while 680R more.

You can adjust the value until you find your sweet spot. :)

Wiring mono into half a Legato is simple. You just needs to remember that the half of the buffalo opposite from the selected channel will have its phase reversed. So for the right side DAC right half output is normal, left is reversed. So to parallel connect Right + to Left - and Right - to Left +. For the left side DAC the process is exactly reverse. :)

Let me know how it goes and/or if you need any further help.

Edited by user Sunday, February 3, 2013 5:49:15 PM(UTC)  | Reason: Not specified

Any preference for resistor type here...like metal film, etc? 1/4 watt 1% good enough?

Edited by user Thursday, February 7, 2013 12:39:09 AM(UTC)  | Reason: Not specified

Russ, what is best procedure for testing output voltage on balanced out? Play a test tone (like 1K), full volume on DAC while connected to a load such as pre amp? Should I see full or half voltage measuring across pos and neg of balanced?