Legato 3, Common Mode DC and You. How do you choose to deal with it?

[cotdt]

Originally Posted by barrows

Here is a link to the Klimax DS measurements:

Linn Klimax DS network D/A processor Measurements | Stereophile.com

note that these are for the earlier version, since then Linn has upgraded to Lundahl's amorphous core transformers, which should have slightly less added distortion.

These are very impressive measurements. But amorphous core transformers will not sound neutral. They will sound different, some will like and others will not.

[cotdt]

Originally Posted by Russ White

Bridged amplifiers are different from a truly balanced (fully differential) input. In a case like that you would be better off using coupling caps or the IVY-III

A fully differential input stage does not need a ground reference. For example my Sympatico design. will work fine with common mode DC voltage up to about 10V - maybe even a bit more.

Can you redesign the Legato to have no common mode DC offset? I prefer the Legato to the Ivy and want to stick to Legato.

[Russ White]

I personally have another I/V stage that does just that. It is called the Counterpoint III. It is not available to anybody but me at the moment, and it may not ever be. It is a DIY project I have taken on for myself.

It uses a folded cascode output with a common mode servo.

[barrows]

really want my -3dB point to be closer to 2Hz, otherwise there is going to be phase shift within the audible bandwidth. Hence, good caps start getting pretty big and expensive. I have Mundorf Supremes (3.9uF) in my BII/Legato2 build right now.
Also, I wonder of some of the good listening experiences I have had with the Linn are due to the isolation factor of the transformers. I guess without trying them it is hard to say. But I am getting suspicious about RF on the output of DACs, and how this RF behaves in the input stages of some amplifiers.

What line level transformers have you tried? From the published specs, some appear to be way better than others.

[Russ White]

The Legato design itself with its LPF is already a good barrier to RF as long as it is wired sensibly .

[Russ White]

A a good 10uf cap is not hard to find.

What impedance are you driving?

[barrows]

Originally Posted by Russ White

The Legato design itself with its LPF is already a good barrier to RF as long as it is wired sensibly .

Russ, I hear ya on that. My build will have considerable output wire length (although balanced), as the only way to properly mount a USB receiver is between the back panel and the Legato/BIII, to keep I2S lines short. I wonder if their is any RF pickup after the filter from airborne RF… This is one reason why I am thinking trannies right at the XLRs might be a good thing. Admittedly, I am picking nits here!

Input impedance depends on the amp used, the lowest would be my Pass at 11K per phase. The 3.9uF caps I have now work pretty well with the Pass, but they are really big physically.

[Russ White]

Honestly - IMO if you were going to use a transformer in that way - the best possible place to put it would be at the power amp.

I personally have never heard a transformer that did not add some color. Not saying it's good or bad. And I have heard quite a few.

[barrows]

Originally Posted by Russ White

Honestly - IMO if you were going to use a transformer in that way - the best possible place to put it would be at the power amp.

I personally have never heard a transformer that did not add some color. Not saying it's good or bad. And I have heard quite a few.

Everything matters: I agree that a transformer will add its own signature, as will differing output wiring, or caps. From a technical perspective though, the measured distortion added is so minimal as to be inconsequential, and I still think there may be benefits: whether these are to taste/preference will be the question. So far, the two DACs which I have listened to quite a bit which used very nice Lundahl output transformers have both been among the best digital sources which I have ever heard, so, for me, I am not too worried about a good Lundahl (1684 amorphous or 1588 Mu metal) degrading the sound from what I have already heard.

[RollE2k]

Russ: will the Counterpoint III be a discrete I/V? also, will this one be released as a product, or like the older ones just a few pcbs?