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-   -   Legato 3, Common Mode DC and You. How do you choose to deal with it? (http://www.diyaudio.com/forums/twisted-pear/224305-legato-3-common-mode-dc-you-how-do-you-choose-deal.html)

barrows 25th November 2012 10:21 PM

Legato 3, Common Mode DC and You. How do you choose to deal with it?
 
I know this has been discussed some, but I was thinking starting a thread with the desire to have a comprehensive discussion in one place could be a good idea.
I am about to start a new BIIIse/Legato 3 build. It appears there are a few possible ways to deal with the offset from +/- to ground on the Legato3

1. Do not worry about it and hope the amplifier is compatible.
2. Add caps and eliminate the problem entirely.
3. Do not connect the signal ground to anything, and hope the amplifier is compatible, floating.
4. Do not connect the signal ground to anything, and transformer couple the output.

I have heard some suggest that #1 will only work in some cases, and that it may not provide the best sonics (why?).
We know why we would like to avoid #2 if possible.
I have read some suggestions that #3 can work in some cases, but may be an undesirable solution as there is a sonic advantage to a signal ground connection between components even in the case of true differential circuits.
Solution 4 is relatively expensive, the Legato's medium output impedance may not drive a transformer ideally, and some feel that the transformer itself will color the signal (although I suspect this would not be a problem with a really good Jensen or Lundahl properly implemented).

I am interested in hearing of others' experiences and opinions on this topic. Currently, I am using a cap (Mundorf Supreme) coupled Legato2, but I would like to eliminate caps in my new Legato3 build, without other compromises if possible. I have a few different amplifiers: Pass Labs X150.5, PS Audio Classic 250, and nCore 400 based, and I only run balanced.

Russ White 26th November 2012 01:48 PM

Actually #1 and #3 are the best possible solutions if your amplifier can handle it.

#2 Is as good as the caps you use.

#4 is only as good as the trafo/scheme you use. :)

pelliott 26th November 2012 05:38 PM

am i missing something?
 
I have had a "no caps" lagato for quite awhile now and once I set the offset ot zero its been there ever since. I check it about once a month and it has not moved. I use TP power supplies that are stock except for a few bypass caps on the lytcs.
I use a calibrated Fluke 77 to measure.

Russ White 26th November 2012 05:40 PM

Good point of clarification. It is very easy to keep the actual offset nulled.

I think Barrows really meant the common mode voltage - which you are right is not really offset.

barrows 26th November 2012 06:00 PM

Yes,
 
Good point Russ. I am definitely referring to the DC (generally 7-8 volts) from +/- to ground.

I am really considering the transformer option, I know some will cringe, but good transformers from Lundahl and Jensen (if one can handle the price, but hey it is cheaper than going dual mono, which is of questionable benefit) add very little distortion, and may actually improve performance of the connected amplifier by blocking RF.

One thing I would like to hear from experts on is the option of not connecting the signal ground at all. I have read some reports where some people claim this approach could somehow reduce the performance of the connected amplifier. I do not really understand this notion in the case of an amp with a truly differential input stage?

cotdt 26th November 2012 06:05 PM

Quote:

Originally Posted by pelliott (Post 3257713)
I have had a "no caps" lagato for quite awhile now and once I set the offset ot zero its been there ever since. I check it about once a month and it has not moved. I use TP power supplies that are stock except for a few bypass caps on the lytcs.
I use a calibrated Fluke 77 to measure.

There should be ~8V DC between Ground and +, and between Ground and - outputs from the Legato. You can only zero out the DC between the + and - outputs.

If outputting to a bridged differential amp, it is a big problem. Unfortunate, because this is an easy way to get 2x to 4x power from an amp while decreasing noise at the same time.

cotdt 26th November 2012 06:14 PM

I would think that a good coupling capacitor would beat the transformer solution.

barrows 26th November 2012 06:21 PM

and...
 
Quote:

Originally Posted by cotdt (Post 3257780)
I would think that a good coupling capacitor would beat the transformer solution.

For the purpose of discussion, why would you think that? One of the best digital sources I have ever heard (the Linn Klimax DS, current version) uses Lundahl transformers to couple its output. This unit has been third party tested by Stereophile, and the measurements are quite good, easy to look up at stereophile.com as well.

When you say "good" coupling capacitors, what would you consider "good": one could pretty easily spend >$1000 on four coupling caps…

Here is a link to the Klimax DS measurements:

http://www.stereophile.com/content/l...r-measurements

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.

cotdt 26th November 2012 06:34 PM

Quote:

Originally Posted by barrows (Post 3257793)
For the purpose of discussion, why would you think that? One of the best digital sources I have ever heard (the Linn Klimax DS, current version) uses Lundahl transformers to couple its output. This unit has been third party tested by Stereophile, and the measurements are quite good, easy to look up at stereophile.com as well.

When you say "good" coupling capacitors, what would you consider "good": one could pretty easily spend >$1000 on four coupling caps…

I'm basing my conclusion (re: caps vs transformers) based on my experiments comparing interstage transformers versus capacitor coupling in tube amplifiers. The capacitors were cheaper and sounded better.

I've also used capacitors at the DAC's output, and it sounded very transparent. One thing I've noticed is that the smaller the value, the more transparent the capacitor. Now I use as small a value as my calculations allow for, with -3dB at 10Hz.

One place to start would be to try Mundorf Silver/Oil and ClarityCap MR. I think they will beat the transformer solution.

Russ White 26th November 2012 06:38 PM

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 (at least the signal need not be centered there - DC offset will be ignored as long as it is within limits). For example my Sympatico design. will work fine with common mode DC voltage up to about 10V - maybe even a bit more.


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