common mode DC in balanced amps question?

[sbelyo]

ok, I'm finishing up a Twisted Pear Buffalo DAC using the soon to be released Legato III I/V output stage. I intend to use the BAL/SE stage in the beginning since all of my headphone amps are single ended.

However I want to build a balanced headphone amp. It will be the balanced dynahi or M³ and I'm concerned about the statement below from Twisted Pear.

One important thing to keep in mind is that while the differential and SE offset can be completely zero'd out, there will be common mode DC on the balanced outputs. Most balanced input amplifiers (Sympatico especially) will not be adversely effected by this at all. If your amplifier cannot handle the small amount of common mode DC (6-7 volts) then you will want to add external coupling caps. But this would be a rare case. I do know of some (mostly tube amps) that have center tapped transformer inputs, those would need caps.

Many of you use single ended outputs and those are also completely free of DC once trimmed. No coupling caps needed here ever.

What does this mean if I build a balanced M³ or dynahi rev2?

[kevin gilmore]

Ti has a lot to say about this. The dac i'm currently testing has between 30mv and 80mv of dc on

each of the outputs depending on temperature. Ti says that the common mode servo needs to be

extremely fast. My bench testing completely disagrees with this statement. balanced dynahi

and balanced dynafet have a slow common mode servo. Further facts, if you are balanced in

ONLY, and balanced out ONLY, then any common mode DC is meaningless anyway.

You should further pay attention to things that are bridged amps (like 4 channel B22) and

fully differential balanced amps (like B24). dynahi and dynafet orignal versions can be

4 channel, newest ones are differential balanced. The issue is whether or not

the input has a common ground or not.

[sbelyo]

Ti has a lot to say about this. The dac i'm currently testing has between 30mv and 80mv of dc on

each of the outputs depending on temperature. Ti says that the common mode servo needs to be

extremely fast. My bench testing completely disagrees with this statement. balanced dynahi

and balanced dynafet have a slow common mode servo. Further facts, if you are balanced in

ONLY, and balanced out ONLY, then any common mode DC is meaningless anyway.

You should further pay attention to things that are bridged amps (like 4 channel B22) and

fully differential balanced amps (like B24). dynahi and dynafet orignal versions can be

4 channel, newest ones are differential balanced. The issue is whether or not

the input has a common ground or not.

Thanks Kevin that makes more sense now. I plan to be balanced in and balanced out only when it comes to using a balanced dynahi or M³. For now I'm not worried because I'll just use the single ended output to my original 2 channel dynahi or a tube amp. But since I've never heard balanced and now I will have a balanced source I will build a balanced amp in the next couple of months.