Why do some amps provide more output power when using a balanced connection?

@almarg  

Hello Al,

so, I have read, re-read, researched, and digested. I found some other threads discussing balanced and unbalanced that you also posted on. I now feel that I have a reasonable working understanding of the concept.

I do have some more questions of course!

1. Is the design that Audio Research used in my LS-25 a reasonable way to deal with a balanced signal?  Or is it a “fake” balanced connection? I see they use the same method in their Ref 2 and Ref 3 line stages, Though my ability to accurately read a schematic  is questionable :-)

2. If balanced connections add more gain, why do you suppose Schiit is showing a difference in wattage output based on the balanced versus unbalanced operation in the Jotunheim?  

3.  Is there a way to test the difference in the driving Power into the headphone?  I mean, should I be able to detect an audible difference in wattage delivery If I compare fully balanced operation to fully single ended up operation?  

If I use RCA cables to connect my audio research preamplifier to the Jotunheim, and the standard single ended headphone cable, I should be getting 571mw of power. If I switch back to XLR from the ARC preamp to the Jotunheim, and use my balanced headphone cable, I should be back up to the 2143mw at 70 ohms that Schiit suggests in the specs (after applying your formula to adjust for the different load) But now after your explanation,  I am confused why the Jotunheim is outputting more wattage with the balanced connection at all.

In fact, I am less interested in the amount of gain that the Jotunheim delivers to my LCD-2 headphones, both settings, low gain or high gain deliver enough volume.  I am concerned about the wattage delivery.  I want the power and the control of the wattage to properly drive the headphones.  Audeze recommends 1-4 watts to drive them, and that is one of the reasons I chose the Jotunheim.

Thanks very much for the nice words, gentlemen!

@marktomaras, regarding your questions:

1)The LS25 has a fully balanced/truly balanced/genuinely balanced internal signal path, and if XLR inputs and outputs are used the entire signal path from input to output will be balanced. The approach of having RCA input and output signals common with one of the two signals in the corresponding balanced signal pair, rather than having them handled by a separate circuit stage, is not at all uncommon, as it significantly reduces cost, complexity, and requirements for internal physical space. The downside of that approach which most commonly arises is that if both outputs are used simultaneously, for example to connect the XLR output to a power amp while connecting the RCA output to a powered sub, impedance compatibility issues and issues involving sensitivity to cable capacitance can arise which can affect the signals to both destinations.

Note, for example, that ARC recommends a minimum load impedance of 20K for the LS25, as they do for most of their other line stages and preamps. Powered subs commonly have line level input impedances in the area of 5K to 20K, which in combination with the input impedance of a power amp would result in the preamp "seeing" a load impedance even lower than that.

2)It follows from some of my earlier comments that the 3000 mw vs. 800 mw difference is most likely just a function of which of the two **output** connectors are used, not which input connector is used. Providing the unit with unbalanced inputs, whether via the RCA input connectors or the XLR input connectors using an adapter cable, will probably affect gain but most likely not maximum output power capability. As I indicated previously, though, I can’t say that with total certainty without specific knowledge of the design.

And if I understand correctly there is no reason you can’t use the headphone’s balanced cable for your Mojo/computer desk application as well as for your main system application.

3)No, I can’t think of a convenient and practical way of determining maximum output power capability, without test equipment that most of us don’t have. But even in what I believe to be the unlikely event that using the Jotunheim with unbalanced inputs/balanced outputs provides you with a maximum output capability of only 571 mw, I don’t see that as being an issue. Calculating from the efficiency spec, 571 mw into the headphones would produce an SPL of 129 db! And regarding your reference to "control," that is a function of output impedance, not maximum power capability. Output impedance is specified as "less than 0.1 ohms, balanced or SE, at both gain settings," which is negligible in relation to the 70 ohm impedance of the phones.

Best regards,
-- Al

"The Jotunheim is balanced differential. Meaning it will not turn a single-ended connection to a "Fake" balanced connection. You may notice more power from using the balanced headphone output with a single-ended input but I cannot speculate on that."

Just a FWIW: this response really doesn't make sense! So I can understand there being some confusion.

If the input to the "Jotunheim is balanced differential", then it will not matter if you are driving it single-ended or balanced. This is because a differential amplifier does not care what its inputs are - it just amplifies what is different between its inverting and noninverting inputs. So if one of  those inputs as at ground as is the case with a single-ended input, it will amplify that. So its gain will not change nor will its distortion. That is, if it is balanced differential.

You won't hurt anything by trying the hookup! Our products are also balanced differential and what this means is that it usually does not matter to the gain of the amp if the XLR or RCA connection is used.

Just a FWIW: this response really doesn’t make sense! So I can understand there being some confusion.

And, unfortunately, that is a common theme where Schiit customer service and technical support are concerned.

Wow, I knew this was an Almarg and Atmasphere question.
B