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

Apology accepted :-)

So the reason I wrote this post, besides sheer curiosity, is that it effects my configuration with my new headphone amp.  

A bit of background: I am using a Schiit Jotenheim to power a pair of Audeze LCD-2 headphones and connect to my main system with fancy DAC, turntable, and ARC preamp, etc. When the baby is asleep and I can’t listen to the speakers in the main system, I will be enjoying music with the headphones. I am connecting the Jotenheim with XLR cables to the tape out on my tube preamp. The entire system is balanced.

Since the Audeze LCD-2 is a bit on the power hungry side, and the Jotenheim is perfect for that, I will also be using the Jotenheim at my computer desk during the work day. That means moving the amp to my home office and connecting to my Chord Mojo running in DAC only mode. The output on the mojo is 3.5mm stereo jack. The amp section will be disabled and it will simply feed an analog line level signal to the Jotenheim.

Finally, I get to my question: Will the Jotenheim deliver the higher output that the specs suggest with the headphones connected to the balanced output, and the input on the Jotenheim using single ended? Or does the balanced input as well as the balanced output need to be used to get the higher watt output? I would like to be sure I am driving the LCD-2’s with the full power available.

I have yet to purchase a cable to connect the Chord Mojo to the Jotenheim. I can get a 3.5mm to XLR cable, if that is required to get the proper output. If not, I can get a simpler 3.5mm to RCA.

I posed this very question to Schiit, and they replied:

"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."

One of the reasons I chose the Schiit Jotenheim was its output power using a balanced config, in my main system.  All good.  The computer desk use is secondary, though I will likely use it more frequently there.  If I can get the 3000mw output rather than 800mw at my computer desk, my headphones will be happier.  Apparently this is not going to work, according to the gent I heard from at Schiit.

Is there anything I am missing?

I use balanced headphones .....balanced from the preamp through the balanced amp and then balanced to headphones.  The sound is quite wonderful...and recommended.  Your question is of less noise and increased clarity with balanced configuration. 

Hi Mark,

The preceding responses are generally correct, but I’ll elaborate on them a little further and then address the Jotunheim question specifically:

When an unbalanced signal is received, usually via an RCA connector, what is sensed by the receiving circuit is the voltage difference at any instant of time between the signal (on the RCA center pin) and ground (on the RCA ground shell). When a balanced signal is received, usually via an XLR connector, what is sensed in most cases is the instantaneous voltage **difference** between the two signals in the balanced signal pair (which are usually provided on XLR pins 2 and 3, with XLR pin 1 being ground). Those signals are usually made equal in magnitude but opposite in polarity, with the difference between their voltages at any instant of time therefore corresponding to twice the voltage of each signal at that instant.

When a component provides both RCA and XLR inputs or outputs, it is often the case that rather than those inputs or outputs being handled by separate and independent circuit stages, the center pin of the RCA connector is simply wired directly to one of the two signal pins on the XLR connector (usually pin 2) and the ground shell of the RCA connector is wired directly to the XLR ground pin (pin 1). In fact you can see in the schematic for your LS25 that exactly that is done, for both inputs and outputs. You’ll find the schematic for both the original and MkII versions of the LS25 near the bottom of the following web page. Click on the left-most of the three schematic pages shown for each version, to expand the relevant figure:

http://arcdb.ws/LS25/LS25.html

So in designs that are implemented in that manner, the relevant voltage on the XLR connector (i.e., between pins 2 and 3) and the relevant voltage on the RCA connector (i.e., between the center pin and the ground shell) will differ by a factor of 2. A factor of 2 voltage difference corresponds to a difference of 6 db, based on the relation db = 20 x log(V1/V2), where "log" is the base 10 logarithm.

Now, regarding the Jotunheim:

First, keep in mind that gain and maximum power capability are two different things, which are not directly related. What gain affects are the volume control settings that will tend to be used, and the volume control setting at which maximum power is achieved, assuming that the gains, sensitivities, and output levels of the components that are involved are such that there is never a need to turn the volume control higher than its max setting.

Most balanced power amplifiers will provide the same maximum power capability when driven single-ended as when driven balanced. But not all. Notable exceptions are many ARC power amps, which are fully balanced and ONLY provide XLR inputs. In those cases, if the amp is provided with an unbalanced signal via an RCA-to-XLR adapter its maximum power capability may be reduced as much as 75%, and sonics will suffer as well. But those designs are unusual in that respect. And since the Jotunheim provides both RCA and XLR inputs I would suspect that it is not similar to those unusual cases.

So while in the absence of specific knowledge of the design I can’t be totally certain, my guess is that you’ll be able to get to 3000 mw into 50 ohms via the RCA inputs of the Jotunheim. (Although that will correspond to about 3000 x 50/70 = 2143 mw into the 70 ohm impedance of the LCD-2). However, I doubt you’ll ever want to do that, given that 1 mw (0.001 watts) into the LCD-2 is spec’d as resulting in an SPL of 101 db!

I’m also basing that conclusion on the Mojo’s spec’d maximum output in "line level mode" of 3 volts, and the specified 14 db max gain of the Jotunheim (although it isn’t made clear which combinations of balanced and unbalanced inputs and outputs that 14 db spec applies to). Those numbers should assure that you won’t run out of range on the headphone amp’s volume control.

Finally, be aware that a 3.5 mm to XLR cable will **not** provide balanced signals to the XLR connectors, since a balanced pair of signals for each channel does exist on the 3.5 mm connector. A 3.5 mm stereo connector provides an unbalanced left channel signal, an unbalanced right channel signal, and a ground connection. The cable would simply route those signals to separate XLR connectors, probably on pin 2 of each XLR connector, and would probably connect the ground of the 3.5 mm connector to pins 1 and 3 of each XLR connector. The result being no benefit relative to a 3.5 mm to RCA cable.

Whew! Best regards,
-- Al

Al, you rock.  I've said it before and I'll say it again.  I think I will have to re-read your post a few times to properly digest it and understand it fully!  You have some serious knowledge.  Thank you!

- Mark

+1 @marktomaras.
Al amazes me every time I read and try to digest some information he provides. Amazing, isn't it? And the best part is, there is ZERO bias in his responses.
Maybe we should have a "Almargopedia" section on audiogon :-)