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
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