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

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

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