Article I- Balanced Drive vs. Unbalanced Drive
We really didn’t know what was going to happen when we introduced the world’s first balanced headphone amplifier, the HeadRoom BlockHead, in 2001. We didn’t know if getting rid of the common ground on headphones would make much difference. We didn’t know how many headphones could be successfully re-cabled to a balanced configuration. We didn’t know if balanced headphone drive was going to sound better---or even if it would sound good. We just knew it could be done and that we had to try. Man, are we glad we did; balanced-drive headphones sound great! It is a new idea though, so we thought we’d help you understand what we’re talking about.
A normal headphone cable plug has three connections on it: the tip is left; the ring is right; and the sleeve is ground. The tip connects to a wire that goes to the positive (+) lead of the left headphone driver coil; the ring connects to a wire that goes to the positive lead of the right driver. The sleeve connects to a wire that goes to both negative (-) terminals of the drive coil; this wire usually has a solder joint in the “Y” or in the earpiece where ground wire from the plug splits into separate wires that are connected to the negative terminals of the driver coils.
Bold lines indicate common return of both left and right channels
The most important thing to note here is that as the left and right channels of the headphone amplifier drive the left and right driver coils, the return current from the drivers gets joined together and travels some distance before returning to the amp’s audio ground. This common pathway has some- possibly significant- electrical resistance from the wire, solder joints, contact resistance at the plug/jack, and so on, which causes a common signal to appear at the negative terminal of both driver coils. This common signal (a low-level summation of the left and right channel) will generate low level cross-talk and distortion in the sound heard on headphones. Just how low depends on the quality of the whole headphone/amplifier system (and, of course, we do a lot to minimize the problem with our amplifiers and re-cabling services), but it will always be there. There is, however, a way to completely get around the problem: balanced headphones.
Balanced headphones are just regular headphones that have been re-cabled in a special way. The normal three-conductor cable with the common ground connection is replaced with a cable that has four conductors: right positive and right negative conductors to the positive and negative connections of the right driver coil; and left positive and left negative conductors to the left driver coil. The cable is terminated in two XLR male connectors, one each for the left and right channel. A special headphone amplifier is used that has “balanced” outputs where each channel has a normal audio drive signal and a mirror image inverted drive signal. The trick here is that there is no “ground” to the headphones anymore, so there is no opportunity for the crosstalk distortion described above.
Well … there’s actually another trick: an amp that makes perfect mirror imaged balanced signals to drive balanced headphones.
>>continue to Article II - The Difference Between Balanced Transmission and Fully Balanced>>