I'm looking at a Nat'l Semi "pseudo-differential mono headphone amplifier with 6db gain," as described in the mfgr literature. P/N LM4915.
Examining the "differential amplifier specification" on page 9, I learned of a very simple inverting amplifier implementation, within. (The idea is to minimize external parts, so the all-important resistors are internal to the device.) Rf = 40kohm, while Ri = 20kohm. That makes for a simple voltage gain of 2...
...PER AMPLIFIER.
If you look at the specification, you'll see that the device offers two such inverting amplifiers, in parallel. Each of the two outputs drive one lead of a simple load, that is, a speaker (spec'd with Zi = 32ohm). But my point is that voltage gain at EACH lead of the speaker is 2. So that means "4," when we take them together, because they add, right?!
OK. Certainly, we might agree that a voltage gain of 2 translates to a power gain of 6 dB. (from dB gain = 20 log Av)
By now, I suppose you see where I'm going with this.
Why doesn't Nat'l Semi describe the gain as 12 dB? After all, each lead contributes 6dB gain.
If one lead was grounded, the remaining lead would constituted a 6dB gain, all by itself. Right?
Thanks for your replies.