Diff, diff

I had mentioned in yesterday's post the standard way to build a balanced output stage was to simply tack on another amplifier that flips the phase around. That's pretty much how it's done, but it isn't best because it's "unbalanced". A purposefully misleading statement. Adding an extra stage for one signal that is not present for the other is more accurately described as asymmetrical—and asymmetries raise the hair on the necks of audio perfectionists. A symmetrical way of making a balanced output is to start with a balanced amplifier. You'll recall that most amplifiers aren't actually balanced at all - they may enjoy a balanced input, or they may sport a balanced output - but that does not a balanced amplifier make them. A true balanced amplifier (see why I hate that term?) is balanced from input to output and symmetrical for both signal phases. In this type of amplifier all that's need for the balanced output is the proper XLR connector, not another stage tacked onto one side and not the other. This is one of our designs and I hesitate to show it because it might be intimidating. There are a lot of squiggly things! For simplicity sake, we colored the inputs and outputs red. Input 1 and 2 are where the XLR cable would connect for the input—a simple diff pair (J1 and J2) as I described earlier. This feeds another diff pair (J3 and J4) that becomes the output. (for those technically inclined, ignore R11, it is not supposed to be there). This is an entire true balanced amplifier with very few parts. Completely symmetrical from input to output, zero global feedback, and cheap. Less than $5 in parts. If we really wanted to use this in a product, we'd want to add some buffers to the outputs so it could drive complex loads, but basically, this isn't all that hard. Tomorrow I'll cover balanced phono cartridges then we'll wrap up the series.