Getting balanced

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In our series on the output stage duties of a DAC or preamplifier, we listed 9 tasks that had to be completed and we've already covered the first. Today we'll cover the second task:removing any common mode distortions and noise if we're running a balanced DAC. As I have covered in the past a balanced amplifier or filter has the ability to ignore anything in common between both halves of the balanced signal. This can be very valuable when it comes to cleaning up an input signal or a power line, because each half of the signal is different - by design - and therefore anything in common is not something we want. Let's take a look at this balancing act and cleanup magic performed by balanced inputs. I am sure we're all familiar with an XLR balanced cable that we might place between an amplifier's input and the output of a preamplifier. Here's what the connectors look like. Notice there are three connections on this cable: two are for the signal and one is for the ground. To make this work we simply place two identical - but out of phase - signals on both pin 2 pin 3. Pin 2 is the same as what would come out of our RCA output on a preamp (in phase) and pin 3 is an inverted copy - meaning as the signal on pin 2 is going positive, the signal on pin 3 is doing the exact opposite, heading negative (out of phase). So if you just took the signal on pin 2 and the ground on pin 1 you'd have exactly what comes out of our standard RCA single ended cable. If you took the signal on pin 3 and the ground of pin 1, you'd have an out of phase (inverted) copy of what comes out of your preamp's RCA connector. As a side note - if you put these two signal into a selector switch and labeled that switch "polarity" or "phase" you'd be able to flip the phase or polarity of anything you're playing on your system - as you can do with many preamps and DACS. Now imagine placing these two out of phase signals into a special type of input that will honor and amplify whatever is present on pins 1 and 2 ONLY if they are doing the exact opposite of each other. Then imagine that this same input will ignore anything that's the SAME between these two pins. That's what a balanced input does and it is sometimes referred to as a differential input because it amplifies only differences. Why do we give a rat's tail about this? In an XLR balanced cable this is cool because you can run extraordinarily long lengths of cable past noisy transformers and environments and have none of that noise in the music! This is because any hum or noise that is radiated from transformers, cell phones, etc. pass through the XLR balanced cable equally and appear on both signal wires. Since our input amplifier won't amplify or honor anything that's on both - they are ignored. Like magic. So, back to DACS and why we might use this balanced approach inside a DAC where there are no long lengths of cables and noise. Each DAC chip that is making the conversion from digital to analog has a common set of distortion characteristics that are unrelated to the music. By using a balanced output DAC chip where one signal output is inverted from the other - the distortions that are common between the two are eliminated in the same manner as our hum and noise. Voila! Lower distortion, lower noise, better sound. Tomorrow we'll cover choices of input types for this amplifier - which have a huge impact on the way it sounds - musical or not musical.
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Paul McGowan

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