Single or double?
Number 7 in our list of design challenges for a DAC or a preamplifier output stage is providing a single ended or balanced output. As in everything we designers do there are multiple ways to achieve this: some simple others more complex and each with different audible results. First let's remember why we would want to have these choices available and what they do for us. A single ended output is the most common output on source and control equipment - it's what we normally call the RCA out because it uses a RCA connector (also known as a coaxial connector). This output has two wires, a ground and a signal or hot wire that the music travels down. Sounds good but has no ability to reject common mode noises. A balanced output is less common but certainly available on a number of products. Most PS Audio products support balanced inputs and outputs because there are certain advantages to using them relative to the single ended ones. This output uses an XLR connector and has three wires: two signal wires and one ground. Balanced is the best sounding option and can remove any noises entering into the connecting cable. If you'd like to learn more about the balanced connections, you can refer back to the start on this series to the postBalanced. From the output side of building a balanced or single ended output it's relatively easy: simply take the single ended output and run it through another IC op amp to invert the signal (flip the polarity upside down) and this gives you the second signal wire necessary to sending a balanced output. This is how many manufacturers do this - but it's not a great idea. Running the output signal through another amplifier stage to flip the phase 180 degrees means that 1/2 the signal going into your power amplifier or preamplifier is different than the original - because it's going through a second amplifier stage - and since a true balanced input amplifies only the differences and not what's the same on both signals, the balanced output of this compromised circuit would probably sound worse than just using single ended output. If the audio designer really cares about sound quality he will design a fully balanced output stage from end to end. This means that there are twice the number of components in the output stage because you basically have two identical output stages - one for each of the two signals needed for a balanced output. The beauty of this approach is that the signal is identical in the length and sound quality of the chain for both halves of the signal. This is the proper method of building a true balanced output stage and it's what every PS product with a balanced output has. Unfortunately, we are among the minority when it comes to this topology - after all it is twice the work, twice the parts to get it right. Whenever the designer chooses to take the easy path in audio design he had better consider all the variables and how they affect the sound before making those decisions. So the next time you want to consider a new DAC or preamplifier that has a balanced output, ask the company the right question. "Is this a true fully balanced design or a single ended one with a phase splitter at the end?" You might be surprised.
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