Conversions

Yesterday we traced the digital audio path from the input of the DAC all the way to the output of the DAC itself, which outputs a series of current steps, and are in the process of converting those current steps to voltage steps.

You can use a simple resistor to make the current to voltage conversion and several manufacturers do, but it’s not an ideal load for the DAC chip. What the DAC chip wants to see on its output is zero impedance: no resistor or resistances at all. But in order to convert current to voltage we’re going to need something that either is a resistor or simulates one and that simulation is typically handled with an op amp.

As we learned, an op amp has two inputs: a plus and a minus. The plus input is high impedance and the minus input is essentially zero impedance. So we use the minus input to connect the output of our DAC up and voila, we get voltage on the op amp’s output. But a few of us nutso high end designers would rather not place an op amp right at this junction because they simple don’t sound great.

The first clue that an op amp right at this position is a sonic no,no happens when you start to swap op amps out to see if the sound changes. Oh my, it changes mightily depending on the op amp’s characteristics. The faster op amps sound better than the slower ones, generally speaking, but all sound a bit bright and transistory in my opinion. Speculation as to why this should have such an impact on sonics usually revolves around the rapid speed at which the DAC is changing it’s current output and how those rapid step responses affect op amp performance.

The whole issue is, however, easy to skirt if the designer wishes to – but unfortunately most don’t. Of the few that do, Nelson Pass and I prefer to use a pair of transistors instead of an op amp – others may have some different scheme I am unaware of. Nelson prefers MOSFETS and I prefer bipolars here, but that’s a matter of choice that is really dependent on everything else in the chain.

The cool thing about using a simple pair of transistors is that if you connect them properly they are still extremely low impedance but don’t have any feedback related hardness issues to contend with as do op amps. Further, we can ensure that they produce low noise, high speed, low distortion outputs that are perfect for feeding into the next stage of our chain: the analog filter.

Tomorrow we get rid of what we don’t want.

Paul McGowan – PS Audio, Intl.

Amperage alchemy

We’ve briefly discussed getting the digital audio stream into the DAC and then decoding those bits back into a form of analog that gets us a lot closer to being able to play it on our systems.

At the output of the DAC chip we have a series of current steps that need to be converted to voltage steps – a sort of alchemy process like turning lead into gold. These current steps can be thought of like different wattage light bulbs if we want to understand them more easily.

Imagine for a moment that we have 24 light bulbs (like 24 bits), each bulb capable of consuming twice the power of the preceding one and in the process getting twice as bright. We start with a 1 watt light bulb, next we have a 2 watt, then a 4 watt and so on. Each of these bulbs consumes more power – or current – than the other and the combination of brightness gets us a large range of light: from the very dimmest to the very brightest and everywhere in between.

This is similar to what is happening at the output of your DAC chip. Now take your imagination one more step forward: imagine instead of 24 light bulbs you have only a single light bulb and you feed it different wattages. The total wattage you feed this single bulb creates a unique brightness level that spans a 144dB range which, in audio terms, covers the range from a single air molecule hitting your ear to the noise of a jet blast and everything in between.

Preamplifiers and power amplifiers need voltage on their inputs, not current (power). So we must convert these power steps to voltage steps at the output of our DAC chip. The easiest way to convert current to voltage is to simply pass it through a resistor. Place one end of a resistor at the output of our DAC and the other end goes to ground. Voila! You have a simple current to voltage converter.

One of our very first DACS used just this approach and, if I remember from reading John Atkinson’s review of the Devialet integrated, that is also what they do. While this works and is simple, it isn’t always the best way to do this because not every DAC appreciates working into a resistor and you start losing linearity and gaining noise.

Most DACS simply place the output of the DAC into a chip op amp’s inverting input. Now, I understand this is technical sounding stuff, but it’s actually fairly simple. An op amp’s inverting input looks like ground to the DAC and it is very happy – somewhat like a zero Ohm resistor. This is the point where a lot of audible trouble can begin to take place.

So important is this conversion process that tomorrow we’ll look a bit deeper into it’s workings and problems.

Paul McGowan – PS Audio Intl.