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Choosing wisely

— By Paul McGowan —
In yesterday's post we decided on the type of output stage we wanted: balanced or unbalanced depending on the topology of the DAC before it or, in the case of a preamp, how extensive the designer wanted to go. In our case we always spend the extra time and effort to go fully balanced wherever we can. One question was asked about tube DACS and preamps: why do many of these not have have XLR balanced connections? Designing a fully balanced input to output tube circuit is far more difficult than that of a solid state design - if for no other reason than the added number of tubes increases the cost dramatically. But balanced is almost always better. And speaking of tubes, today's post is about choices for the input stage itself. I mentioned in the last post how important the choice of input devices are to any amplifier stage and I wanted to give a few examples of that today. The audio designer has essentially four device types to choose from:
  1. An IC op amp. This not only covers the input but takes care of everything else. Drop in and connect a few things and you're good to go. By far the most popular because of the ease of design. Not the first choice for sound quality.
  2. A bipolar transistor. If you're going to build a discrete stage, a bipolar transistor pair is perhaps the most common. It is the least expensive option. They are a current amplifying device. Definitely a better choice than an IC op amp from a sonic standpoint.
  3. A FET transistor. This has always been my choice although one not shared by too many others. They are far more expensive than bipolars, somewhat trickier to design and match. Unlike a bipolar they are a pure voltage amplification device and IMHO a very musical warm sounding option.
  4. A vacuum tube. A few DAC designers will place vacuum tubes here but it's rare. The are expensive, take lots of real estate, require exotic power supplies, fragile and microphonic. They are, like the FET, a pure voltage amplification device and a wonderful sound quality choice.
So you can see, there are choices and each has a significant impact on the way the DAC or preamplifier sound. Designers make choices based on their tastes, cost constraints and overall goals. In my case I certainly prefer FETS and within the FET category, there are two types: JFETS and MOSFETS. JFETS sound much more like music to me than do MOSFETS and both sound more musical and natural than bipolars and certainly IC op amps. Tubes are just too painful to design, build and they degrade over time and use. Many people love their sound, as do I, but I cannot abide by their shortcomings and weakness.

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