Shortening the path

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Over these last few days, I've been writing a very long series on the path to understanding and appreciating volume controls in preamplifiers. Without hesitation I would maintain their design and implementation remains one of the bigger challenges in high-end audio when sound quality is the primary concern. To date we have covered many ways to control level: pots, stepped attenuators, light dependent resistors, and transformers. All have one thing in common. They require extra parts in the signal path. The best volume control would be no volume control. As I have written over the years, we designers can rarely make things better—though Power Plants would refute that. Generally, we do our best to cause less harm and the ubiquitous volume control is a great example. No volume control element of any design can improve the signal. Best you can hope for is as little damage as possible. But what if we could eliminate the extra circuitry in the signal path altogether? Control the volume without an additional element? This is where an idea I came up with years ago comes into play. Instead of adding a pot, or attenuator in series with the signal, would it be possible to simply change the gain of the amplifier within the preamp? In fact, it is, but it turned out that's not an easy thing to do well. All active preamplifiers have an amplifier at their heart. Generally low gain, these amplifiers are the core of any device. Remember back to the first post in this series? There I described simply the internals of a preamp.
  • Input selector
  • Volume/balance control
  • Gain stage
The question I had asked myself was simple. Instead of a three element design might it be possible to eliminate the middle and create a two element design?
  • Input selector
  • Gain stage
In so doing we obviate the problems associated with the volume balance control. Because if you don't have one to muck up the sound, you've scored! I'll delve deeper starting tomorrow.
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Paul McGowan

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