Linear performance

One of the items in our list of 9 for the output stage requirements of a DAC or preamplifier is linear performance. So, what's that mean and why is it important? Linear performance means what goes in comes out in identical form - just bigger. Seems simple, right? Rarely is anything quite so simple in designing amplifiers. If we take the simplest amplifier, which is a single stage device like one transistor or one tube, there is a range of signal where we get this linear performance - as well as a range where it is not so linear. The not so linear areas of a single device are usually at the extremes: the lowest levels and the highest levels. The middle range of signal is reasonably linear - where the voltage coming in is the same as the voltage coming out - just bigger. Notice in this graph example how the middle section is linear and the extremes to the far left and far right are curved. The vertical scale on this example is rising voltage so when the device has low voltage signals coming in it's not linear and at higher voltages it gets close to clipping at the end of the device range. What you may also notice is the majority of this device's performance is linear (let's call it 70%) and the balance of 30% is non-linear. We can extend this linearity by adding feedback - a corrective action technique - or we can increase the voltage across the device so that the signal range we're looking to cover happens more in this linear region than not without the aid of feedback. Raising the voltage across the device and throughout the entire amplifier seems to have one of the best sonic improvements we've ever played with and preferable to trying to solve this with feedback. You can think of this in terms of headroom of a power amplifier if you like. I am sure we've all experienced the change in sonic qualities of the music when the signal gets quite loud - especially with a lower powered amplifier. The sound can change from open and musical to closed and harsher at the top of dynamic peaks with a power amplifier that's bordering on too low power for a given loudspeaker system. This same phenomena is also why highly efficient loudspeakers can sound so effortless and dynamic, relative to lower efficiency loudspeakers. Whenever we get closer to the voltage or power limits of a given amplification device the music changes its character and we hear audible compression - even though from a measurement standpoint there is no clipping or loss of output. Keeping the signal away from the amplifier's extremes by a wide margin is a good idea because it lowers audible compression effects which are certainly undesirable as we approach the extremes of a devices' linear range.