Due in part to our recent threads on impedances and power amps we've seen a few questions from readers digging deeper into how things work (the curious mind is a wondrous thing). Specifically we've had a few readers ask about power amp volume controls and what effect they have on the performance of an amp.

In typical power amps the "volume" control is actually an input sensitivity control. In other words it has no direct effect on the output stage of the amp, but rather determines how much resistance there is to the incoming input signal getting to that stage. By making it more difficult for signals to reach the power amp one is effectively turning the amp down, but it's important to understand that the full power range of the amp is still available if the input signal rises high enough to use it. Some people have claimed over the years that lowering the volume of a power amp reduces the signal to noise ratio and dynamic range, but with an understanding of the above you can see that this isn't necessarily true. The input sensitivity of an amp can be lowered to make the amp behave more appropriately when hit with unusually high input signals. This is often a better alternative than turning the levels down on upstream gear (which often does reduce the signal to noise ratio).

In sound reinforcement applications the amplifier is often the last gain stage to be set. Basically the engineer will introduce a source (such as pink noise) into a system and use it to set up the gains of every upstream device so they all have the same amount of headroom (basically they all clip at the same level), then while each of those upstream devices is right at clipping they will set the input sensitivity controls of the amps so they are just clipping as well (use earplugs while doing this). This way the engineer can look at the signal at any point in its path and know where he stands in terms of the overall headroom of the system. The same principle applies to recording levels.