I'm fairly new to having a head amp. Can someone explain to me what the the power amp is? I'm confused.
When you get an amplification system there are typically three main components: Preamp, Power Amp, and speakers.
The Preamp receives the signal and allows modification to the signal, such as EQ or built in FX. Preamps typically add a 'color' to the sound (basically tone), which is a big part of the deciding factor when purchasing one amp over another. Sometimes that choice is made based on the preamp adding little or no color to the sound and sometimes it is based on a very specific tone, i.e., a jazz amp will offer a completely different tone than a heavy metal amp.
The Power Amp takes the signal from the Preamp and adds power to the signal, measured in watts. This power is used to drive the speakers.
The speakers will have a specific resistance, measured in ohms, and a certain wattage rating. The wattage that is output from the power amp is relative to the resistance of the speakers. So a power amp's output may have a specific wattage listed but that is for a specific resistance, e.g., if the amp says 400 watts at 8 ohms and you plug it into a speaker cab with a resistance of 16 ohms, then the wattage would be 200. This is a somewhat confusing concept that took me a while to fully grasp, so I would recommend researching a bit to fully understand if you intend to be making purchases without a knowledgeable party assisting.
You will typically see all of these pieces come together in 3 different variations: Combo Amp, Head + Cab, or Preamp/power amp/cab all separate pieces. The combo amp includes everything in one. With the Head + Cab, the head will contain the preamp and power amp, which will be plugged into the speaker cab. It is less common to see in a guitar amp (or similar instrument amplification) but you can also get all three pieces separately. This can be good because it allows you to use a preamp with the tone you like and not be dependent on its power amp specifications. Typically you will see the three pieces separated in a PA setup, though some low end PAs will include the preamp and power amp in one.