Direct Outputs

When recording in a sound room or playing live in large or open areas, you need to gather the sound to send it to the recording table or to the Live mixer, respectively. In slang we say mike the guitar : in fact one or more microphones are used to gather the sound developed by the cones of the guitar speakers. The choice of microphones and the relative position in front of the cone is very important. This has been, and probably always will be, the best system to amplify the instrument.
However, using microphones has its problems: undesirable resonances, return from nearby instruments, and also the work required to find the correct placement of the microphone. This is why the Direct Outputs (also called Recording Outputs, Full Range Outputs, etc.) were created. It's generally a stereo output, that receives a balanced signal which is then sent to the Full Range systems (voice systems, mixers, and recorders), simulating the sound that a microphone would pick up if placed in front of the speaker cone. This system is included in most modern preamplifers. In some cases you can even imitate different types of speakers: you can opt for an open speaker with a 12 inch cone, or for a closed 4x12 (typical Marshall speaker). This system is called Speaker Cabinet Emulator, and is a circuit placed on the exit of the Direct Outputs. It's can also be a rack system or a simple box (probably the most famous is the Red Box by Hughes & Kettner). Never connect the normal L and R preamplifer outputs to the mixer. These are designed for giving a calibrated signal for a power amp of a guitar, which passes the sound to the loudspeakers, that are deliberately lean, and are not able to diffuse the entire sound spectrum of the frequencies (Full Range). It's actually the combination of this system that gives the guitar the sound. Thus, if we send the signal from the line outputs to a mixer, it won't be able to interact perfectly: using distorsions we will get the typical "mosquito" effect, while clean sounds will be blessed with good dynamics. Be carefule of the Line Out present on lower cost amplifiers, which theoretically could be connected to a mixer, but rather should be connected to a microphone.
The use of Direct Outputs can have some notable advantages, which are the opposite of some of the possible drawbacks for the microphones: The fact remains that the Direct Outputs only simulate the sound of the microphone, whose sound quality is superior. The job of the microphone is similar to that of the human ear. The Announcer Emulator, on the other hand, can interact with the environment, which is of particular importance for the overall sound. Even though the simulation can be quite good, it's still a simulation and therefore the choice varies according to the demands.

The Power Amplifier

We come to the final stage of amplification: the Power Amp. This amplifies the entire input signal by a considerable amount, slightly modifying it in some cases, then sends it to the speakers, which allow a subform of sound to be heard. The constructive philosophy of a power amp is very similar to that of a typical power amp contained in an amplifier, with the difference being that it is a single unit like a rack system, and is generally stereo, because it has two channels with their relative controls.
n power amp can be built with valve circuits, transistors or both. The typical controls for each channel are the volume and the presence.
The volume, obviously checks the output level of the channel.
The presence checks the presence of the signal, useful in fine tuning the overall sound when playing live, where the middle-high frequencies are emphasized to provide more clarity during the concert.
Normally these controls are located on the front panel of most power amps, which are more that enough to correctly amplify the sound. Selection controls may also be present, which cut the low and middle-high frequencies.
On the back panel there can be one or more Line Outs for every channel. These are the outputs that are connected to the speakes. Pay attention to the resistance of these outputs, which must be compatible with the resistance of the speakers. We could have 4, 8 and 16 ohm outputs (depending on the type of power amp and amplifier), in order to create different combinations between the power amp and the speakers. In some cases there can be some confusion regarding the combination of resistances between power amps and speakers. We will try to make things clearer using the following graph:

Equal resistances can be connected to each other, but a power amp with an 8 ohm output for instance, can be connected to a speaker that has a 16 ohm input. This is true for every output channel of the power amp. Transistor power amps don't really suffer in cases where the connections with the speakers are wrong (there could be a slight loss of volume), but valvular power amps, especially older ones, can suffer serious damage. Always consult the manuals that come with every power amp. When the output resistance of the power amp is adapted to that of the speakers, all the energy from the power amp is transferred to the speakers without loss of signal, allowing the system work as best as possibile (and avoiding damages). The above described values don't cause problems, even if it is always better to connect two resistances of equal value. Using more complex systems (i.e. many speakers connected to the same power amp), you need to make sure that the total resistance value of the loudspeakers corresponds to that of the power amp.
There are two types of connections: in series or in parallel.
To connect two loudspeakers in series means to unite a positive and a negative terminal of both and connect the remaining two terminals to the amplifier. Their values are added: for example, two loudspeakers of 8 ohms in series is 16 ohms.
When the loudspeakers are connected in parallel, the terminals of the same charge are united among themselves. To calculate the total value you need to use a formula, using R1 and R2 as the values of the two speakers, (R1 x R2)/(R1 + R2). For two 8 ohm loudspeakers, we have: (8x8)/(8+8) = 64/16 = 4 ohm. Therefore connecting two equal loudspeakers in parallel cuts the value in half. The system of connection in parallel is often already included in the power amp (see the manual!), and accordingly allows elaborate connection situations.
Besides the outputs for the speakers on the back panel, there is one or more protection fuses, and in some cases Foot Switch outputs. To finish, I would like to remind you that valvular power amps are delicate, and it is therefore important that the inside of a rack has a good circulation of air, because the temperature of the valves is very high, and they must be cooled before moving the power amp.