As seen in fig.2, the guitar doesn't directly enter the preamps, but is sent to an active Buffer/Splitter that gives better linearity and frequency response to the signal, considering the fact that this has to operate in a long path through various equipment. There is also a convenient output for the tuner. The signal coming from the Patch-Bay is sent to the effects which spits and sends it to the Speaker Emulator and finally to the Power Amp connected to the speakers (in this example, two 4x12: each speaker contains four 12 inch cones). The Speaker Emulator makes a copy of the processed stereo signal and sends it to the mixer, while the origonal continues to the power amp.

Use of the mixer during setup
In the example above the signal directly entered the effect instead of passing from the send-return of the preamp: this was a mandatory choice because there were two preamplifers. But if there were only one, the send-return would not have been the best choice because there would be too much equipment to connect. In these cases it is interesting to use special guitar mixers. Note: most rack mixers are adapted to use with keyboards or Public Address systems but not for the guitar.
Every channel of a guitar mixer has to have an input level within the range of +4db to +20db (or more) to be able to receive the signal directly from the preamplifer. If someone is working with professional line signals, +4db will be the system standard for the pre, the effects and the mixer. Lets look at some of the main models on the market:

• Custom Audio Electronics Dual Stereo Line Mixer, built by Bob Bradshaw, is one of the leaders in the construction of rack systems for guitar, which many famous guitarists use
• Rocktron G612 Line Mixer
• Rocktron Rack Interface
• Rane FLM82 Line Mixer
• Rane Sm 82 Stereo Line Mixer
• System MixPlus Guitar Audio Mixer, Digital Music Corporation
• Advance Tube Tech

The principal function of the line mixers, besides the various possibilities of connection and splitting offered by some models, is to maintain the maximum linearity and fidelity in the mixture of the effect with the dry mono signal (also in stereo). This system could slightly digitalize the sound if you are not using high level equipment. The valve mixer identifies this possible inconvenience by passing the signal through its valve circuits. The type of system to use, as usual, is a personal choice.
Lets look at the signal path in figure 3. The pre sends two equal signals: one directly to channel 1 of the mixer, the other to the input of the first effect, which will send the effected signal directly to channels 2 and 3. The send A of the mixer sends a copy of the dry signal from the preamp (taken from channel 1) to the input of effect 3; the stereo signal of the effect returns to channels 6 and 7. The dry signal leaves the send B to the split which divides it in two, in case effect 2 needs to work with a input stereo signal coming from the split. The effected signal returns to channels 4 and 5. We need to make the proper adjustments to the input levels of the mixers various channels in order to transfer a comprehensive stereo signal of the direct sound as well as the dry signal through the master output of the power amp.