Signal processing within the modeling concept can be realized with analog or digital circuitry, or combinations of the both. Digital amplifier modeling may appear as software, such as plugins for DAWs which may be aided by computer hardware accelerators, or may be part of a standalone device or amplifier. As part of a digital audio workstation, amplifier modeling may be applied "after the fact", to a guitar signal that was recorded "clean", in order to achieve the sound of an amplifier being used. This process has the advantage of being dynamic—the amplifier settings can be adjusted without forcing the musician to re-record the piece. Today many analog modeling circuits may have a digitally-controlled interface, and the analog signal paths within such units are often "re-routed" and reconfigured with aid of digital logic and semiconductor-based switching circuitry. In addition, many "digital" modeling devices that employ DSP may also employ analog modeling circuits.
A good example of a moderately complex analog modeling circuit is Peavey's "T-Dynamics" power amplifier design, which emulates complex clipping and bias-shifting characteristics of push-pull tube power amplifiers, as well as the typically high-ish output impedance of such. Vox "Valve Reactor" power amplifier, Hughes&Kettner "Dynavalve" power amplifier, Mesa Boogie Triaxis Tube Preamp, Pritchard guitar amplifiers and Quilter musical instrument amplifiers are other examples of units that feature analog circuit designs of similar nature. Roland's earliest "Blues Cube" amplifiers employed analog tube modeling circuitry, though Roland did not model specific tube amplifiers, more so the overall characteristics of a generic tube-based preamplifier circuit. Peavey's "TransTube" preamplifiers are designs of similar nature. Pritchard amplifiers also model characteristics of tube-based circuits in general and without attempt to model any "amp-specific" tones per se. Roland and Line 6 employ analog power amplifier emulation in some of their amplifier models. Peavey's "Vypyr" series of modeling amplifiers utilises analog "TransTube" circuit instead of a digital waveshaper, and Vox Valvetronix amplifiers have throughout their history presented a marriage of semiconductor and vacuum tube -based analog modeling circuitry and digital signal processing circuitry.
