Most first-class mic pre-amps use input transformers. Transformers afford the best interface between a microphone and a pre-amp because it correctly matches the microphone, provides quiet gain and gives the best common mode rejection and ground isolation of any connection scheme. Almost all tube circuits like Fairchild, Pultec and Teletronics, used them. API, Neve and Trident used them. They are usually the most expensive components in an audio device.
Discrete amplifiers are amplifiers made with individual parts such as transistors, capacitors and resistors. In a discrete circuit, all parts and values can be optimized and the audio path can be very short. Most modern consoles use integrated circuits (ICíS) Ė little plastic packages with hundreds of components in them. One integrated circuit may contain more transistors than an entire channel of a classic discrete console. Discrete amplifiers typically run at much higher voltages and deliver more current than ICís so they can have much greater headroom. Modern consoles have many ICís in the audio path of each channel. Audio does not improve as the chain gets longer.