Microphone preamp
The first gain stage after the microphone, lifting a faint signal up to line level.
A dynamic mic puts out somewhere around −60 dBu, and the rest of the chain wants +4 dBu. The preamp therefore has to supply 40–60 dB of gain without adding noise of its own, which is harder than it sounds: any hiss introduced here gets amplified by every stage that follows. Quality is quoted as EIN, equivalent input noise, with good designs around −128 dBu. The same circuit usually supplies phantom power to condenser mics.
Every audio interface has preamps built in, and modern ones are technically excellent. Yet there is a whole industry of standalone units, often modelled on Rupert Neve's 1073 from around 1970, sold on their colour rather than their cleanliness. Transformers and a few harmonics, in other words.