In the early days of solid state electronics for high end audio just about all preamps had capacitors on both their inputs as well as between their outputs and their mating power amplifiers' input (and the power amp inputs had them as well).
Most tubes circuits of the day required them but solid state designs didn't - they were mostly added as "good practice" by engineers who were making the transition from tube design to solid state. And it was at the beginning of the second golden era of audio.
It was at about this time many designers were starting to figure out the idea that "less is more" that everything in the signal path mattered and that certainly included the capacitor. Further, we started to discover that if you had a capacitor in the music path the quality of that capacitor mattered greatly - although there was no capacitor like no capacitor at all.
"No capacitor at all" earned itself a marketing phrase which we still use today: Direct Coupled. I say marketing phrase because it's not like you're adding something extra or special to the circuit, in fact, you're removing something that really didn't need to be there in the first place. Direct Coupling simply means the lack of an unnecessary component in the signal path - as marketers when we remove something we call it out as a feature - simply because ours sounds better without it.
Just what is a capacitor and why would we have used one in the first place?
Tomorrow we'll dig in.