Pack Your Rack with This Modern Classic Mic Pre Start with a 500 Series mic preamp circuit then give the engineer the option to switch in premium gain-compensated Lundahl input and output transformers. That's the enticing premise behind the Fredenstein F601A. A fully discrete Class A gain stage with a unique design the F601A delivers clean warm low frequencies and superior transient response for silky detailed highs without a trace of harshness.