AT-3 REVIEW with Barry Rudolpf & Mix Magazine Luxurious, Single-Channel Tube Mic Preamp By Barry Rudolph The AT-3 is an all-tube mic preamp and a worthy companion to the company's AT-101 Stereo Limiter, a rendition of the classic Fairchild 670 Limiter I reviewed back in The 2010 issue of Mix . The AT-3 furthers the company's philosophy of a no-expense-spared design and build.