The AT-1 is a mono limiter closely based on the classic Fairchild 660 using all the original parts specified in original models of the day. The circuits for these units are the same as the 660 models which include the use of the 6386 tubes .
The Fairchild 660 is not one half of the stereo 670 version as it behaves slightly differently in operation to its stereo counter-part. These models have a differant tube power supply and differant tubes for the AC threshold, as well as transformers. Standard addititions to this limter include; Bypass, External *Key* input and 21 stepped AC threshold as well as the ability to link each unit together.