API Audio 527 Compressor/Limiter - 500 Series, 2510 and 2520 discrete op amps, transformer output, continuously variable detented threshold, attack, release, ratio and output fader control. 10-segment VU/gain reduction meter with selector switch, overload LED, "Link" switch for multiple unit interdependent compression linking, "New/Old" switch for feed-forward or feed-back compression modes, "Hard/Soft" compression curve knee switch, patented THRUST switch for frequency dependent side chain control, "In" switch with hard relay bypass (in the "Out" position), must be installed into an API Lunchbox (API 10-position 500V vertical rack) or compatible system.