The 525 is a fully discrete feedback type compressor (a la 1176, 660, etc.) and was first manufactured 30 years ago.
Compress or limit. Unique ceiling control linking threshold and make-up gain on one pot, there is also a de-ess switch.
The 525 goes beautifully before the 550B EQ for killer kicks and snares, or after the 512C mic pre for rich vocals. Phat sound.
+4 balanced XLR I/O.