SN 1739 rev 4 modification. With PS MOD version 4 transformer. With deep bass enhancement, tunes transformer installed in vactrol. Voiced compressor to lessen a "crowded" mix in spatial midrange imaging. Transformer and enhancement now comes into mix when 2 LED's are lit on compressor. Super calibration, low noise channel spec matches + .5db over 58 db range. 9+ condition, new. Since people keep asking, I use an equalized transformer with capacitors and resistors �tuning� the transformer voice midrange slightly down is installed into the Vactrol circuit. This circuit now allows more bass depth and impulse response, as well as insures that high frequency and presence will not be compromised excessively. Also a little 2nd and 3rd harmonic is imparted on the audio to give a slightly thicker sound to the low midrange and up, somewhat like a tape recording or vintage tube/ transformer pre-amp. Except now everything is cleaner, less noise and stays calibrated same way every day every hour, you can lightly or heavily run the compressor for dramatic effects.. I've sent out this design to quite a few personal studios and the unit is some what tweaked for Rock and Roll or modern multi-instrument acoustic music. Some may have felt its most effective use is on tracking vocals.. Its really about combinations of mics and other matters when operating together.