Optical bias followed the Series II. Balanced inputs were introduced with the e series.
Many of the Series II units were upgraded to optical bias, and some units were upgraded to quasi e series with balanced inputs, though unlike the true e series with their dual trannies, they were balanced inputs only and not true balanced.
Many of the Series II units were upgraded to optical bias, and some units were upgraded to quasi e series with balanced inputs, though unlike the true e series with their dual trannies, they were balanced inputs only and not true balanced.