Threshold's

From what I remmeber reading all of the S series double down in 4 ohms. I have some info for my S/150 II which says it is also burned in for 3 days continuous into a 2 ohm load. I have a S/150 series II + a series I and the PCB boards do look different and the connection layout on the back is different and the S/150 II has a pos + neg rail fuse where as the S/150 I has a poss rail fuse and then an output speaker protection fuse which is in the signal path. the series II does not have the fuse in the signal path. Ar-t you may be getting older but your memorey is still pretty good. lol,lol

Many of the S 500II are optical bias.
The series I is noted for the grey top and rear. The circuit was early attempts at bias control. They tend to run cold, since they easily to into thermal runaway. I do have a fix for that.. The later units (series II) have "overall feedback free" on the faceplate. They have a better bias contol, but still like to runaway if you are not careful with the bias procedure. The optical bias version optained better control of the bias and as the amp runs into a difficult the load the bias will back off to maintain a relatively contant temperature. Hence the name "sliding bias" came to be used by many to explain the control mechanism.
I hope this helps,
Jon Soderberg

Jon, can you confirm one way or the other, that the pre-optical bias boards are blue and the optical bias boards are gold?