Threshold's

The S series Threshold amps are still highly regarded amps and can still hold their own against many of the modern designs. All that I know is that the S500 was rated at 250 watts RMS at 8 ohms, I assume the II series was some type of upgrade. Best to contact Threshold Audio or maybe Nelson Pass as he was the one who designed it. I have a Threshold S350e power amp and have the original owner's manuals. The manual does not give the power ratings of 4 or 2 ohm loads but I remember reading somewhere that they will double their power into a 4 ohm load as most higher quality amps do. As far as the class A/AB ratings I will quote what my manual says.

"The tracking accuracy of the optical bias circuit has allowed the biasing of Threshold S/model amplifiers to such levels that are now designated class A/AB in operation. This means that they will function as pure class A amplifiers until about 20% of their instantaneous power rating into 8 ohms and operate as class AB beyond that point."

So a 250 watt S500 should put out roughly 50 watts pure class A power. Hope this helps.

Thanks Thorman, Arnold, and especially David and Sean who offered detailed e-mail advise! Though I haven't heard one I am very encouraged by all this information. Now if I could only find one.

Wish I could tell you exactly. All I can remember is that it involved changing out the rear panel, as the arrangement of the connectors was different. The PCB was obviously different.

I do remember that it happened around the time he came out with newer versions of the FET-I and FET-II preamps. I seem to recall that those upgrades used some small-signal MOSFETs in the gain stage. I do not think the S-300 upgrade was as extensive as the preamps were. But that was over 20 years ago.

Looking for manuals, schematics, and service manuals for anything from 350e, 450e, or 550e. Photocopies or what have you. Henry Sjaardema

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