Threshold's

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

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

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

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

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.

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.

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.

Unsound-
I have a very nice copy of the original Threshold manual that covers the S/150 series II through the S/1000 series II and also the Model SA/1 stasis and SA/2 stasis.
If you get a Threshold S/500 let me know and you can copy the manual.
The manual doesnt exactly answer your questions though.
Here's what I found.
Threshold S/series Stasis amplifiers employ pure class A operation in all signal portions of the amplifier except the current sources comprising the current bootstrap section of the output stage,where AB operation is used.In these amplifiers the STASIS section is required to effect domination over the AB characteristics of the current bootstrap section without the use of global feedback loop.

In SA/series Threshold STASIS amplifiers the addition of a new constant current bias circuit has allowed increasing bias of the current bootstrap section to pure class A.This improvement of the current bootstrap section significantly lowers the nonlinearities in its operation prior to coupling with the stasis section.While tis exacts a penalty in the form of cost,efficiency and physical size relative to rated power it results in a instrument whose extraordinay musical accuracy sets new boundries for ther classification "state of the art"

S/500- 250 wpc ( I think it does double down to 4 ohm but doesnt say)
20 trsnsistors per channel
output current-16 continuous.40 peak amperes/channel
Power supply- 1,000 watt toroidal transformer.

hey,give me a holler!

I've noticed that these amps display class A/AB on the face plate. Does anyone know how many watts are offered in class A before they go into class AB. These amps are rated as 250 watts per channel into 8 Ohms, how much power per channel do they produce into 4 Ohms?

I had a series 2 S/300 ( same era ) and there was a slight upgrade at the time but nothing major. It was also a time that they raised the purchase price . I think the more important thing about either would be how well it was taken care of and the actual condition of either unit.Either will get the job done fine,depending on condition....The "S" series amps were and are considered some of the best amps around for that era....