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RobertS61 19th April 2009 07:57 PM

Threshold S/1000 Series II
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Hello (again),

Just acquired a pair of Threshold S/1000 Series II.
1. Know that it has a bridged topology to get the rated 500 Watts at 8 Ohm that's why I wonder if it's stable at the lower loads? They have to drive Acoustat 2 + 2 Electrostatics.
2. Should I perform any prefailure mods with these 25 years old amps?
3. Does anybody know the difference between the initial Series and the Series II?

Thanks in advance,


Nelson Pass 19th April 2009 11:09 PM

I would go ahead and use them and see what you get before
worrying about rapleacing capacitors, etc.

I don't recall a Series 2. I cruised through the old brochures up
to '91 when I left, and I don't see mention of it.


RobertS61 20th April 2009 08:30 AM

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Thanks for your information.
I have some pictures of these Series II designation.
The first Series II S/1000 amps were produced in 1983 according to the serial badge at the back.

According to the serials these sampels I own are produced in januari and March of 1984 and are designated as Series II also on the Threshold stylish T badge in the front it says Series II.

Can you recall the difference between the first offerings of these amps and the Series II.
Do you think the bridged topology is stable enough to drive low efficient and low impedance Acoustats. I know J. Gordon Holt got good results with the massive parallel SA/1 on the SoundLab A1 or A3.
The picture I attached says: 84030030
Probably means April 1984 number 30?

RobertS61 20th April 2009 10:00 AM

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Sorry do not know how to attach multiple Images in one reply:
Here a image of the back of the amp.
Must be March of 1984 of course not April :)

Nelson Pass 20th April 2009 06:34 PM

Frankly, I don't recall what was meant by Series II back in 1983.

If you compare these models to the original Stasis 1,2 and 3 you
will find that besides a silver faceplate, the circuit has deleted the
cascoding of the output stage. They should not have difficulty driving
the Sound Labs.


RobertS61 20th April 2009 07:09 PM

Thanks again. It's indeed a long time ago.
You together with Dan D'Agostino took the effort to mention a major update (or was it René who saw it as commercial viable) on your products. The MLAS amplifiers I know of, the ML-2 and ML-3 had a total of four Series but never mentioned that, only on their schematics. They also got no people later asking about the differences ;)

I'll check the railvoltages, DC and bias/temperaturelevels on both units.

Bye Robert

georgehifi 20th April 2009 09:19 PM

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I belive the difference was the Series II had opto bias where the series I did not.

Cheers George

georgehifi 20th April 2009 09:20 PM

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And the Series I

Cheers George

RobertS61 20th April 2009 09:42 PM

Thanks George,
Off course I did some homework and scanned every thread which came up with my Searchquery 'Threshold'.
so I was pleased to find your schematics and copied them to my disk already yesterday. Don't want to bother you guys with questions that could be answered by reading this information on this Forum. So thanks again.
As I recall those amps from that era have been reincarnated in three different versions:
The first one with obviously no Series designation on them, Series II and the optical bias versions.
Only the substituded S/150 (by the S/200) and the S/1000 never made it to the latest incarnation, the optical bias version. The SA/1, 2 and 3 appeared on the scene simultaneously with the Series II versions of
the AB class amps so these Class A designs never bore that designation (Series II), later they also
got updated to the optical bias version.
If Nelson does not recall the differences between the two first Series I'm afraid nobody does, maybe Grey could be of some assistance in this matter.

RobertS61 20th April 2009 10:18 PM

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This is a special about Nelson Pass from a Japanese Audio Magazine in the early eighties, or maybe a architectural one with a photo of somebodies interior at that time and the interior does not look like as your typical cramped Japanese room but more like a spacier American one.
I don't know how to read Japanese but I do know three of the Audio items depicted:
Fet One pre-amp and two S/1000's.
So I reckon it must be Nelson's home or that of Rene Besné operating a pair of these amps that seems like orphans now...

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