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mike m 15th January 2003 09:21 PM

Theshold S500II
Purchasing this amp. Wondering what mods is there for it and can it be turned into a mono amp?

GRollins 15th January 2003 10:00 PM

Mods? How crazy do you want to get? Add caps to the power supply? Swap transistors in the front end? Rebuild it as an X? (Don't take me up on that last one. I've got too many projects on my plate already.)
The amp can be bridged by inverting the input to one channel and using the positive outputs <i>only</i>. Leave the negative outputs alone. The power will be x4 until you exceed the available current; it's a good idea to stick with 8 ohm loads so that this dosn't happen too quickly.
I've got two of the original S-500s. They make good low frequency amps. The mids and highs are a little cloudy, but still match or exceed a number of the units on the market today. Ditto for imaging and detail retrieval.


Nelson Pass 16th January 2003 04:29 AM

I have a nice fix for that: new front end and higher bias.
I believe I've already posted the last (never to be produced)
front end for the Stasis series, delivered as I walked out
the door.

If you can't find the reference, let me know and I'll repost it.
It was remarkable for being totally DC coupled with almost no
drift at all and was improved in every respect over its

mike m 16th January 2003 08:14 AM

I cannot find the modification please repost > Thank You.

Nelson Pass 20th January 2003 09:43 PM

mike m 21st January 2003 11:28 PM


Originally posted by Nelson Pass
I cannot open that file can anyone help!

planet10 22nd January 2003 12:06 AM

2 Attachment(s)

Originally posted by mike m
I cannot open that file can anyone help!

Nelson Pass 22nd January 2003 04:37 AM

Yep, I was walking out the door about then, and this would
be the last piece of schematic I left, along with my white
paper "IGBT's: Threat or Menace"

Yeah, I know, I really amuse myself with that kind of humor.

I believe this schematic made it into some updates, but not
production. Its primary virtue was that there was almost
no thermal drift, as all the temperature coefficients of each
component were carefully measured and calculated and the
circuit was designed to null the PN junction drift throughout.

macka 22nd January 2003 05:23 AM

April 17 1990.

Thats a while go now, quite a few millcentons (Vulcan calander)

"IGBT's: Threat or Menace"

So what was the paper pointing out, or did you build a better mouse trap?


Nelson Pass 22nd January 2003 05:40 PM

The paper discussed the difficulties of matching the
devices due to their dual device nature and the
inability to bias the mosfet part of the part at a
greater current than the Base current of the Bipolar
part. In an ordinary circuit, the driver is biased at
higher than the draw of the following part, which
improves the linearity and speed.

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