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Nelson Pass 22nd August 2002 08:38 PM

800A Schematic
So, for today we have a special treat:

The 800A was designed in 1975 and was my first
commercial product. A total of 150 or so were built,
the majority going to Asia.

The Bag O Fun posting shows the original brochure with
the relevent specs, story, and pictures.

These were hand built by Rene Besne and I, initially in
my garage and living room. I made the PC boards myself.

The design is notable for several things:

Like H.G.Wells' martians, the output stage is a study in
threes: A triple-series, triple parallel, triple darlington
follower. I think it was the first and last of its kind.

The output stage was idled at about 200 watts, or about
1/2 of what you would expect from a Class A circuit.

The output devices were Motorola 2N5878/76, chosen
for their higher bandwidth (at the time) over 6031's
and 5631's (1 MHz). Interesting to note that the
Levinson ML-2's later used the 5800 type chips also.

I believe this was probably the first solid state amp to
eschew the use of an output inductor.

The bias circuit was the infamous "dynamic bias" which
adjusted the bias voltage in real time to keep both halves
of the output stage in conduction to 200+ watts. You can
see it in Q5, Q6, Q10, and Q11. Its function was helped
along by D22 and D23 and R73 and R74 which provided
a breakpoint in the transfer curve, smoothing the cutoff

You can see variations of this type of circuit in later work
by Hawksford, Cordell, and of course companies like JVC
and Pioneer and a host of lesser known brands.

You will see VI limiters in Q7 and Q12, but they have been
set at fairly high current values and have integrators to keep
them from tripping into electrostatic speakers.

The front end is prosaic enough, and was designed to be
driven by a buffer which was on a separate assembly in
the amplifier.

Several of the resistor values are pulled from memory here,
as the only schematic copy I have is of very poor quality.

Threshold built these amps for about 3 years, along with
the 400a and the 4000, but then abandoned the approach
because by then there were too many imitators offering
circuits which called themselves Class A but which idled at
almost no current, tarnishing the image of the product.

The notion of idling output stages at low bias and ramping
it up on demand has lived on; check the literature on Krell's
Sustained Plateau Bias.

thylantyr 22nd August 2002 09:21 PM

I feel like i'm in the candy store.
so much goodies, which one to buy................ lol

kristijan-k 22nd August 2002 09:54 PM

2 Attachment(s)
Hello Mr.Pass,

What is the difference between 800 A and T 800 D ?
Are they same amplifiers ?

At your opinion, what is the best amplifier that you ever made so far ?

Best regards,
Kristijan Kljucaric

Nelson Pass 22nd August 2002 10:38 PM

What is the difference between 800 A and T 800 D ?

The difference of which I can be certain is that I had nothing to
do with the T800, and I have no information on it, other than
that it was produced long after I sold my interest in
Threshold and exited.

As far as my best amp, they all are, each in its own way. :)

cocolino 23rd August 2002 12:26 AM


The notion of idling output stages at low bias and ramping it up on demand has lived on; check the literature on Krell's Sustained Plateau Bias
And I always thought Krell has been the inventor of this kind of circuits......:cool:

Mr. Pass:
I can imagine that pure Class A is still superior than automatic bias level adjustment (though it works in "real time") but I wonder if this method of biasing basically could have any real advantage sonically over a fairly high biased ordinary Class A/B amp (of course proper application is presupposed)?
If that were the case why it isn`t found more often in commercial (or DIY as well) designs?

SkyChu 27th August 2002 03:09 AM

Hello Mr. Pass,

What's function of D22 and D23 ? I can found it in GAS ampzilla and SAE X25 schematics ? but your other amp don't have it.

Sky Chu

Nelson Pass 27th August 2002 10:00 PM

D22 and 23 operate as described in first posting. They
limit drop across the resistors in parallel.

Samuel Jayaraj 2nd September 2002 09:21 AM


Can this schematic be used with upgraded semiconductors to provide high quality music in PA and Disc rigs? Will it better the sound of the Leach SuperAmp?

I have MJ21193/21194 devices in mind for the outputs, while I have not investigated into the other stages' devices.

Any comments.

Samuel Jayaraj 3rd September 2002 09:01 AM


I failed to catch your attention (or anybody else's, for that matter) in my previous post.

Nelson Pass 4th September 2002 07:34 PM

I don't know the answer to your questions. Certainly you
can build such amplifiers with later design semiconductors,
but how they will stack up against another product is
anybody's guess.

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