Introduction
I have been building power amplifiers for over forty years now, and at this late date I wasn't
expecting much in the way of surprises. In the past few years at First Watt I had been
working with low power designs using some of the new power Jfet transistors from Lovoltech
and SemiSouth which offered some performance improvements with simple circuits, so at
least I felt like I was still making some incremental progress.
The problem was, however, that while you can make really good sounding amplifiers with
simple linear circuits, it tends toward a certain amount of sameness as they measure better.
It is difficult to create something subjectively magical ” about the sound, something that
makes people wake up and say “
What the hell is that?
”
As a tradition, there is really no formula for such a thing, as you are dealing with the incredibly
complex neural system that sits between people's ears, and it doesn't respond to sound in the
same way as our distortion analyzers and oscilloscopes.
If you have experienced ears and if you know what you like when you hear it, then the
approach that works is to try everything. That is largely what I do, remembering Edison's
dictum “invention is 2 percent inspiration and 98 percent perspiration.”
Sometimes, though, things just fall into your lap. A couple years ago I was talking to Jeff
Cassidy at SemiSouth and he mentioned that at one time they had made a special run of
Static Induction Transistor
devices on some kind of military/industrial contract, and that one of
their technical people had remarked that they were nearly ideal for use in audio amplifiers.
“
Really
”, says I, “
Do you have any of them left over?
” No the customer scooped them all up,
but they would be interested in making more. The price of a small run was astronomical (to
me, at least) and I spent quite a bit of time pondering the risk.
And then I wrote the check and didn't look back.
After a few months I had a small batch of SITs with my name on them, and I started playing.
It took about a year and a half to arrive at the designs of the SIT-1 and SIT-2 amplifiers, and it
has been a revelatory experience.
The SIT devices allow operation with only one gain stage. In fact the SIT-1 has only one
transistor in the entire amplifier, without feedback and even without degeneration. It's as
simple and raw an amplifying circuit as you could imagine.
I have built such simple amplifiers before using Mosfets and power Jfets, but I was not at all
prepared for the quality of sound that I got with the SITs. It was like magic.
The SIT-1 and SIT-2 are special amplifiers. They have single gain stages – the input signal
goes into the Gate of the SIT and comes out the drain amplified and driving the loudspeaker.
There is no feedback or degeneration. They run single-ended Class A, and the SIT-2, biased
by a current source, has an efficiency of about 10%, and the SIT-1, biased through 800 watts
of power resistors, has an efficiency of about 5%.
Содержание SIT-2
Страница 1: ...First Watt SIT 2 Power Amplifier...
