************* Patent 6,737,915 *************
Tube Input JFET Output (TIJO) zero feedback audio amplifier
U.S. Patent Documents:
4163198 July,1979 Berning 330/277
5017884 May,1991 Perandi 330/3
5022305 June,1991 Butler 84/711
5450034 Sept.,1995 Werrbach 330/3
Hamm, Russell O., "Tubes Versus Transistors - Is There an Audible
Difference", Presented September 14, 1972, at the 43rd Convention
of the Audio Engineering Society, New York.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of electronic
amplifiers. More particularly, the present invention relates
to the use of vacuum tubes with low supply voltage to provide
amplification of musical electronic signals.
2. Description of The Prior Art
Russell O. Hamm, in his brilliant paper "Tubes Versus
Transistors - Is There an Audible Difference", says: "Vacuum-
tube amplifiers differ from transistor and operational
amplifiers because they can be operated in the overload region
without adding objectionable distortion. The combination of
the slow rising edge and the open harmonic structure of the
overload characteristics form an almost ideal sound-recording
compressor." He also states that vacuum tube circuitry only
makes a definite audible difference when it is used at the
Vacuum tubes are rarely used along with transistors, so called
hybrid circuits, because of the different supply voltage
requirements. Power supplies for vacuum tubes are generally
much more expensive, and dangerous, because of the need for
high voltages and a separate filament heater voltage.
It would be an improvement over prior art to develop a hybrid
circuit that uses a single low voltage transformer, for
example 6 to 24 volts AC, to power the heater filament(s) and
to produce a common B+ load voltage for the vacuum tube and
transistor devices, for example 12 to 48 volts DC. However,
vacuum tubes operated at low voltage typically have low gain
due to increased plate resistance and resulting lowered
transconductance. The few cases of prior art that attempt
to solve the low transconductance problem use various
transistor devices that do not fully preserve the pleasing
harmonics and overload compression of vacuum tubes.
BRIEF SUMMARY OF THE INVENTION
The present invention is a hybrid vacuum tube and JFET audio
amplifier, hereinafter called TIJO (for Tube Input JFET
Output), using a simple, low voltage power supply. The
present invention solves the low transconductance problem
associated with operating vacuum tubes at low voltages
by connecting the tube output to a JFET (junction field
effect transistor) while preserving the pleasing harmonics
and overload compression of the vacuum tube.
The primary objective of the TIJO is to provide a transducer
interface audio amplifier incorporating a device that produces
low order harmonics when overdriven. Said transducer may be a
microphone, guitar, or even a digital to analog convertor.
Vacuum tubes produce low order harmonics where each subsequent
harmonic (higher frequency) is much lower in level than the
previous harmonic. This characteristic is most relevant to
the input stage which is connected directly to the transducer
(via a coupling capacitor) as this is where transient peaks
are most likely to be encountered.
A second objective of the TIJO is to disregard the standard
practice of using negative feedback to reduce distortion and
increase bandwidth, but instead celebrate the native response
of the amplifying device by using zero (no) feedback. Also
the use of operational amplifiers (op amps) is completly
avoided as they are too clean and sterile sounding, i.e.
they are extremely low distortion and have very wide
bandwidth, and thus are very accurate but not very musical.
A third objective of the TIJO is to simplify the power supply
by using a single voltage transformer to feed the filaments
and produce a single low voltage B+. This is done with a
12 Volt AC transformer, a voltage doubler, and a zener
regulator to provide about 30 Volts DC to the tube. This
also enhances usability by allowing external placement
of the tube as no high voltages are present.
The fourth objective of the TIJO is to provide a line driver
audio amplifier that interfaces well with a vacuum tube
operated at low voltage by presenting a high impedance to the
tube output to minimize loading. The JFET is electronically
and harmonically similar to the vacuum tube, except that it
typically operates at lower voltages and higher currents, thus
being optimum for the output stage to drive difficult loads.
The fifth objective of the TIJO is to allow the vacuum tube
to be easily changed to various types including the 12AX7,
12AU7 and 12AT7. The different tubes provide different gains,
plate resistances, bias responses, and, in general, overdrive
differently to produce different tonal palettes.
Described generally, the TIJO comprises a vacuum tube having
at least one grid for receiving an input signal, a plate run
at low voltage for delivering a plate voltage responsive to
the input signal, and a cathode connected to ground. The
TIJO also comprises a JFET (Junction Field Effect Transistor)
having a P-type region, or gate, diffused into an N type
channel with a source or drain at each end for receiving
an input signal at the gate from the tube with minimal
loading and responding to said input signal by varying the
resistance between source and drain, hence converting input
voltage into output current. Both the vacuum tube and the
JFET are supplied from a common low voltage B+. The TIJO
utilizes zero feedback and is non-inverting. The vacuum
tube is mounted externally. There are various other
devices to support the TIJO circuit like load resistors,
coupling capacitors, and bias resistors which follow
standard practices from prior art.