listener defined low frequency bass impact without bass boom. This
circuit has been used in different forms in many products including
Rockford Fosgate Car amplifiers. Jim Fosgate received US patent
number 3,883,832 for this circuit in 1975. It is called a "SINGLE
ELEMENT CONTROLLED TWIN T FILTER" and has a much steeper
curve than a bass or loudness control. It changes frequency and
amplitude (Boost) with a single control. When the control is set in the
“Min”position, the boost is applied at a lower frequency than when
the control is set to the “Max”position.
In order to infuse the sound with a natural sound stage, Jim
integrated his surround circuitry design, part of his US patent
5,307,415 “Panorama Control” filed in April 1994. This circuit applies
an out of phase cross blend to create a sense of depth and space
and moves the dimensional soundstage outside of your head. The
control has “Min”, “Max”, and “Off”positions. In the center “Off”
position the surround circuit is bypassed so the signal is unaffected
and provides normal stereo output.
The Fosgate Signature Tube Headphone Amplifier’s elegant circuit
has the tube amplifiers, buffers, bass EQ, and surround processing
configured in a single stage. Negative feedback is applied around this
stage in a single loop. Output offset is controlled with DC servos,
allowing the video buffers to directly drive the headphones with no
output capacitors in the signal path. There is only one coupling
capacitor per channel in the signal path, and it is enclosed within the
feedback loop, thus capacitor colorations are eliminated. Distortion
and noise are below the threshold of hearing. The bandwidth is very
wide allowing audio signals to pass through without colorations.
Bandwidth is an amazing “ten times” the threshold of human hearing,
2 HZ to 200KHZ at -3DB.
The power supply is similar to the one used in our Fosgate Signature
Tube Phono stage. It uses separate oversize storage capacitors on
each amplification path of each channel to provide a rock solid supply
voltage without voltage regulators to affect the signal. The circuit is
configured so that audio signal from one channel cannot couple to
signal from the other, preserving soundstage width and depth. This
power supply provides the benefits of both dual mono and batteries
but in a simple AC powered circuit.
A 35 second muting circuit eliminates turn on/off thumps fromthe
direct coupled output circuit. Two stereo inputs and a stereo tape
output are provided for added flexibility.
