Sections I and II
Paragraphs 1-1 to 2-7
1-2. The Model 450A is a general-purpose, stabi-
lized, fixed-gain amplifier for use with low-level
signals from 2 cps to 2 mc. It provides two calibrated
gain factors, lOX (20 db) and 100X (40 db), selectable
by a toggle switch on the front panel. Each gain factor
is accurate to within
1/8 db in the audio frequency
range; frequency response is given in the table of
specifications (figure 1-1). Principal characteristics:
stable gain with smooth attenuation beyond rated fre-
quency range, wide frequency range, low distortion.
1-3. Typical uses include increasing the sensitivity
of ac test equipment (voltmeters, oscilloscopes,
bridges, etc), obtaining larger signals across lower
impedances in test bench setups and permanent system
An electronically-regulated power
supply and large amounts of degenerative feedback
around the entire amplifier circuit provide very re-
liable and stable operation in case of changing line
voltage, load resistance, or tube characteristics.
1-4. The amplifier is constructed on a single chassis
with removable cover and bottom plate.
carrying handle is provided on the left side of the
The front panel is finished in light
grey enamel; the rest of the cabinet is finished in
dark grey wrinkle paint. Operating controls and ter-
minals consist of toggle-type power and gain switches
and binding post type input and output terminals on
the front panel.
Binding posts are spaced 3/4 inch
on-centers to receive dual banana-plug connectors.
The power cable is permanently attached to the rear
of the amplifier and is terminated in a 3-prong,
A fuse is provided on the rear
and can be replaced externally.
1-6. This handbook contains complete operating and
servicing instructions for the 450A Amplifier and
conforms to the format specified in MIL-M-5474C.
When unpacking the amplifier,
inspect it for any sign of physical damage.
cabinet is damaged, remove cover and bottom plate
and inspect chassis parts for further damage. Re-
port all damage to the carrier and keep amplifier
intact for inspection by carrier and insurer. All in-
struments shipped by the Hewlett-Packard Company
are insured against shipping damage. See Warranty
at rear of manual.
Electrical inspection consists
of testing certain electrical characteristics of the
amplifier to determine that it functions normally after
having been stored or transported. Only one test is
required; full instructions are given in paragraph
4-20 steps a through f.
2-5. INPUT CONNECTIONS. The amplifier can be
connected to a signal source through either twisted
pair leads or shielded cable.
Keep input leads as
short as possible to avoid excessive capacitive shunt-
ing of the signal source.
necessary, use coaxial
cable to prevent unwanted signal pickup from stray
electric and magnetiC fields.
00 NOT connect the
amplifier input to circuit potentials greater than
400 volts unless an externall-j..lfcapacitorhaving suf-
ficient voltage rating is used in series with the input
2-6. OUTPUT CONNECTIONS. The amplifier output
can be connected through any convenient lead set or
cable. The low output impedance of 150 ohms permits
shielded cable to be used freely without capacitive
loading in the audio range and permits twisted pair
leads to be used with much less effect from stray
The amplifier is designed to be used with
resistive loads of 3000 ohms or more. Loads below
3000 ohms reduce amplifier gain bandwidth, and maxi-
mum output voltage available. For higher frequencies,
the load must have small capacitive reactance topre-
serve full output signal quality and stability.
connect the output terminals to dc potential of more
than +50 or -300 volts, or output capacitor ratings
will be exceeded.
2-7. CONSIDERATION FOR LOW SIGNAL LEVELS.
When amplifying low-level signals it may be necessary
to eliminate an electrical ground loop formed by the
power cable ground lead and signal ground lead between
this electrical path is completed
(typically a combination of the signal ground and power
line ground leads do), line frequency currents flow in
the signal ground lead and develop voltages across the
leads which are in series with the desired signal. To