Technical Description
The C 504 is comprised of six basic sections. They
are: phono amplifier, high level amplifier, equalizer
amplifier, output/headphone amplifier, input selector
switching, and power supply. Their circuit descrip-
tions follow.
PHONO AMPLIFIER
The phono amplifier uses a high technology in-
tegrated circuit operational amplifier. Its differential
input stage has been optimized for low noise and low
distortion performance. Open loop gain of this in-
tegrated circuit is 100,000. With high open loop gain a
large amount of negative feedback can be used
around the phono amplifier to further reduce noise
and distortion. The feedback network also provides
precision RIAA frequency compensation. The net-
work uses 1% metal film resistors and 5% poly film
capacitors. To achieve low noise performance it is
essential that the feedback network have very low im-
pedance. As a consequence, the preamplifier must be
capable of operating as a power amplifier to drive this
impedance. The actual power output capability of this
preamplifier stage is more than 100 milliwatts, a great
margin beyond that which is required.
Input sensitivity of the phono amplifier is 2.2
millivolts. The gain of the amplifier is 41.1 dB at 1000
Hz. The phono amplifier has a very wide dynamic
range. At 1000 Hz the phono input circuit will accept
100 millivolts without overload, a voltage far greater
than the output of any current magnetic phono car-
tridge. Phono input overload therefore is virtually im-
possible. A signal level of 10 millivolts at the phono
input at 1000 Hz will produce 1.13 volts at the tape
output. The tape output has a source impedance of
200 ohms and is designed to operate into a load im-
pedance of 5,000 ohms or greater.
HIGH LEVEL AMPLIFIER
At the input to the high level or loudness amplifier
the signal passes through the MONO/stereo switch,
then through the volume control, and into the
amplifier. In the past, loudness controls have typical-
ly used simple passive circuits connected to a tap on
the volume control. As a consequence, compensation
accuracy was dependent on many variables such as
volume control position and differences in the input
level. The C 504 uses active circuitry. The same type
of integrated circuit operational amplifier that is used
in the phono amplifier is used here. It has two feed-
back loops. One feedback loop is flat. The other feed-
back loop conforms to the Fletcher-Munson equal
loudness compensation. A potentiometer is placed
between these two feedback loops making it possible
to select any combination of the two from a flat
response to full loudness compensation. The overall
gain of the stage is 20 dB at mid-frequencies and is
-10-
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