not affected by the p o s i t i o n of the loudness c o n t r o l .
EQUALIZER AMPLIFIER
The equalizer amplifier is c o n s t r u c t e d w i t h another
of the same type low noise operational amplifier as
used in the phono amplifier. Three other operational
amplifiers are arranaged in circuit c o n f i g u r a t i o n s that
are the equivalent of three series t u n e d circuits. One
is resonant at each of the three equalizer center fre-
quencies. These series tuned circuits are inserted via
the control p o t e n t i o m e t e r s in either the input circuit
or feedback circuit of the equalizer amplifier thereby
providing a boost and cut capability of 12 dB for each
band of frequencies. The equalizer amplifier has a flat
response gain of 0 dB.
OUTPUT/HEADPHONE AMPLIFIER
The output amplifier is a push-pull c o m p l e m e n t a r y
class AB amplifier using a signal inverting differential
stage at its input. This amplifier drives the main out-
puts and the headphone jack. More than 10 volts RMS
output can be p r o d u c e d with typically no more than
0.003% harmonic d i s t o r t i o n .
A turn-on delay circuit is located ahead of the out-
put amplifier. This circuit uses a light e m i t t i n g
diode/light dependent resistor network that f u n c t i o n s
to transmit no signal until t w o s e c o n d s after the
power s w i t c h is t u r n e d on and to remove the output
signal almost instantly upon power turn-off. Thus the
turn-on and turn-off is transient and noise free.
INPUT SELECTOR SWITCHING
Low level, noise critical input selector s w i t c h i n g is
done electronically using field effect transistor
analog s w i t c h e s . The front panel selector simply swit-
ches small a m o u n t s of control DC w h i c h turn the FET
analog s w i t c h e s off or on. This design e l i m i n a t e s
degradation of frequency response and noise pickup
from long signal paths necessary w i t h c o n v e n t i o n a l
s w i t c h i n g .
POWER SUPPLY
To minimize m a g n e t i c hum radiation and thus im-
prove signal to noise ratio the C 504 power
transformer is triple shielded. Shielding includes a
copper strap, a silicon steel strap, and finally a steel
outer shell. The transformer output voltage is fed to a
full wave bridge rectifier and 3,300 microfarad filter
capacitors to provide the + 2 4 volts for powering the
plus regulator and - 2 4 volts for the m i n u s regulator.
Integrated circuit voltage regulators supply the ± 1 8
volts needed for low level amplifier stages.
-11-
