Technical Description
AUDIO SECTION
Each channel of the MA 6200 has four basic sec-
tions. They are: phono preamplifier, high level and
loudness amplifier, equalizer amplifier, and power
amplifier.
PHONO PREAMPLIFIER
The phono preamplifier 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
integrated circuit is 100,000. With high open loop
gain a large amount of negative feedback can be us-
ed around the phono preamplifier to further reduce
noise and distortion. The feedback network also pro-
vides precision RIAA frequency compensation. The
network uses 1% metal film resistors and 5% poly
film capacitors. To achieve low noise performance it
is essential that the feedback network be very low
impedance. 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 re-
quired.
Input sensitivity of the phono preamplifier is 2
millivolts. The gain of the amplifier is 42 dB at 1000
Hz. The phono preamplifier 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 1.2 volts at the tape
output. The tape output has a source impedance of
200 ohms. For most efficient signal transfer the tape
output should operate into a load impedance of
5,000 ohms or greater.
HIGH LEVEL AMPLIFIER
At the input to the high level or loudness amplifier
the signal passes through the mode switch, then
through the volume control, and into the amplifier. In
the past loudness controls have typically used sim-
ple 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 input
level. The MA 6200 uses active circuitry. The same
type of integrated circuit operational amplifier that
is used in the phono amplifier is used here. It has
two feedback loops. One feedback loop is flat. The
other feedback loop conforms to the Fletcher-
Munson equal loudness compensation. A poten-
tiometer is placed between these two feedback
loops making it possible to select any combination
of the two from a flat response to full loudness com-
pensation. The overall gain of the stage is 20 dB and
is not affected by the position of the loudness con-
trol at mid frequencies.
EQUALIZER AMPLIFIER
The equalizer amplifier is constructed with the
same operational amplifiers as used in previous
amplifiers. Five other operational amplifiers are
each arranged in an active circuit configuration that
is the equivalent of a series tuned circuit, one at
each of the five center frequencies. Each series tun-
ed circuit is inserted, via the equalizer control poten-
tiometer, in either the input circuit or feedback cir-
cuit of the equalizer amplifier thereby providing a
boost and cut capability of 12 dB for each band of
frequencies.
POWER AMPLIFIER
The input impedance of the power amplifier is
22,000 ohms and requires 2.5 volts rms to drive the
amplifier to rated output. The short jumpers on the
rear panel of the MA 6200 connect the equalizer
amplifier output [PREAMP OUT] and power amplifier
input [AMP IN]. At the input of the power amplifier
two transistors are connected as a differential
amplifier. The two input signals to the differential
amplifier are the input signal and the negative feed-
back signal from the power amplifier. The differen-
tia! amplifier permits the best use of negative feed-
back to maintain low noise and low distortion perfor-
mance. The outputs of the differential amplifier are
combined in a current mirror to a single output. The
current mirror feeds a linear voltage amplifier which
in turn drives two medium power driver transistors.
The drive transistors feed the output stages. The
output section is arranged as a fully complementary
direct coupled series push/pull amplifier. The power
transistors used in the output circuit are selected for
their high power dissipation capability, wide fre-
quency response, and large safe operating area. The
power transistors are mounted on large black
anodized heat sinks to assure that under normal
operating conditions the transistors will operate at a
low temperature. If operating temperatures should
increase due to a shorted speaker or restricted ven-
tilation, an automatic sensing device turns the
speaker circuit off. The speaker circuit will turn on
again when the temperature has returned to its nor-
mal limits. This additional protection assures you of
reliability even under the most extreme operating
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