The significance of this equation is that it tells us
the noise in a MC preamplifier is largely determined
by e
n
, transistor noise voltage and not i
n
, transistor
noise current, since R
g e n
is small. This dictates the
transistor design. For a transistor to have low e
n
it
must have low base diffusion resistance, r
bb
. This
requires that the transistor must have a large base
diffusion area with a long case perimeter and use a
low resistivity silicon material. Also, surface
leakage must be absolutely minimum.
The complementary input stage transistors in the
MCP 1 have these characteristics. The equivalent
r
bb
can be lowered further by parallelling a number
of transistors. Eight transistors are parallelled in the
MCP 1 which gives a reduction of 9 dB in noise level
over using just one transistor. This design ap-
proaches closely the theoretical optimum. To
guarantee performance each input transistor is in-
dividually tested for noise voltage and noise current
at 100 Hz, 1 kHz, and 10 kHz before it is mounted in
the MCP 1. This testing is time consuming but
assures noise free operation.
Each channel of amplification in the MCP 1 uses
two stages of amplification. The first stage has eight
low noise bipolar transistors in a parallel com-
plementary push-pull configuration. This stage
amplifies the signal between 2.5 and 20 times (8 to
26 dB) depending on which input impedance is used.
The second stage uses a pair of complementary
bipolar transistors with a gain of 1.8 times (5 dB).
The over all amplifier is described as a noninverting
parallel cascaded complementary low noise bipolar
transistor amplifier circuit.
The two cascaded stages of amplification in the
MCP 1 preserve the phase of the input signal and in-
crease the signal handling ability or dynamic range.
The input stage inverts the signal and the second
stage restores the phase to zero. The inverting input
stage has an important additional noise advantage.
If the input is open circuited the noise output
reduces instead of increasing as in some other
designs.
The power supply circuit required a new approach
with very careful design. Many MC amplifiers use
batteries to avoid hum and noise problems. These
amplifiers, of course, require battery replacement
and usually the noise and distortion performance is
compromised to allow better battery life. The MCP 1
uses a line voltage power source to eliminate the
need to compromise its noise performance. But the
usual problem of a power transformer had to be solv-
ed first. Power transformers which use 50/60 Hz have
external magnetic fields which can modulate the
minute signal in the moving coil cartridge, add hum
and noise to the cartridge, to the cartridge connec-
ting cable and even to the main preamplifier. The
design of the MCP 1 eliminates all of these pro-
blems. Knowing that a 50/60 Hz hum field simply
could not be tolerated the Mcintosh design group
chose a totally new approach. A high frequency,
completely shielded, solid state switch mode power
supply was used.
The AC power line feeds a reactive voltage divider
and a full wave bridge rectifier. The rectifier pro-
duces 24 volts DC which is filtered and supplied to a
15 volt series voltage regulator. This regulator has 70
dB ripple rejection and reduces the power line hum
to a negligible level. The 15 volts DC powers a stable
multivibrator/frequency divider with an output fre-
quency of 200 kHz. The 200 kHz drives a push-pull
power switching amplifier followed by a ferrite core
toroidal transformer. This transformer has two
secondary windings bifilarly wound. Each winding
feeds a separate rectifier and filter system. The two
outputs are completely independent. One feeds the
left amplifier channel and the other feeds the right.
Filters and shielding are used to contain the 200 kHz
switching within the power supply itself and there is
no unwanted interference radiation or conduction.
The MCP 1 has three independent grounding
systems to prevent ground loops when connecting
input and output cables. The steel metal enclosure
connects to a ground post for grounding to the turn-
table frame and associated amplifying equipment.
The left and right channels have separate grounds.
There is no hum or noise introduced as a result of
the routing of input and output cables.
How To Design A
Head Amp
The input transistors, input circuit configuration,
and the power supply required critical design con-
sideration since noise and hum must be held to an
absolute minimum.
Transistor noise is characterized by two noise
components; e
n
, Equivalent Short-circuit RMS Noise
Voltage, and i
n
, Equivalent Open-circuit RMS Noise
Current. The resistance or impedance of the MC car-
tridge and resistance in the input circuit of the
amplifier also contribute Thermal Noise Voltage, e
r
,
as a result of random electron movement within
these resistances. The combined noise voltage, e
n
,
appearing at the input of the amplifier can be
calculated as:
4
