If, for example, a condensor microphone is to be used, a 50 ohm source is
connected directly to the 600 ohm primary of a 600/600 ohm low leve1
transformer (eg. TRIAD A67J), and the 600 ohm secondary is connected to the
Mode1 101. The microphone will see 600 ohms, which loads the microphone with
12 times its source impedance (ideal loading), while the amplifier sees 50 ohms
(1:1 transfer), and gains 3 dB in noise over a 600 ohm loading.
A dynamic microphone (150 ohm source) may be 1oaded by connecting to 150
ohm/600 ohm transformer, which matches the source load of the microphone and
provides 600 ohm source for the Model 101. The step-up of the transformer
increases the signal by 6 dB at the input of the amplifier, and in effect,
lowers the equivalent input noise at the microphone to -133 dB.
Signal-to-noise ratio for a microphone signa1 of -50 dB in the above two
cases is 80 dB and 83 dB respectively.
ACTIVE ISOLATION TRANSFORMER
The Model 101 Audio Amplifier contains an integral active electronic
isolation transformer with l:l transfer ratio. This is unique among amplifiers
in that the ground is not continuous through the amplifier (see Figure 1).
Two operating conditions of major significance are made possible:
l. Complete single-ended systems without transformers
2. High frequency stability
Since the active isolation transformer contains no band width compromises
associated with convention transformers, it does not exclude RF (without
capacitor filter), and thus transformers are recommended at the input and
output of the system with no transformers within the system.
GROUNDING
In order to preclude the output modulating the input as a direct result of
input ground potential fluctuations at high frequencies, and thus cause high
frequency instabi1ity, the length of any lead carrying signal common to both
output and input should be as short as possible (see Figure 2). Such an
improper termination produces system oscillation at frequencies in the 1-5 M Hz
range and injects hum (ambient 60 cycle riding on oscillation carrier) into the
system. Where ground returns exceed three feet in length, shielding is
recommended (see Figure 3).
Wherever branching ground circuits are designed, the amplifier itself may
be employed as an active isolation transformer thus eliminating ground loops
that would normally be encountered with conventional amplifiers in single-ended
circuitry. Figure 4 illustrates the grounding system required - the on1y
difference being the incorporation of a capacitor to al1ow high frequency
grounding without disturbing low frequency ground isolation. This capac1tor
(.47 MFD, l0V) should be terminated at the amplifier connector with short
leads, and all common path 1eads to ground in Figure 4 are shown with physical
length, for illustration only.
