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TM
dm10
The first stage is followed by a high input impedance (FET input) low noise
amplifier with a gain selectable to x 4 or x 8. The output of this stage is fed to
an inverting amplifier with a gain selected to be x -2 if the high input
impedance amplifier stage gain is selected to be x 4, or x -1 if the high input
impedance amplifier stage is selected to be x 8. (The “minus” indicates
inverting.) The output of the inverting amplifier is connected back to the high
input impedance amplifier stage input via a continuously variable
potentiometer. A continuously variable capacitor is also connected to this
input and to ground. The variable resistor and capacitor set the load
impedance of the high input impedance amplifier stage and act to load
moving magnet cartridges.
This configuration forms an active load. The variable capacitance sets the load
capacitance and the load resistance is 1/8
th
of the value of the potentiometer
resistance. This configuration lowers the noise floor at high frequency, and its
attributes are described below.
The output of the high input impedance amplifier stage is, as well, fed to a
15Hz 3
rd
order high-pass Bessel filter. This acts as a “rumble-filter.”
Following this is the RIAA equalization stage. The RIAA equalization adheres
to the RIAA specification within 0.25dB at all frequencies.
The output of this RIAA stage is fed to a low noise input selector stage which
has a gain of 2. In truth, if a phono input is selected, this gain can be set to
either 1 or 2, depending upon the gain of the selected phono input. Some
inputs are balanced inputs and one input is a current input with a load
impedance of 51 ohms, for optimal inter-connect load matching. The
equivalent input noise of the voltage inputs is approximately 3.5nV/sqrt(Hz),
and that of the current input is 13pA/sqrt(Hz).
The volume control is implemented so as to include two identical stages in
series. This minimizes output noise.
The dm8 & dm10 preamplifiers feature balanced, unbalanced, current mode
and bridged current mode outputs. The latter consists of a non-inverted and
inverted current output. There is also an unbalanced output for tape and
headphones.
