Product Design; Electronic Design - Halcro dm10 Owner's Manual

Product Design

Electronic design

by Bruce Candy
In keeping with my philosophy of design, my
basic aim was to produce a preamplifier with
which I, personally, am quite satisfied. I
sincerely hope that others will share my
satisfaction with these products.
As I saw it, the major issues were:
• zero compromise on Transparency
• flexibility
• ease of use
• progressive design concepts
In order to meet this challenge, the resulting
circuits have particular characteristics:
First, all circuits in the preamplifier have
distortion so low that it is immeasurable,
whether THD, IM and so on.
Second, all circuits in the preamplifier exhibit
exceptionally low noise (see specifications).
Third, there is no compromise whatever with
the choice of components (Vishay resistors,
FKP1 capacitors and so on).
Fourth, the circuits are highly immune to
electromagnetic interference. Some inputs
and outputs include both first order filters and
common mode chokes.
Fifth, the power supply and microprocessor
circuits are designed
electromagnetic emissions (extensive 2
th
6 order filters).
Sixth, the power supply switching frequency
was chosen to be much higher than the audio
band (>200 kHz).
Seventh, the power
exceptionally well regulated, double regulation
in fact (switch-mode and linear servo loops).
Eighth, components and design are selected
for high reliability.
The figure below shows a functional block
diagram of the preamplifier. As can be seen,
the phono stage has four stages. Firstly, there
is an ultra low noise input stage with a gain of
x 20 for moving coil cartridges. This can be
switched out of the circuit train.
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
capacitor is also connected to this input and
to ground.
4
for minimal
nd
to
supply rails are
variable