Page 4 of 26
TM
dm8
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:
Ÿ
Zero compromise on Transparency.
Ÿ
Flexibility.
Ÿ
Ease of use.
Ÿ
Progressive design concepts.
In order to meet this challenge, the resulting circuits have particular
characteristics:
Ÿ
First, all circuits in the preamp have distortion so low that it is
immeasurable, whether THD, IM etc.
Ÿ
Second, all circuits in the preamp exhibit exceptionally low noise (see
specifications).
Ÿ
Third, there is no compromise whatever with the choice of components
(Vishay resistors, FKP1 capacitors etc).
Ÿ
Fourth, the circuits are highly immune to electromagnetic interference.
Some inputs and outputs include both first order filters and common
mode chokes.
Ÿ
Fifth, the power supply and microprocessor circuits are designed for
minimal electromagnetic emissions (extensive 2
nd
to 6
th
order filters).
Ÿ
Sixth, the power supply switching frequency was chosen to be much
higher than the audio band (>200kHz).
Ÿ
Seventh, the power supply rails are exceptionally well regulated,
double regulation in fact (switch-mode and linear servo loops).
Ÿ
Eighth, components and design are selected for high reliability.
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.