DYNAMIC SOUNDS ASSOCIATES Phono III
Page 15 of 38
USER MANUAL
5.1
Amplifier Chain
The block design of the
Phono III
is shown in
Figure 6
. It consists of four all JFET balanced (differential)
amplifier gain stages, two of which can be adjusted to provide the required gain to accommodate
virtually any phono cartridge. In addition, each gain stage employs its own precision, temperature
compensated, low noise constant current source and voltage regulator for the ultimate in stability,
signal control, and isolation.
Figure 6 –
Phono III
Block Diagram
As can be seen from
Figure 6
, the first and second gain stages, as well as the third and fourth gain stages
are differential pairs. The fourth gain stage is also connected differentially to the unity gain output
driver which provides a balanced output through a standard XLR connector and a single-ended output
via a standard RCA type phono jack. The two switches shown at the input represent the process that
allows the input stage to accept either unbalanced or balanced inputs. The solid lines represent the
unbalanced mode configuration and the dashed lines the balanced configuration.
Using two differential pairs of gain stages has significant advantages for noise reduction which will be
discussed in more detail in
Section 4.5
. The downside of differentially coupling the first and second gain
stages is that two matched LP filters are required for the HF component of the playback equalization
curves. However, the filters are of a simple design and can be easily balanced to ensure proper playback
equalization. The balancing and calibration of these filters is performed by DSA during the “break-in”
period after a new unit is assembled and is not a user accessible adjustment. Once balanced and
calibrated, there is no requirement to perform any further adjustments on these filters.
While the
Phono III
has no coupling capacitors in the signal path, it still does not have any significant
response at DC which prevents the propagation of DC drifts and offsets. This “DC block” occurs at the
input to the third gain stage, the first stage in the second differential pair shown in
Figure 6.
This “DC
block” combined with the gain dependent low frequency response of the first stage results in a
frequency “roll-off” at very low frequencies as shown in
Figure 7.
This very low frequency roll-off is
relative with respect to the absolute gain and any playback equalization being used. If the filter function
has been activated, that response will be added to the frequency response shown in
Figure 7
.
