DYNAMIC SOUNDS ASSOCIATES Phono III
Page 33 of 38
USER MANUAL
Table B.1
The first switch for EQ selection determines the "group" to be used (G1, or G2), and the second
switch selects the appropriate curve (EQ1, or EQ2) from that group. The switch for group
selection has a central "off" position which selects the RIAA EQ curve for that input, and the EQ
switch for that input is disconnected. For both switches, "down" is the "1" selection, and "up" is
the "2" selection.
B.3.2 Designing the EQ Module
The design of the EQ module with the five EQ curves was based on how the RIAA EQ was
incorporated into the
Phono III
. As shown in the block diagram for the
Phono III
(Figure 5), the
RIAA equalization is divided into two low-pass filter sections. The portion of the equalization that
is >1 kHz follows the first gain stage, while the portion <1 kHz follows the second gain stage. The
sole purpose of the 2nd gain stage is to add gain such that a low-pass filter can be used for the
equalization of the LF portion of the audio spectrum. The EQ module that was designed for the
Phono III
encompasses both of these two low-pass filters, but with the appropriate components
selected by miniature relays.
B.3.2.1 EQ above 1 kHz
The EQ for frequencies >1 kHz is accomplished with a series resistor (R) followed by a precision
capacitor (C) which shunts the signal to ground such that the value of R x C = desired time
constant, t3. With this configuration, only the values of C must change for the different EQ
curves, as shown in
Figure B.3
. The RIAA curve (Sw
0
& C
0
) is used as "baseline" for the final
"trimming" of R to obtain t3 = 75 μsec, after which only the values of capacitance must be
selected using relays Sw
1
- Sw
3
to obtain the desired EQ curves.
16
B.3.2.2 EQ below 1 kHz
The EQ for frequencies <1 kHz is more complicated since there are two time constants to be
considered.
Figure B.4
shows the final design concept to meet the requirements as closely as
16
The trimming of R to obtain an accurate RIAA response is performed by DSA prior to shipping the unit to
the user.
GROUP
EQ Curve
NAME
t1
t2
t3
---
--
RIAA
3180 μsec
318 μsec
75 μsec
G1
EQ1
Columbia-Pre RIAA
1590 μsec
318 μsec
100 μsec
G1
EQ2
Decca/FFRR - Pre RIAA
1590 μsec
318 μsec
60 μsec
G2
EQ1
Columbia 78RPM
---
530 μsec
100 μsec
G2
EQ2
Decca/FFRR 78RPM
---
530 μsec
25 μsec
