4301PA View Datasheet(PDF) - THAT Corporation
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4301PA Datasheet PDF : 12 Pages
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THAT 4301 Dynamics Processor IC
Page 9
Applications
The circuit of Figure 14, Page 9, shows a typical
verting stage. If, for some reason, more than 0 dB
application for THAT 4301. This simple compres-
gain is required when the VCA is set to unity, then
sor/limiter design features adjustable hard-knee
the resistors may be skewed to provide it. Note that
threshold, compression ratio, and static gain1. The
applications discussion in this data sheet will center
the feedback capacitor (C2) is required for stability.
The VCA output has approximately 45 pf of capaci-
on this circuit for the purpose of illustrating impor-
tance to ground, which must be neutralized via the
tant design issues. However, it is posslble to config-
ure many other types of dynamics processors with
THAT 4301. Hopefully, the following discussion will
imply some of these possibilities.
Signal Path
As mentioned in the section on theory, the VCA
input pin is a virtual ground with negative feedback
provided internally. An input resistor (R1, 20kW) is
required to convert the ac input voltage to a current
within the linear range of the 4301. (Peak VCA input
currents should be kept under 1 mA for best distor-
tion performance.) The coupling capacitor (C1, 47 mf)
is strongly recom-
47 pf feedback capacitor across R2.
The VCA gain is controlled via the EC– terminal,
whereby gain will be proportional to the negative of
the voltage at EC–. The EC+ terminal is grounded, and
the SYM terminal is returned nearly to ground via a
small resistor (R3, 51 W). The VCA SYM trim (R5,
50 kW) allows a small voltage to be applied to the
SYM terminal via R4 (300 kW). This voltage adjusts
for small mismatches within the VCA gain cell,
thereby reducing even-order distortion products. To
adjust the trim, apply to the input a middle-level,
middle-frequency signal (1 kHz at 1 V is a good
mended to block dc cur-
rent from preceeding
stages (and from offset
voltage at the input of
the VCA). Any dc current
into the VCA will be
modulated by varying
gain in the VCA, showing
up in the output as
“thumps”. Note that C1,
in conjunction with R1,
will set the low fre-
quency limit of the cir-
cuit.
The VCA output is
connected to OA3, config-
ured as an inverting cur-
rent-to-voltage converter.
OA3‘s feedback compo-
nents (R2, 20 kW, and
C2, 47 pf) determine the
constant of cur-
rent-to-voltage conver-
sion. The simplest way
to deal with this is to
recognize that when the
C1
47uF
+15
CCW
R12
10K
CW
THRESHOLD
R11
383K 1%
R10
2M00 1%
-15
R8
4k99 1%
IN
+15
C7 100n
C8 100n
C3
47uF
-15 R6
10K0 1%
COMPRESSION CW
R13
10K
CCW
R14
1K43
1%
C6
22uF
R1
20K0 1%
VCA SYM
R5 +15
50K
R9
10K0 1%
CR2
CR1
-15
R3
51
R4
300K C2 47pF
R2
20K0 1%
-
OA1
+
SYM
IN VCA OUT
EC-
EC+
VCC THAT4301
VEE
IN RMS OUT
IT
CT
GND
R7
2M00
1%
C4
10uF
-15
R15
10K0
1%
-
OA3
+
OUT
+
OA2
-
R16
4k99 1%
C5
100N
R17
590K
1%
+15
GAIN
CW
R18
10K
CCW
-15
VCA is set for unity
(0 dB) gain, the input to
output voltage gain is
simply R2/R1, just as in
the case of a single in-
Figure 14. Typical Compressor/Limiter Application Circuit
1. More information on this compressor design, along with suggestions for converting it to soft-knee operation,
is given in AN100, Basic Compressor Limiter Design. The designs in AN100 are based on THAT Corporation’s
2150-Series VCAs and 2252 RMS Detector, but are readily adaptable to the 4301 with only minor modifications. In
fact, the circuit presented here is functionally identical to the hard-knee circuit published in AN100.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
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