Applications
The circuit of Figure 14, Page 9, shows a typical
application for THAT 4301. This simple compres-
sor/limiter
design
features
adjustable
hard-knee
threshold, compression ratio, and static gain1. The
applications discussion in this data sheet will center
on this circuit for the purpose of illustrating impor-
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 (R
1, 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 (C
1,47 mf)
is
strongly
recom-
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 C
1,
in conjunction with R
1,
will
set
the
low
fre-
quency limit of the cir-
cuit.
The VCA output is
connected to OA
3, config-
ured as an inverting cur-
rent-to-voltage converter.
OA
3‘s
feedback compo-
nents (R
2,20kW,
and
C
2, 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
VCA
is
set
for
unity
(0 dB) gain, the input to
output voltage gain is
simply R
2/R1, just as in
the case of a single in-
verting stage. If, for some reason, more than 0 dB
gain is required when the VCA is set to unity, then
the resistors may be skewed to provide it. Note that
the feedback capacitor (C
2)is required for stability.
The VCA output has approximately 45 pf of capaci-
tance to ground, which must be neutralized via the
47 pf feedback capacitor across R
2.
The VCA gain is controlled via the E
C– terminal,
whereby gain will be proportional to the negative of
the voltage at E
C–. The EC+ terminal is grounded, and
the SYM terminal is returned nearly to ground via a
small resistor (R
3,51 W). The VCA SYM trim (R5,
50 k
W) allows a small voltage to be applied to the
SYM terminal via R
4 (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 at1Visa good
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
THAT 4301 Dynamics Processor IC
Page 9
EC+
EC-
IN
OUT
SYM
VCA
+
OA3
-
OUT
1%
20K0
R2
51
R3
R16
1%
4k99
+
OA2
-
GND
C5
100N
1%
590K
R17
1%
10K0
R15
+15
R18
CCW
CW
GAIN
10K
-15
1%
1K43
R14
COMPRESSION
CW
CCW
R13
10K
C3
47uF
R6
-15
10K0 1%
22uF
C6
IN
IT
OUT
CT
RMS
R7
2M00
1%
-15
10uF
C4
VEE
VCC
THAT4301
+15
100n
C7
C8
100n
1%
4k99
R8
+
OA1
-
-15
2M00 1%
R10
1%
383K
R11
CR1
CR2
10K0 1%
R9
R12
CW
10K
CCW
THRESHOLD
+15
20K0 1%
R1
47uF
C1
300K
R4
-15
+15
R5
50K
VCA SYM
C2
47pF
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.
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