4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 12

• Manufacturer Part Number: 4305-DEMO
• Description: Audio Modules & Development Tools Low-Cost Dynamics Processor Demo Board
• Manufacturer: THAT Corporation

Specifications of 4305-DEMO

• Description/function: Audio DSPs
• Operating Supply Voltage: 15 V
• Product: Audio Modules
• For Use With/related Products: THAT4305
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Page 12 of 20
In this datasheet, we will show detailed circuits for
the 4305 in a relatively simple above-threshold
compressor, and in two simple companding systems.
As mentioned above, many other configurations of
the 4305 are possible. THAT intends to publish
additional circuits in forthcoming applications notes.
Please check with THAT's applications engineering
department to see if your application has been
covered yet, and for personalized assistance with
specific designs.
Feedforward Compressor/Limiter
knee, feedforward compressor/limiter. In addition to
compression ratio, the sidechain includes controls
for threshold and make-up gain as well.
The Signal Path
ground, and R14 converts the input signal into a
current flowing into the VCA. The maximum total
signal current, (I
supplies, so R14 is sized to keep the maximum
current at unity gain to below this level. With peak
input voltage swing limited by the ±15 V supply
rails, the 20 kΩ resistor at R14 limits maximum i
to about 750 μA. At 0 dB gain, this will cause the
The circuit in Figure 16 shows a typical hard-
The input of the VCA (pin 15) is a virtual
In
100p
C16
10u
10u
C2
C1
IN
+ I
R14
R28
6k2
20k
33k
R1
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
OUT
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
) is 1.8 mA with ±15 V
+15V
15
VCA In
THAT4305
11
EC-
C13
2
22u
EC+
RMS In
+15V
9
12
-15V
U1B
Vee
Figure 16. 4305-based feedforward compressor
Vcc
8
RMS Out
Threshold
4
CT
THAT4305
Gnd
VCA Out
6
U1A
10u
C
-40 dBu
20 dBu
TIME
5
+15V
-15V
13
Applications
10k
Increase
R10
R17
5k1
IN
430k
R7
22p NPO
22p
C6
C4
R13
1N4148
20k
D1
C15
22p
same current at the VCA output, which is converted
back to 15 V by U2 and R13 (also 20 kΩ). C2
provides AC coupling, required to block any DC
currents that might otherwise flow into the VCA
input. This prevents changes in gain from modulat-
ing this current, which could produce audible
"thumps".
required for stability. The VCA must “see” a source
impedance no greater than 5 kΩ above 1 MHz. R28
in parallel with R14 accomplishes this. C16 is
chosen to prevent the added noise gain of the lower
source impedance from increasing noise within the
audio band.
when the voltage-to-current converting resistor (R14)
is 5 kΩ or less. For example, if the input signal were
limited to lower voltages, the input voltage-to-current
converting resistor (R14) could be reduced in value,
possibly eliminating the need for R28 and C16.
impedance amplifier that converts the VCA's output
current into a voltage. C4 prevents the VCA's output
capacitance from destabilizing the op-amp in this
configuration.
U2
1N4148
10k
R2
U3
D2
The compensation circuit of R28 and C16 is
Note that such compensation is unnecessary
U2, along with C4 and R13 forms a trans-
Out
Compression
INF:1 CR
1:1 CR
Ratio
THAT4305 Pre-trimmed Analog Engine®
+20 dB
-20 dB
Gain
R11
10k
+15V
-15V
1k43
R9
R12
10k
Increase
620k
10k
R8
R3
R18
5k1
100n
U4
C5