that1570 THAT Corporation, that1570 Datasheet - Page 5

• Manufacturer Part Number: that1570
• Description: Low-noise, Differential Audio Preamplifier Ic
• Manufacturer: THAT Corporation

THAT1570 Low-Noise
Differential Audio Preamplifier
this is ~255 mV. As well, if the wiper of the pot loses
contact with the element, gain will abruptly change to
unity, with an attendant change in dc offset as well.
To minimize dc offsets, consider the circuits of
Figures 5 and 7, which ac couple R
Improving Noise Performance
To achieve this feature, the input transistors are
large-geometry NPN types, biased at high (~1 mA)
collector current. In order to maintain the low
voltage-noise performance of the 1570, designers
should take care not to present too high a source
impedance to the input pins. A high impedance
generates its own self-noise when not shunted by the
impedance of the source connected to the input pins.
and any noise in that base current, must be drawn
through the bias network (R
connected to IN1 and IN2 (which connect to the
bases of the input transistors). Any input current
noise will be drawn across the source impedance (as
seen by the inputs), which turns it into a voltage that
is amplified by the gain of the device. Too high a
source impedance can easily spoil the noise of the
device. The 1kΩ resistors used at R
Figure 2 provide a low source impedance for the
1570 even when the input is open, and provide a
2 kΩ (differential) load for the microphone. Higher
The 1570 has extremely low input voltage noise.
Additionally, the input transistors’ base current,
IN+
IN-
PROTECTION
Figure 3. 1570 Typical Application Circuit Using Dual-Gang Pot for Gain Control
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RFI
1
Copyright © 2009, THAT Corporation; All rights reserved.
and R
G
.
1k0
R1
2
1
in Figure 2)
and R
R2
1k0
R
8.66
GF
2
R
R
in
GV2
Page 5 of 12
GV1
5k
5k
CW
CW
source impedances will increase noise seen (and
heard!) with open inputs.
that noise at low gains is dominated by the noise of
resistors R
equivalent input noise at 6 dB gain (the minimum pot
setting) is ~21.3 nV/√Hz, or -108.3 dBu. Much lower
noise (~ 11.8 nV/√Hz) can be achieved if R
are reduced to ~2.5 kΩ, but to achieve 60 dB gain,
this requires R
assumes R
preamps with internal R
resistors may be varied at once. See the circuit of
Figure 3, which uses a dual-gang potentiometer as
the variable element.
while simultaneously increasing R
advantage of this approach is that R
naturally be lower for low gains, without requiring
such a low value for R
circuit, 60 dB gain occurs with R
is 1.1 nV/√Hz, or -133.9 dBu with a zero ohm input
termination. With a 150 Ω input termination, the
EIN, dominated by the 150 Ω resistor, is 2 nV/√Hz or
~ -128.7 dBu.
fully CCW. At this gain, the input-referred noise is
~13 nV/√Hz, or -112.5 dBu with a shorted input, and
One disadvantage of the single-pot approach is
An alternative offered by the 1570 (and not by
In Figure 3, high gain occurs by decreasing R
This circuit’s minimum gain is 3dB, where R
THAT
1570
IN1
R
R
IN2
G
G
GV
1
2
A
2k5
2k5
R
+15V
R
and R
-15V
= 5 kΩ.)
10k
Vcc
Vee
R3
A
B
10k
R4
G
100n
100n
C10
OUT1
OUT2
C9
to be = 5 Ω. (This analysis also
B
. For the circuit of Figure 2, the
G
to achieve high gains. In this
A
and R
Document 600132 Rev 01
G
B
= 8.66 Ω, and EIN
) is that all three
OUT+
OUT-
A
A
and R
and R
A
and R
B
. The
B
GV
will
is
G
B