that1570 THAT Corporation, that1570 Datasheet - Page 8

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

Document 600132 Rev 01
in addition to offset.) D
around the input stage in the 1570 further prevent-
ing damage.
made large to minimize high-pass filtering of the
signal based upon the sum of the values of R
well, keeping their reactance low relative to the exter-
nal microphone's source impedance will avoid
increasing the effects of low-frequency current noise
in the 1570 input stage.
their approach to the gain potentiometer. The single-
gang pot shown in Figure 7 may be a little less expen-
sive to implement, but the dual-gang pot of Figure 8
will deliver better noise performance at low gains, for
the reasons noted above. Note that Figure 8 features
minimum gain of 3 dB, compared to Figure 7 at
6 dB. The low-frequency corners are about the same
(~5 Hz) in the two circuits.
many pro audio products include, a zener diode
arrangement instead of the bridge rectifier to V
V
recommend the zener approach, because we find that
R
order to limit peak currents enough to protect
reasonably sized zener diodes (e.g. 1/2 W). Such
large series input resistors will limit the noise
performance of the preamp. The ultimate floor is set
by the impedance of the microphone, but any
EE
3
IN+
IN-
and R
The series combination of C
As in Figures 2 and 3, Figures 7 and 8 differ in
Other manufacturers have recommended, and
as shown in Figures 7 and 8. THAT does not
Figure 7. 1570 Typical Phantom Power Application Circuit With Single-Gang Pot for Gain Control, AC-Coupled R
4
PROTECTION
must be made much larger (e.g., 51 Ω) in
C1
22p
C2
22p
RFI
220p
C3
PHANTOM POWER
6k81
R5
+48V
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
R6
6k81
5
+
+
47u
47u
C4
C5
and D
Copyright © 2009, THAT Corporation; All rights reserved.
4
R4
10
PHANTOM POWER
R3
10
and C
PROTECTION
6
+15V
-15V
steer currents
FAULT
D3
S1DB
D4
S1DB
S1DB
5
S1DB
should be
D1
D2
1
+15V
+R
-15V
CC
2
. As
and
1k2
Page 8 of 12
R2
1k2
R1
PROTECTION
additional
performance.
Schottky diodes for D
longer do so. Schottky diodes appeal because of their
fast turn-on behavior and low forward voltage drop.
However, aside from their higher cost, our experi-
ence is that they tend to leak much more than
conventional, glass-passivated power diodes, and
that their fast turn-on behavior is unnecessary in
practice.
Audio Engineering Society preprint "The 48 Volt
Phantom Menace," by Gary K. Hebert and Frank W.
Thomas, presented at the 110th Audio Engineering
Society
Menace Returns” by Rosalfonso Bortoni and Wayne
Kirkwood, presented at the 127th AES Convention.
Outputs
-610 mV. The differential output has low offset, but
this common-mode dc offset must be considered in
connecting the 1570 to subsequent circuitry. Most
high-performance A/D converters require a dc offset
at their inputs for proper operation. In such cases,
drive circuitry should be configured to add the
appropriate dc offset to the 1570 outputs in order to
match the converter.
unity,
C6
100p
5%
C7
100p
5%
RFI
220p
C8
Additionally, while at one time we recommended
For further insights into this subject, see the
Each of the two 1570 outputs has a dc offset of
As well, the 1570 has common-mode gain of
regardless
(AES)
R
10k
GV
series
R
10 3300u
Convention,
GF
1N4148
1N4148
D6
D5
C
resistance
1
Differential Audio Preamplifier IC
G
~D
of
4
THAT
1570
in Figures 7 and 8, we no
its
IN1
R
R
IN2
G
G
1
2
and
+15V
THAT1570 Low-Noise
R
Vcc
Vee
-15V
5k
R
A
5k
differential
further
B
OUT1
OUT2
100n
100n
C10
C9
“The
G
degrades
Phantom
OUT+
OUT-
gain.