THAT1512S08-U Datasheet, PDF (6/8 Pages) List of Unclassifed Manufacturers – Low-Noise, High Performance Audio Preamplifier IC

Document 600031 Rev 08
Page 6 of 8
THAT 1510/1512 Low-Noise
High Performance Audio Preamplifier IC
currents. (Higher values further limit current flow,
but introduce additional source impedance and
noise.) D1 through D4 prevent the IC’s inputs from
significantly exceeding the supply rails. D5 and D6
steer currents around the input stage in the
1510/1512, preventing damage.
The series combination of C4 and C5 should be
made large to minimize high-pass filtering of the
signal based upon the sum of the values of R1+R2. As
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 1510/1512 input stage.
Other manufacturers have recommended, and
many pro audio products include, a zener diode
arrangement connected to the bridge rectifier instead
of the connection to V+ and V- as shown in Figure 5.
THAT does not recommend this approach, because
we find that R3 and R4 must be made much larger
(e.g., ≥ 51 Ω) in order to limit peak currents enough
to protect reasonably sized zener diodes (eg. 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
additional series resistance further degrades
performance.
For further insights into this subject, see the
Audio Engineering Society preprints "The 48 Volt
Phantom Menace," by Gary K. Hebert and Frank W.
Thomas, presented at the 110th AES Convention and
“The 48 Volt Phantom Menace Returns”, by Rosal-
fonso Bortoni and Wayne Kirkwood presented at the
127th AES Convention.
Impedance and Line Input Configurations
A higher common-mode input impedance is
desirable (compared with that of Figures 3 and 4)
when input coupling capacitors (C4 and C5) are used
to block phantom power. At low frequencies where
the reactance of C4 and C5 become significant
(compared to the common-mode input impedances),
the two capacitors interact with the common-mode
input impedance (seen looking to the right-side of
both capacitors) to form voltage dividers for
common-mode signals. Differences in the two capaci-
tors' values leads to different voltage dividers, spoil-
ing the low-frequency common-mode rejection of the
stage. Since C4 and C5 are generally large, electrolytic
types, precise matching is difficult and expensive to
achieve. High common-mode input impedance
reduces the matching requirement by decreasing the
frequency at which the capacitive reactance becomes
significant inversely with the common-mode input
impedance.
The "T-bias" circuit (R1, R2, and R7) shown in
Figure 5 accommodates this objective. In this circuit,
R1 and R2 are connected to a third resistor R7, boost-
ing the low-frequency common mode input
+15
C6
-In
-In ¹⁰⁰ⁿ
C1
470p ^R1
RG1 _V+
1k
RG
Out
Out
C2
+In
470p
CG
Ref
RG2 ^V-
+In
U1
THAT
1510/1512
C3
47p
R2
1k
C9
100n
-15
Figure 4. Basic 1510 / 1512 Circuit with Variable Gain
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