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Eisen Audio |
BA512
Discrete Operational
Amplifier
Eisen Audio
Version 1.2
June 9th, 2014
BA512 is a discrete operational amplifier introduced by Rupert Neve™ & Company Ltd. in 1976, as a drop-in
replacement for its BA440 serial feedback amplifier. We have re-engineered the BA512 for use in a variety of
third-party applications, conforming to a more popular form factor and pinout. Specifications are as follows.
* Serial feedback amplifiers, also known as quasi-operational amplifiers, lack a true differential input, instead
utilizing the emitter of a lone input device as an inverting input.
BA512 Electrical Characteristics at Vcc = +/-15V, Tamb = +20 ̊C
Symbol
Vio
Voo
Iib
En
CMRR
Icc
Icc
Vopp
Zi
Ro
SR
Parameter
Input offset voltage, Rs = 600Ω
Output offset voltage
Input bias current
Equivalent input noise, f = 1kHz, Rs = 25Ω
Typical
3
30
3
-147
Common mode rejection ratio, Rs = 600Ω
Supply current, no load
Supply current, 150Ω load
Output voltage swing, RL = 150Ω
Input Impedance
Output resistance
Slew rate
Vi = +/- 1V, RL = 600Ω, CL = 100pf, unity gain
70
+/- 16
+/- 36
+/- 14
460
3
10
Unit
mV
mV
uA
dBu
dB
mA
mA
V
kΩ
Ω
V/uS
AOL
Open-loop voltage gain
Vi = 100mV, RL = 150Ω, f = 10kHz
72
dB
GBP
Gain bandwidth product
Vi = +/- 10mV, RL = 150Ω, CL = 100pf, f =
40kHz
2
MHz
All content Copyright © 2013 Eisen Audio, LLC and/or respective owners. All rights reserved. Terms and
specifications are subject to change without notice. Neve™ is a trademark of AMS Neve Limited Corporation
United Kingdom. Eisen Audio is not affiliated with this party. www.eisenaudio.com
BA512
Eisen Audio
page 2
Architecture
The BA512 discrete operational amplifier is composed of the following three stages:
1. A differential Long-Tail Pair input stage, with Constant Current Source biased for ~2.2mA and emitter
degeneration resistors, feeds [2.] from the loaded collector of its non-inverting input device.
2. A Voltage Amplification Stage, with its own Constant Current Source biased for ~4.3mA, provides lev-
el-shifting and feeds [3.] as follows. Single pole miller compensation is employed.
3. A voltage source of two times Vbe comprises the VAS collector load, passing the same constant
~4.3mA branch current. From either side of this "base spreader" a complementary pair of medium
power output transistors is connected, yielding class A/B push-pull operation.
Power Supply
The BA512 is optimized for bi-polar DC power supplies of +/- 12 to 20V, with the typical offering of +/- 16VDC
being ideal. It will draw approximately +/-16mA of DC current under idle conditions. [Note: The BA512 can easi-
ly drive AC coupled output loads presenting a nominal impedance of 140Ω or greater, but will draw more DC
supply current, accordingly.]
When tested with +/-24V power supplies, the BA512 shows an increase in idle current to +/-22mA,
and output offset voltage increases to 100mV. With the exception of a greater Vop, audio
performance shows no significant changes.
On-board 0.01uF bypass capacitors make local power supply decoupling unnecessary. However, we recom-
mend best practice for decoupling a discrete op amp, e.g. local reservoir capacitors of 22uF or greater, in paral-
lel with 0.1uF film bypass capacitors.
Feedback and Stabilization
Typical frequency response for non-inverting applications is flat from 10Hz to 100kHz. Low value capacitors
(e.g. 100pF) at any junction (i.e. power bypass, output load, series feedback, input shunt) will not significantly
alter this high-frequency response, and are not necessary for stability, so long as the BA512 is operated within
application guidelines from 0 to 50dB of voltage gain.
Application Hints
The BA512 maintains a DC offset of ~3mV at its non-inverting input, depending on source resistance. While
this amount is negligible in most circumstances, the following configuration considerations must be made:
1. Input transformers having a secondary winding DCR of less than 200Ω, and/or unusually thin and sen-
sitive coil wire, should either not be DC coupled to the BA512, or have a resistor of ~1kΩ inserted in
series with the BA512 input.
2. When configured for variable gain, or any fixed gain greater than 1, an adequately sized (i.e. for
1/(2pi•R•C) = better than -3dB @ 10Hz) electrolytic blocking capacitor should connect the shunt feed-
back resistor to ground. BA512 voltage gain thus occurs only in the AC domain, avoiding an increase
in DC offset. This also protects a sensitive gain adjust potentiometer from DC damage.
All content Copyright © 2013 Eisen Audio, LLC and/or respective owners. All rights reserved. Terms and
specifications are subject to change without notice. Neve™ is a trademark of AMS Neve Limited Corpo-
ration United Kingdom. Eisen Audio is not affiliated with this party. www.eisenaudio.com
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