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The article(s) listed below fall into the category of material which has enjoyed continued interest over
the years since original publication.
published as AES preprint 1252.
1979. This series of articles had prior roots in the above 1977 AES presentation as well as
the earlier 1977 series within The Audio Amateur.
the Journal, yours truly received an AES fellowship in November of 1979, cited as: "for his
publications on the subjects of audio applications of integrated circuit operational amplifiers
and the analysis of distortion". Walt Jung AES Fellowship
describes various design approaches to audio line driver stages. While some of the articles
have been stand-alone pieces, others have been in the context of a larger work, such as the
ADI seminar notes of 1992, 1993 and 2002.
as audio line driver stages. This article series is evolutionary, beginning with the first from
1992 and progressing with the most recent, from 2002. The basic topology used is a
combination of two op amps, each optimized for either the input or output stage function.
Thus, it can be referred to as a composite amplifier, since the two amplifier combination
serves functionally as a single driver amplifier.
resultant. In this case, the target goal is a low distortion audio line driver, with ample output
power to drive either long lines or headphones, for example. At the input side, a low
distortion FET input device is typically used, to offer minimal loading to the source (and thus
The articles and their original publication are briefly summarized below.
"Applications for Amplifiers in Audio," Ch. 5 within Walt Kester, Editor, 1992 Amplifier
Applications Guide, Analog Devices, Inc., Norwood, MA, 1992, ISBN 0-916550-10-9,
pp. 18 – 22. A High Performance Audio Composite Line Driver Stage
Although not cited as an author here, it should be noted that AD744 op amp designer
Scott Wurcer provided the original inspiration for the two amplifier concept in use,
which employed the AD744 and the AD811 as a composite driver.
2. Walt Jung, 'High Performance Audio Stages Using Transimpedance Amplifiers',
within Gary Galo, "POOGE-5: Rite of Passage for the DAC960," The Audio Amateur,
issue 2, 1992, pp. 15-18. High Performance Audio Stages Using Transimpedance
DAC-960. It illustrated the basic composite line driver using the AD744 and the
AD811 op amps. It also had some measurement discussions on DAC output levels,
and showed a method of using a transimpedance (current feedback) type amplifier
as an integrator in a DAC I/V stage.
Analog Dialogue, 27-1, 1993, pp. 14 - 17. Op Amps in Line-Driver and Receiver
Circuits, Pt 2
This article expanded upon the basic two amplifier concept, showed alternate line
drivers, and discussed topics related to high performance line drivers.
4. Walt Jung, ‘Audio Line Drivers’, from "Audio Applications," Ch. 8 within Walt Kester,
Editor, 1993 System Applications Guide, Analog Devices, Inc., Norwood, MA, 1992,
ISBN 0-916550-10-9, pp. 8-63 – 8-100. Audio Line Drivers
This work expanded substantially on numbers 1 and 3 above. Also discussed are
housekeeping details such as driving capacitive loads and bypassing. A special note
here for page 8-64—the non-inductive bypasses should be film, not ceramic
capacitor types, i.e., polyester or PPS would be examples. Other application
examples are both single-ended and differential drivers, as well as transformer
drivers with and without feedback. A useful measurement technique shown includes
a non-inverting test to examine input stage non-linearity with regard to amplifier
sensitivity to source impedance.
Special Issue, June 24, 1996, pp. 78-80. Composite Line Driver with Low Distortion
This article is a focused application of the two-amplifier line driver concept, suitable
for headphone use or very high current line driving. It added impedance
compensation to the input stage op amp for lowest distortion for a given source
impedance. Interestingly, it also showed how thermal distortion can be generated by
high current outputs, a phenomenon that is intrinsically addressed by use of the
6. Walt Jung, "Walt’s Tools & Tips: ‘Op Amp Audio – Minimizing Input Errors (Part 4)’”,
Electronic Design, December 14, 1998, pp. 80-82. Op Amp Audio - Minimizing Input
This article, the final installment of Walt’s Tools and Tips in Electronic Design (also the
fourth in the Op Amp Audio series), showed a complete composite amplifier
optimized for audio use. It employs the basic two-stage amplifier, but with stage one
modified to operate with lower open loop bandwidth via the use local feedback.
within Walt Jung, Editor, Op Amp Applications, Analog Devices, Inc. 2002, ISBN 0-
916550-26-5, pp. 6-48 – 6-78. Audio Buffers and Line Drivers
This, the most recent work on this topic, expanded considerably on numbers 1, 3,
and 4 above. Again discussed are housekeeping details such as driving capacitive
loads and bypassing. Application examples are both single-ended and differential
drivers, as well as new distortion canceling transformer drivers using feedback. The
measurement technique using a non-inverting test to examine input stage non-
linearity with regard to amplifier sensitivity to source impedance is expanded/
updated with the performance of current devices.
The link following is the entire (210 page) chapter from Op Amp Applications which
contains this work. It is also recommended as a broad resource on amplifier
techniques. Chapter 6 from Op Amp Applications