Tubes, Discrete Solid State Devices, and Integrated Circuits 341controlled amplifiers (VCAs). However, through the
1970s all these integrated circuits were intended more
for use in consumer and industrial applications than
professional audio.
In the mid-1970s, semiconductor manufacturers
began to recognize that professional audio had signifi-
cantly different requirements from the needs of
consumer audio or industrial products. The Philips
TDA1034 was the first op-amp to combine low noise,
600 : drive capability and high slew rate—all impor-
tant characteristics to pro audio designers. Shortly after
its introduction, Philips transferred production of the
TDA1034 to the newly purchased Signetics division
which re-branded it the NE5534. At about the same
time, Texas Instruments and National Semiconductor
developed general-purpose op-amps using a combina-
tion of bipolar and FET technology (the TI TLO70- and
TLO80- series, and the National LF351-series, some-
times called “BIFET”). These parts offered high slew
rates, low distortion, and modest noise (though not the
600 : drive capability of the 5534). While not specifi-
cally aimed at pro audio, these characteristics made
them attractive to pro audio designers. Along with the
NE5534, these op-amps became pro audio industry
standards much like the 12AX7 of the vacuum tube era.
Op-amps are fundamentally general-purpose
devices. The desire to control gain via a voltage, and the
application of such technology to tape noise reduction,
in particular, created a market forICs that were dedi-
cated to a specific function. This paralleled the way that
phono preamplifiers spawned ICs designed for pream-
plification. In many ways, the VCA drove the develop-
ment of early pro audio ICs.
The design of audio VCAs benefitted from the early
work of Barrie Gilbert, inventor of the “Gilbert Cell”
multiplier, who in 1968 published “a precise four-quad-
rant multiplier with subnanosecond response.”^1 Gilbert
discovered a current mode analog multiplication cell
using current mirrors that was linear with respect to
both of its inputs. Although its primary appeal at the
time was to communications system designers working
at RF frequencies, Gilbert laid the groundwork for
many audio VCA designs.
In 1972, David E. Blackmer received U.S. Patent
3,681,618 for an “RMS Circuit with Bipolar Loga-
rithmic Converter” and in the following year patent
3,714,462 for a “Multiplier Circuit” useful as an audio
voltage-controlled amplifier. Unlike Gilbert, Blackmer
used the logarithmic properties of bipolar transistors to
perform the analog computation necessary for gain
control and rms level detection. Blackmer’s develop-
ment was targeted at professional audio.2,3
Blackmer’s timing could not have been better as the
number of recording tracks expanded and, due to
reduced track width coupled with the effect of summing
many tracks together, tape noise increased. The
expanded number of recorded tracks also increased mix
complexity. Automation became a desirable feature for
recording consoles because there just were not enough
hands available to operate the faders.
Companies such as dbx Inc. and Dolby Laboratories
benefited from this trend with tape noise reduction tech-
nologies and, in the case of dbx, VCAs for console
automation. Blackmer’s discrete transistor-based rms
level detectors and VCAs, made by dbx, were soon used
in companding multitrack tape noise reduction and
console automation systems.
The early Blackmer VCAs used discrete NPN and
PNP transistors that required careful selection to match
each other. Blackmer’s design would benefit greatly
from integration into monolithic form. For some time
this proved to be very difficult. Nonetheless, Blackmer’s
discrete audio VCAs and Gilbert’s transconductance cell
laid the groundwork for dedicated audio ICs. VCAs
became a major focus of audio IC development.
Electronic music, not professional recording,
primarily drove the early integration of monolithic VCAs
and dedicated audio ICs. In 1976, Ron Dow of Solid
State Music (SSM) and Dave Rossum of E-mu Systems
developed some of the first monolithic ICs for analog
synthesizers. SSM’s first product was the SSM2000
monolithic VCA.^4 Solid State Music, later to become
Solid State Microtechnology, developed an entire line of
audio ICs including microphone preamplifiers, VCAs,
voltage-controlled filters, oscillators, and level detectors.
Later, Douglas Frey developed a VCA topology known
as the operational voltage-controlled element (OVCE)
that was first used in the SSM2014.^5 Doug Curtis, of
Interdesign and later founder of Curtis Electro Music
(CEM), also developed a line of monolithic ICs for the
synthesizer market that proved to be very popular with
manufacturers such as Oberheim, Moog, and ARP.^6
VCAs produced for electronic music relied on NPN tran-
sistor gain cells to simplify integration.
In the professional audio market, Paul Buff of Valley
People, David Baskind and Harvey Rubens of VCA
Associates, and others in addition to Blackmer also
advanced discrete VCA technology. Baskind and
Rubens eventually produced a VCA IC that ultimately
became the Aphex/VCA Associates “1537.”^7
Blackmer’s VCAs and rms detectors used the precise
logarithmic characteristics of bipolar transistors to
perform mathematical operations suitable for VCAs and
rms detection. The SSM, CEM, and Aphex products