Handbook for Sound Engineers

342 Chapter 12

used variations on the linear multiplier, where a differ-
ential pair, or differential quad, is varied to perform
VCA functions and analog voltage-controlled filtering.
Close transistor matching and control of tempera-
ture-related errors are required for low distortion and
control feed-through in all VCA topologies.

The Gilbert multiplier, the CA3080-series of OTAs,
and the VCAs produced by SSM, CEM, and Aphex all
relied solely on NPN transistors as the gain cell
elements. This greatly simplified the integration of the
circuits. Blackmer’s log-antilog VCAs required, by
contrast, precisely matched NPN and PNP transistors.
This made Blackmer’s VCAs the most difficult to inte-
grate. dbx finally introduced its 2150-series monolithic
VCAs in the early 1980s, almost six years after the
introduction of the SSM2000.^8
Many of the earlier developers of VCAs changed
ownership or left the market as analog synthesis faded.
Analog Devices currently produces many of the SSM
products after numerous ownership changes. THAT
Corporation assumed the patent portfolio of dbx Inc.
Today Analog Devices, THAT Corporation, and Texas
Instruments’ Burr Brown division are the primary
manufacturers making analog ICs specifically for the
professional audio market.

12.3.4.1 Voltage-Controlled Amplifiers

Modern IC VCAs take advantage of the inherent and
precise matching of monolithic transistors that, when

combined with on-chip trimming, lowers distortion to

very low levels. Two types of IC audio VCAs are

commonly used and manufactured today: those based

on Douglas Frey’s Operational Voltage Controlled

Element (OVCE)^9 and those based on David

Blackmer’s bipolar log-antilog topology.^10

The Analog Devices SSM2018. The Frey OVCE gain

cell was first introduced in the SSM2014 manufactured

by Solid State Microtechnology (SSM).^11 SSM was

acquired by Precision Monolithics, Inc, which was itself

acquired by Analog Devices, who currently offers a

Frey OVCE gain cell branded the SSM2018T. Frey’s

original patents, U.S. 4,471,320 and U.S. 4,560,947,

built upon the work of David Baskind and Harvey

Rubens (see U.S. Patent 4,155,047) by adding correc-

tive feedback around the gain cell core.12,13,14 Fig.

12-42 shows a block diagram of the SSM2018T VCA.

The OVCE is unique in that it has two outputs: VG

and V1-G. As the VG output increases gain with respect

to control voltage, the V1-G output attenuates. The result

is that the audio signal pans from one output to the other

as the control voltage is changed.

The following expressions show how this circuit

works mathematically:

(12-59)

and

Vout 1 =VG

= 2 KVu in

Figure 12-42. A block diagram of the SSM2018T VCA. Courtesy Analog Devices, Inc.