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THAT Corporation Design Notes contain detailed information on how to use our products
in specific real-world circuits.
In contrast, our
Application Notes tend to deal with our products in greater depth
and from a more theoretical viewpoint.
Application
and Design Notes are
provided
in Adobe' s PDF format. You need to have Adobe
Acrobat Reader installed on your computer to view or print the file. To get a free copy of Acrobat Reader follow this link.
Application Note 100A
Basic
Compressor/Limiter Design
(113k)
THAT Corporation's
2252 RMS Level Detector and 2180/2181 Series
Voltage-Controlled Amplifiers (VCAs) are ideal basic
building blocks for compressor / limiter designs. This
application note describes in detail the circuitry for
two basic compressor / limiter designs using these
devices.
Application Note 101A The Mathematics of Log-Based Dynamic Processors
(64k)
Dynamic Processors
(compressors, expanders, gates, etc.) may be easily
constructed from THAT Corporation Voltage-Controlled
Amplifiers (VCAs) and RMS-Level Detectors. The detector
is used to sense signal level and control the gain of the
VCA to produce compression or expansion. Both feed-back
and feed-forward configurations are possible. This application note offers insights into
circuit designs based on these
log-responding components.
Application
Note 102 Digital Gain Control
with Analog VCAs
(106k)
In many cases, a fully
analog signal path provides the least compromise to sonic
integrity, and ultimately delivers the best sounding
audio products at the lowest cost. What is often needed,
however, are methods for varying the gain and other
parameters of the analog circuitry, under digital
control.
While several methods are available to the designer, the
best combination of sound quality, control range, and
control resolution can be obtained from two easily
combined building blocks -- a high performance,
exponential-control, Voltage Controlled Amplifier (VCA),
and a low-cost Digital-to-Analog Converter (DAC).
Application
Note 103 Signal Limiter for
Power Amplifiers
(95k)
Power amplifiers, when
driven out of their linear range of operation, sound
particularly bad, and can produce damage to themselves or
the transducers to which they are connected.
The design of traditional protection circuits is
complicated by the various performance, cost, and sonic
tradeoffs involved. There is certainly no one right
answer to the limiter puzzle. The circuits presented
here, however, are designed to maintain a high level of
sonic integrity, while remaining cost-effective.
Application Note 104
Improving
Loudspeaker Signal Handling Capability
(217k)
One advantage of a powered speaker is that the amplifier
always drives a known load. Designers can further exploit
the opportunity this presents by providing some
"intelligent" control of the signal driving the
load to further optimize performance and headroom.
This application note describes a method for increasing
perceived headroom by compressing bass frequencies above
some threshold signal level. If performed judiciously,
the headroom of the speaker system can be extended
without noticeable degradation in sonic performance.
Application Note 105
A Low Parts Count,
Two-Slope
Compressor (78k)
Compressing the dynamic range of an
audio signal can be of significant benefit in certain play-back
applications. In multimedia sound systems, for example, compressing
the subwoofer signal can lend added richness to the sound,
particularly at lower signal levels. With automotive sound systems,
compression can raise low-level signals above the relatively high
ambient noise level. Speaker protection is also important, and a
dynamic range limiter is an essential component of any system that
attempts to handle overloads gracefully (inaudibly). This paper
describes a combination compressor and limiter, with adjustable
threshold and compression ratio, for just these types of applications.
Design Note 100
A Fully Adjustable Noise Gate(69k)
Gates are useful for
suppressing background noise in the absence of masking source
material, but for a gate to sound “natural”, it can be desirable
to control one or more of the following parameters: the threshold
below which the gate acts; the hold time which prevents gating action
during brief pauses in the source material; the release rate during
which the gain is smoothly “faded” down. The circuit shown in
DN100 is a feedforward design with independent, variable control of
each gating parameter.
Design Note 101 Peak
Detection with the THAT4301
(37k)
The circuit described in DN101 shows the level detector of
a THAT4301 configured as a peak detector. The detector, which normally
responds in true rms fashion, is re-configured for peak operation by
making C1, the timing capacitor, quite small, thereby disabling the
logarithmic filtering.
Design Note 102
Adjustable
Ducker using the THAT4301
(76k)
Duckers are usually used to reduce the level of the main
program material during announcements. In this circuit, while the
"voice over" signal is below threshold, the main program
passes through the VCA at a fixed gain, determined by the position of
a potentiometer. The RMS detector, which senses the level of the
"voice over" signal, is set for a zero dB reference level of
-10dBu.
Design Note 103 / 104 Improving
VCA Performance I
(101k)
Say for
instance that we want to achieve 100 dB of off isolation with a VCA
circuit. The VCA specifications seem to indicate that this should be
possible, but lab measurements show poorer isolation, particularly
with increasing frequency. DN103/104 discusses how to improve
isolation by minimizing the parasitic capacitance. Though this discussion is specifically about off isolation,
the conclusions can be applied to other signals coupled into the
summing node of the VCA's output trans-impedance amplifier.
Design Note 105
Low-Voltage
Compressor / Limiter
(100k)
Design Note 105 details the circuit for a
compressor / limiter adapted to run off +5 volts and -4 volts. The
remainder of the circuit is a standard compressor / limiter with some
minor enhancements.
Design Note 106 What to look for when
the distortion in 2180s doesn't match the specs
(57k)
All too often, we get frantic calls from
designers measuring THD and/or noise well above the specifications on
the 2180XX and 2181XX data sheets, wondering if layout is a factor.
Well, sometimes it is, but it could be the result of other factors as
well. All 2180s and 2181s are 100% tested for noise and THD+N, in
addition to a number of other parameters. Their specifications are
conservative, and most should perform substantially better than
"worst case".
Design Note 107 / 111
A Simple Effective
Soft-Knee Compressor / Limiter
(113k)
The schematic shown in Design
Note 107/111 is a basic soft-knee compressor/limiter circuit that can
be used as-is or as the basis for a more sophisticated design. The
circuit consists of a THAT2180C VCA, a THAT2252 RMS level detector, a
few op-amps, and a handful of passive components. An alternative
design using a single THAT4301 (comprised of a VCA, RMS-Detector, and
three general purpose op amps) and some passive components is also
diagrammed.
Design Note 108
Single Chip Automatic
Gain Control (73k)
AGCs, as a rule, maintain a
constant output signal level while the input signal level varies. This
circuit is essentially a limiter operated most of the time above its
threshold. Gain control is accomplished by connecting the output of
the RMS detector to the negative control port of the VCA. In a feed
forward topology, the gain is then reduced by the same amount that the
input level increases, keeping the output level constant.
Design Note 109
Microphone Preamp
using a THAT 120 Transistor Array(87k)
The high-quality microphone
preamp presented in the Design Note 109 uses a THAT120 with two
transistors paralleled on each half of the differential input. Q1A and
Q1B are a general purpose matched pair configured as current sources,
which bias the differential input.
Design Note 110
Improving VCA
Performance II and III (73k)
Design Note 110 addresses two
important elements in designing a superior circuit with a VCA. First
is the problem of noise modulation, the condition whereby a signal
passing through the VCA is multiplied by noise on the control port.
Second is VCA oscillation which can occasionally result when using op
amps with limited gain-bandwith product (like the LF353) which
exhibit inductive output impedance at relatively low frequencies.
Design Note 112
Led Bar-Graph
Compression Indicator
(83k)
The circuit described in Design
Note 112 is a simple schematic for indicating dynamic range
compression with an LED bar-graph. It also illustrates the basics of
using bar-graph indicators.
Design Note 113
THAT4301 Gain
Reduction Indicator
(33k)
This circuit assumes that you
are using the THAT 4301 in a typical compressor / limiter application.
The circuit has an RMS detector, a threshold amplifier, and a control
port buffer nearly identical to the compressor / limiter schematic
shown in our 4301 data sheet. Additionally, the circuit uses a
comparator, ˝ of an LM393, to sense the output of the threshold
amplifier. When this voltage goes below ground, the LM393 switches low
to indicate that the voltage at the control port is changing.
Design Note 114
Adaptive Attack and
Release Rates using THAT RMS Detectors
(48k)
Here is a schematic which shows
a THAT2252 RMS detector incorporating a non-linear capacitor circuit.
Single-band compressor designs must contend with the trade off between
fast detector response, and low frequency distortion. A non-linear
capacitor can make this trade-off unnecessary by acting as a large
timing capacitor for slow moving signals and as a smaller timing
capacitor for fast moving signals.
Design Note 115
Fully Adjustable
Compressor/Limiter
(46k)
This design demonstrates a
full-blown compressor/limiter with a THAT4301 at its heart. Excellent performance is coupled with reduced
parts count and minimal cost. A non-linear capacitor circuit provides low distortion for slow moving signals, but fast action in
the presence of rapidly changing signal levels; Threshold is
switchable from conventional hard-knee response to
"soft knee"; and two timing modes are available -- auto
(with linked attack and release rates) and manual.
Design Note 116
Techniques for Stereo
Volume Control
(140k)
Design Note 116 offers a
variety of schematics using VCAs and RMS detectors to control stereo
volume. Circuits included are: controlling two channels with a single linear volume
control potentiometer; a dual slope volume control with breakpoint; a
volume control with switch selectable sensitivity; RMS detectors
connected for true RMS power summing; and a VCA volume control with
compressor.
Design Note 117
Input Limiter for
ADCs
(120k)
This circuit is composed of an
input attenuator to accommodate both professional and consumer levels,
a THAT2252 level detector with a non-linear timing capacitor for
optimum response, a side-chain, and a THAT2181XA VCA.
Design Note 118
High Performance
Stereo AGC
(90k)
Here we present a good, general
purpose audio AGC which includes: A compressor, compressing at
between 2:1 and 8:1; A limiter that protects signal integrity and
keeps the signal within the systems operational boundaries,
while sounding
natural; and a hold feature that keeps the AGC from raising its
gain, along with the noise floor, during pauses in source material.
Design Note 119
Wide Ranging dB Meter
(177k)
DN119 defines a dB meter which
uses a THAT4301 as a 2:1 feedback compressor effectively doubling the
dynamic range of the THAT4301's RMS detector by compressing the VCA's
120dB dynamic range through the 80 dB dynamic range of the level
detector. This results in a true RMS meter with ~120 dB dynamic range
and an output linear in dB, suitable for all but the most demanding
metering applications.
Design Note 120
VCAs in a Pan
Potentiometer Application
(105k)
The pan potentiometer allows users to steer
an audio signal between two channels and, unlike standard
potentiometers which have a linear taper, or audio potentiometers
which have something like a log taper, the elements in a pan pot will
have something approximating a sin vs. cos taper. The intent is to
yield a constant power sum of the two output channels as the pan
control is swept.
Design Note 121
VCA Symmetry Auto-Trim Circuit
(129k)
This design note presents two
"auto-trim" circuits that will allow VCAs which would
normally require an external symmetry trim (the THAT2181 series) to
achieve low distortion levels with no external adjustment.
Design Note 122
5.1 Channel Volume Control
(67k)
Voltage controlled
amplifiers (VCAs) provide a superior alternative to a six section
ganged potentiometer. The exponential control input of log/anti-log
VCAs is considered by many to be a superior control function in
comparison with a ganged, audio taper potentiometer.
Design Note 123
Operating Log/Anti-log VCAs
Off ±24V Supplies (32k)
This circuit enables a VCA specified for
±18V to operate on ±24V supplies. By taking advantage of the VCA’s
current-mode topology, the signal is passed through the VCA without
any supply-related reduction in the signal’s dynamic range. This
circuit could even be modified to operate off higher voltages using
principles outlined in the design note.
Design Note 124
Interchanging the THAT218x
and the THAT215x Series VCAs (43k)
With very little effort, engineers
can design their products to accept both the 2181x and 215x VCA
series. Doing so provides some semblance of second sourcing, and helps
ensure continued supply of VCAs even when one or the the other series
is in short supply.
Design Note 125
The "One Knob
Squeezer" (90k)
The typical compressor seen in the
pro audio industry can be an intimidating device to many end users. An
alternative to this high level of complexity is a circuit we call the
"One Knob Squeezer". The circuit allows for the continuous
adjustment of gain, threshold and compression ratio with a single
potentiometer.
Design Note 126
Replacing the SSM2120 Level
Detector (77k)
A significant number of designers have
used our VCAs in conjunction with the dual level-detectors available
in the SSM 2120 (having disabled the SSM 2120’s VCAs). Since the
2120 has been discontinued, users need an alternative to this device.
Design Note 127
Upgrading Modular VCAs
(90k)
While the performance of early modular
VCAs is rather lacking by today’s standards, many of the early SSL
and Quad-8 consoles built during that era are still in operation. When
a channel fails, most operators opt to repair the channel strip rather
than replace the entire desk.
Design Note 128
What’s a PTAT Temp.
Coefficient, & Where Does It Come From? (64k)
THAT Corporation’s VCAs and RMS
detectors exhibit temperature coefficients that are proportional to
absolute temperature (PTAT). This fact is occasionally disconcerting
to some users, but is rarely an issue in audio applications. When it
is an issue, it is relatively easy to compensate.
Design Note 129
A Low-Cost VCA Limiter
(47k)
THAT Corporation’s VCAs and RMS
Detectors allow the design a variety of compressors and expanders, but
are often the two most expensive components in a given dynamics
processor. A common low cost application for these devices would be in
protecting sub-woofers, and here we demonstrate a technique for doing
so in a cost effective manner.
Design Note 130
VCA-Controlled 1st Order State
Variable Filter (174k)
VCAs are widely used throughout the audio
signal chain to control amplitude, especially where dynamic gain
control is desired. A less well known application of these devices is
in dynamically controllable filters. VCAs ease the design of these
circuits, and avoid the ”zipper noise” which plagues circuits
using stepped attenuators.
Design Note 131
Dealing with the PTAT
Coefficient (75k)
The nature and origin of the PTAT
coefficient in THAT Corporation’s VCAs and RMS detectors was
previously discussed in Design Note 128. Here we present specific
circuits designs to deal with the PTAT coefficient.
Design Note 132
Alternate Method of Indicating
Compression (52k)
A low-cost method of indicating an
above-threshold condition with an LED was discussed in Design Note
113. This design note presents three alternative approaches using an
bi-color LED which displays green for the under-threshold condition
and red for above-threshold.
Design Note 133
Achieving Optimum CMRR with
Differential Input A/D Converters (54k)
Unbalanced source impedances are a widely
recognized source of degraded common mode rejection performance, the
THAT InGenius® input stage described in this design note reduces
these effects and provides better performance in terms of price,
specifications, and board space, for most applications.
Design Note 137
Substituting THAT 218x VCAs for 215x VCAs in Existing Designs(68k)
THAT Corporation’s 2180- and
2181-series VCAs are pin-for-pin compatible, improved performance
replacements for the 2150-series VCAs. Designers may convert existing
2150-series designs to use the 2180/2181 VCAs without making any
changes to existing PCB layouts.
Design Note 138
Configuring gain with the THAT
1510 and 1512
(271k)
The 1510 and 1512 are low
noise, wide bandwidth microphone preamplifiers available in several
different industry-standard packages. They allow designers to upgrade
existing designs to take advantage of the superior performance of
these new ICs. This design note offers some guidance on gain control
to designers who are considering the 1510 or 1512, first for new
designs, but also for replacements in existing circuits.
Errata Sheet V1 - Corrections
to Application Notebook Volume 1 (28k)
Errata Sheet V2 - Corrections
to Application Notebook Volume 2 (40k)
Design Notes DN100 through DN133 were previously compiled, printed,
and distributed in our Application Notebooks Volume 1 and 2. It should
be noted that original printing of these documents included a few
errors. These errors have been corrected in the PDF versions of the
design notes found here on the web site. However, if you would like to
see a list of the specific corrections, please download the above
errata sheets.
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