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Design Notes

THAT Corporation design notes contain detailed information on how to use our products in specific real-world circuits. The design notes are provided in Adobe' s PDF format.
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THAT Offers Alternatives to the

Analog Devices recently discontinued their SSM2164 quad VCA. To ensure your production doesn’t miss a beat, we have an alternative that is worth considering, our THAT2162 dual Blackmer VCA. Some of the design issues which you will need to consider are addressed here.
Design Note 00A - 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. (formerly named Application Note 100A).
Note: The THAT2252 is no longer available. Consider a THAT Analog Engine instead.

Design Note 01A - 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. (formerly Application Note 101A)
. Note: The THAT2252 is no longer available. Consider a THAT Analog Engine instead.

Design Note 02  - Digital Gain Control with Analog VCAs (153k)
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). (formerly Application Note 102)

Design Note 03 - 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. (formerly Application Note 103)

Design Note 04 - 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. (formerly Application Note 104)

Design Note 05 - 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. (formerly Application Note 105)

Design Note 100  - A Fully Adjustable Noise Gate (60k)
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 (56k)
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 (82k)
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 (69k)
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 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 (146k)
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 (84k)
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 320 Transistor Array (97k)
The high-quality microphone preamp presented in the Design Note 109 uses a THAT320 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 (63k)
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 (57k)
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 (46k)
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, ½ 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 (53k)
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 (63k)
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 (124k)
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 (108k)
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.
Note: The THAT2252 is no longer available. Consider a THAT Analog Engine instead.

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 (126k)
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 (89k)
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 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 (106k)
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 (265k)
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 (195k)
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 (70k)
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 (81k)
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 (248k) 
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.

Design Note 140 - Input & Output Circuits for THAT Preamplifier ICs (244k)
DN140 describes practical input and output circuits for THAT microphone preamplifier ICs which satisfy many of the challenging requirements for these devices including low noise performance with low source impedances, high signal-handling capability, high radio-frequency (RF) immunity, high common-mode signal rejection, and variable differential gain over a range of 1 to 1,000.
For owners of printed versions of THAT Corporation Application Notebooks Volumes 1 & 2 (now out of print)
Design Notes DN100 through DN133 were previously printed, and distributed in our Application Notebooks Volume 1 and 2. The original printing of these documents unfortunately included a few errors. The errors are corrected in the PDF versions of the design notes found here on the web site. Furthermore, the above design notes contain other, more current revisions not included in the errata below.
For the latest information please refer to the above design notes. That said, if you would still like to see a list of the specific corrections to the printed notebooks, please download the below errata sheets.

Errata Sheet V1 - Corrections to Application Notebook Volume 1 (38k)

Errata Sheet V2 - Corrections to Application Notebook Volume 2 (40k)


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