dbx Model 3BX

Introduction

Why Use An Expander

Many music lovers and audiophiles notice that recorded or broadcast performances often lack the excitement of live events. This is primarily due to the dynamic range of the performance being restricted to fit the limitations of recording and broadcast mediums. The dbx 3BX is a sophisticated, three-band expander designed to restore this dynamic range and excitement to recordings or radio broadcasts, significantly enhancing the listening experience. By expanding dynamic range, the 3BX reduces the characteristic noise levels of tapes, records, or FM broadcasts, restores the impact of loud passages, and enhances quiet passages. It can revitalize old record collections and make FM broadcasts more enjoyable. When used with a dbx tape noise reduction system, it allows for tape copies that surpass the original quality.

Dynamic Range

Dynamic range is the difference in level, measured in decibels (dB), between the loudest and quietest parts of a program. For recordings, it's typically defined as the difference between the loudest parts and the noise level, as quiet passages are often limited by noise.

Restricting Dynamic Range

Live performances can reach up to 120 dB SPL. Quiet sounds, however, are often masked by ambient room noise (around 30 dB SPL in a quiet auditorium). This leaves a usable dynamic range of about 90 dB for live performances. Recording studios, with less ambient noise, can achieve over 100 dB. Recorded programs, however, are intentionally restricted to fit medium limitations. Studio-quality tape recorders offer about 60 dB of dynamic range, limited by tape noise and saturation. Home recorders, especially cassettes, have even less, often only 45 dB. dbx tape noise reduction systems can nearly double the dynamic range capabilities of tape recorders.

Figure 3: Dynamic Range Limitations of the Tape Recording Process
This diagram illustrates how the dynamic range of a program is limited during tape recording. It shows an input signal with 100dB of dynamic range being compressed into a 50dB usable dynamic range "window." This limitation is due to factors like tape saturation (limiting the loudest sounds) and the tape's noise floor (limiting the quietest sounds). The waveform shown is an "envelope" representing volume changes, not a pure sine wave.

How the 3BX and Other Expanders Work

Expander Function

An expander increases the level of loud musical passages and decreases the level of quiet musical passages, effectively stretching the dynamic range. The EXPANSION RATIO defines how much the dynamic range is increased (e.g., 1:1.4 means a 1.0dB input change results in a 1.4dB output change). For example, an expander with a 1:1.4 ratio can expand a 60dB input dynamic range to 84dB.

Transition Level (Threshold)

This control determines the signal level at which the expander begins to increase or decrease program levels. When the signal is above the threshold, the expander increases its level; below the threshold, it decreases it.

Detection Methods

Expanders use different methods to detect signal levels:

• Peak Detection: Senses musical peaks, but can react erratically to noise spikes or transients.
• Average Detection: Senses the average signal level, which is more stable but may respond too slowly to rapid changes.
• RMS Detection: The 3BX uses RMS (Root-Mean-Square) detection, which accurately reflects how the human ear perceives sound levels. This method avoids over-reaction to transients and noise spikes, providing natural-sounding expansion.

Attack and Release Times

These parameters control how quickly the expander reacts to changes in signal level (attack) and returns to normal (release). The 3BX features program-controlled, automatic attack and release times that adapt to the music's envelope for a smooth, natural sound, unlike fixed-time units.

Single vs. Multi-Band Expansion

Single-band expanders affect the entire frequency spectrum at once. Multi-band expanders, like the 3BX (which uses three bands), can control expansion for different frequency ranges separately. This multi-band approach avoids issues like "breathing" (audible noise changes) that can occur with single-band units on programs with strong low-frequency content.

The 3BX Advantage

The dbx 3BX utilizes three-band linear dB expansion, RMS detection, and program-controlled attack/release rates. It avoids the coloration of "weighted" expanders, the "breathing" of single-band units, and the "noise modulation" of systems dividing the spectrum into many bands. It enhances dynamics by adjusting gain during quiet and loud passages, achieving natural-sounding expansion without subtracting frequency components.

Figure 7: Linear Expansion
This graph demonstrates linear expansion, showing how it operates across the entire dynamic spectrum. It illustrates that loud passages become louder and quiet passages quieter, reducing audible noise. The diagram indicates that expansion ratios are adjustable, with examples like 1.0 for 0% increase, 1.2 for 20% increase, and 1.5 for 50% increase in dynamic range.

Connections

AC Power

Connect the Model 3BX to a 117V AC, 50 or 60Hz power source only. Models for other power sources are available. The unit requires a maximum of 30 watts. Connect the AC power cable only after all signal connections are made.

Signal Connections

The 3BX connects to your audio system via various configurations, typically involving a preamplifier, integrated amplifier, or receiver. The rear panel features INPUTS, RECORDER OUTPUTS, RECORDER INPUTS, and a REMOTE CONNECTOR.

Connection Diagrams

Several diagrams illustrate connection methods:

• Figure 8: Connecting the 3BX with a Receiver/Preamplifier/Integrated Amplifier that has a Tape Monitor Loop.
• Figure 9: Connecting the 3BX when the amplifier lacks a Tape Monitor Loop but has PREAMP-OUT/MAIN-AMP-IN jacks.
• Figure 10: Connecting the 3BX with a separate Preamplifier and Power Amplifier, without a Tape Monitor Loop.
• Figure 11: Connecting the 3BX with an Equalizer or Speaker Equalizer that has a Tape Monitor Loop.
• Figure 12: Combining the 3BX with a dbx Tape Noise Reduction System.

Notes for Connections: Do not connect tape recorder jacks if no tape recorder is used. When using PREAMP-OUT/MAIN-AMP-IN or between preamp/power amp, set tone controls to "flat" when recording. If using a dbx tape noise reduction system, refer to page 16.

Operation

Front Panel Controls

POWER: Push once to turn ON, push again for OFF.

REMOTE: Push once (IN) to activate the optional remote control unit and disable front panel controls. Push again (OUT) to reactivate front panel controls. If no remote unit is used, this switch acts as a bypass: OUT for normal operation, IN to bypass.

REMOTE INDICATOR [LED]: Glows when the REMOTE CONTROL UNIT is active.

PILOT [LED]: Glows when the 3BX is ON.

EXPANSION: Slide control for desired expansion degree (e.g., 1.4 for 40% increase in dynamics).

GAIN CHANGE [LEDs]: Indicate the amount of gain change in each of the three frequency bands. Red LEDs indicate upward expansion (volume increase), yellow LEDs indicate downward expansion (volume decrease).

TRANSITION LEVEL: Adjusts the level at which the 3BX expands upward or downward. Set so red LEDs glow on loud passages and yellow LEDs glow on quiet passages. This adjustment primarily controls LED activity.

SOURCE: Push to expand programs from sources like FM tuners or phonographs.

TAPE: Push to expand programs from tape recorders.

PRE: Push to expand a program before recording.

POST: Push with TAPE to expand a program played back from a tape recorder.

Summary: PRE + SOURCE expands before recording; TAPE + POST expands playback. Pushing PRE and POST simultaneously bypasses expander functions.

Remote Connector

This connector is for the optional remote control unit, allowing independent control of expansion ratio and transition level for each of the three frequency bands from a distance. If the remote unit is not used, do not plug anything into this jack.

Remote Switch Function

When no remote control unit is connected, the REMOTE switch acts as a bypass. In the OUT position, the 3BX operates normally. In the IN position, the 3BX is bypassed.

Expansion: FM Broadcast or Conventional Phonograph Disc

1. Turn system power on with amplifier volume down.
2. Select source (FM or Disc).
3. Press SOURCE and PRE buttons on the 3BX.
4. Set EXPANSION RATIO and TRANSITION LEVEL controls to mid-position.
5. Adjust TRANSITION LEVEL while music plays: red GAIN CHANGE LEDs should glow on loud passages, yellow LEDs on quiet passages. This adjustment is not critical.
6. Slowly increase amplifier volume to the desired listening level.
7. Readjust EXPANSION control for desired expansion (e.g., 1.1-1.2 for classical, 1.4-1.5 for compressed FM). Adjust until the sound is natural.

To Expand a Tape During Playback

Follow the same steps as above, but press TAPE and POST buttons instead of SOURCE and PRE. If a tape was already expanded during recording, re-expansion during playback is not necessary.

How to Expand and Tape Copy a Program

Note: Expanding a program before recording might exceed tape recorder dynamic range, potentially causing distortion or noise. dbx recommends using a dbx tape noise reduction system in conjunction with the 3BX for this purpose.

1. Set recorder to "record ready" (RECORD and PAUSE) with volume controls down.
2. Select desired source.
3. Press PRE and SOURCE on the 3BX.
4. Play source, set TRANSITION LEVEL (red LEDs for loud, yellow for quiet), and set EXPANSION control.
5. Bring up amplifier volume.
6. Adjust tape recorder input levels for normal VU meter readings (0 VU for reel-to-reel, -3 VU for cassette).
7. Record normally. If tape monitoring is used, press the TAPE switch on the 3BX.

Combining 3BX Expansion with dbx Tape Noise Reduction

This combination can yield dramatic results, allowing high-quality tape copies of discs or broadcasts with enhanced dynamic range and reduced noise. Connect as shown in Figure 12 and follow the "How to Expand and Tape Copy a Program" instructions.

Simple Precautions When Using Any Program Expander

The 3BX places greater demands on power amplifiers and speakers. Ensure your system can handle wide dynamic range to avoid clipping distortion. Use good speakers and a reasonably large amplifier. If distortion occurs, reduce the expansion ratio. Like any powerful tool, the expander should be used properly to achieve optimal listening enjoyment.

Specifications

*SMPTE stands for the Society of Motion Picture and Television Engineers.

Specifications subject to change without notice or obligation.

dbx Product Warranty

All dbx products are covered by a Limited Warranty. Consult your warranty card or local dealer for full details.

Factory Service

The dbx Customer Service Department is available for assistance with product use, interfacing, and special applications. Contact them by phone at 617-964-3210 or by mail:

dbx, Inc.
71 Chapel Street
Newton, MA 02195
Attn: Customer Service Department

For factory servicing:

1. Repack the unit with a note describing the problem and purchase date.
2. Send freight prepaid to: dbx, Inc., 224 Calvary Street, Waltham, MA 02154, Attn: Repair Department.
3. Insure the package and use UPS where possible.
4. Direct all inquiries to the dbx Customer Service Department.

Outside the United States, contact your nearest dbx dealer for authorized repair center information.

Schematic

A schematic diagram of the dbx Model 3BX is included in this manual for technical reference.

Glossary

Asperity Noise

A swishing background noise on tape recordings caused by minute imperfections in the tape's magnetic coating, especially noticeable when low-frequency signals are present and high-frequency signals are absent.

Attack Time

The time it takes for a note to reach its full volume (in music) or the interval during which an expander/compressor changes its gain from initial to 63% of final value.

Aux Input (Aux Level)

Low-sensitivity jacks for preamplified signals, typically used for tape outputs, tuner outputs, or dbx play outputs. They are less susceptible to hum than microphone inputs.

Bandwidth

The range of frequencies between specified upper and lower limits, or the absolute value of that range.

Bass

The low audio frequency range, generally below 500Hz.

Bass Boost

Accentuation of lower audio frequencies, making them louder.

Biamplified

A sound system using a crossover network to divide the audio signal into ranges fed to separate amplifiers for different speakers (e.g., woofers and tweeters).

Bias

A high-frequency signal mixed with the program during tape recording to achieve linear magnetization of the tape.

Clipping

Distortion occurring when an amplifier's output capabilities are exceeded, resulting in a flattened waveform.

Clipping Level

The signal level at which clipping begins to occur.

Compression

A process that reduces the dynamic range of program material by decreasing the level of loud passages and increasing quiet ones.

Compressor

An amplifier that decreases its gain as input signal level increases, reducing dynamic range.

Crossover Frequency

The frequency at which an audio spectrum is divided for distribution to different speakers or amplifiers.

Crossover Network

A circuit that divides the audio spectrum into frequency bands for speakers or amplifiers.

Damping Factor

The ratio of loudspeaker impedance to amplifier output impedance, indicating the amplifier's ability to control speaker movement.

DB (Decibel)

A unit used to express ratios of power, voltage, or sound pressure levels. It is the smallest change in loudness the human ear can detect.

Decay Time

The time it takes for a compressor/expander's circuitry to return to normal gain after a signal change.

Decoder

A circuit that restores an original program from a specially treated (encoded) version.

De-emphasis & Pre-emphasis

Processes used to reduce audio noise. Pre-emphasis boosts high frequencies during encoding, and de-emphasis attenuates them during decoding to counteract noise.

Dynamic Range

The range of signal levels in a program, from the softest to the loudest, expressed in dB. In equipment, it's the difference between the noise floor and the maximum undistorted signal level.

Encoder

A circuit that processes an original program to create a specially treated (encoded) version.

Envelope

Describes the change in average signal level of a musical note over time (attack, peak, decay, sustain, release).

EQ (Equalization)

Intentional change in a circuit's frequency response, used for boosting or cutting specific frequencies.

Expander

An amplifier that increases its gain as input signal level increases, stretching the dynamic range.

Expansion

The process of increasing a program's dynamic range, often used to restore range lost to compression.

Fundamental

The basic frequency of a musical note, around which harmonics are built.

Ground Compensated Output

An output circuit that corrects for potential differences between the unit's ground and the connected device's ground to reduce hum and noise.

Harmonic Distortion

Signal components at the output that are whole-number multiples of the input signal frequency, expressed as T.H.D.

Harmonics

Overtones that are integral multiples of the fundamental frequency.

Headroom

The "space" in dB between the nominal operating signal level and the maximum signal level before distortion.

I.M. (Intermodulation Distortion)

Signal components at the output resulting from interaction between two or more input frequencies, not harmonically related to the input.

Impulse Response

A circuit's ability to respond to sharp sounds or transients.

Level Match

The signal level at which an encoder/decoder passes the signal without change. In dbx systems, this is a 1dB "window" for transition.

Limiter

A compressor with a high compression ratio (10:1 or greater) that sets a fixed "ceiling" on maximum signal level.

Line Level (Line Input)

A preamplified audio signal level, typically higher than mic level.

Modulation Noise

Swishing noise on tape recordings related to the recorded signal level, which can become audible when hiss is reduced.

Octave

An interval between two frequencies with a 2:1 ratio.

Overshoot

An expander/compressor circuit's tendency to over-react to signal level changes, causing audible distortion.

Peak Level

The maximum instantaneous voltage or level reached by an audio signal.

Phase Shift

A delay of certain frequencies within an audio program relative to others, which can affect sound quality.

Power Amplifier

An amplifier that boosts a medium-level signal to drive loudspeakers.

Preamplifier

A device that amplifies small or medium signals and routes them to a power amplifier, often including tone and volume controls.

Receiver

A unit combining tuner, preamp, and power amplifier sections.

Rise Time (Attack Time)

The time it takes for a circuit to respond to a sudden increase in signal level.

RMS Level

A measurement of signal level derived from the Root-Mean-Square of instantaneous voltages, reflecting perceived loudness.

Sub Harmonic

A frequency that is a sub-multiple of the fundamental frequency.

Sub Woofer

A loudspeaker designed to reproduce very low audio frequencies (20Hz-100Hz).

Synthesizer

An electronic device that generates and modifies sounds, often used for creating unique musical effects.

Tape Saturation

The point at which magnetic tape can no longer accept more signal energy, leading to distortion.

T.H.D. (Total Harmonic Distortion)

The total power of all harmonic components generated by a circuit, expressed as a percentage of the total output power.

Threshold

The signal level at which a compressor or limiter begins to change gain.

Tracking Accuracy

The degree to which one circuit's output levels match another's, or how accurately a noise reduction system's encoded/decoded signals match the original.

Transition Level

The signal level at which a circuit passes through without gain change (unity gain), similar to a threshold but defining a range.

Triamplified

A sound system using three amplifiers for bass, mid, and high frequencies, fed by a crossover network.

Tuner

A unit that receives radio broadcasts and converts them into audio signals.

VCA (Voltage Controlled Amplifier)

An amplifier whose gain is adjusted by an external DC control voltage, allowing for wide dynamic range control.

Woofer

A loudspeaker that reproduces low frequencies.