The basic principle behind the dbx expander range was to increase the difference between loud and quiet parts of a programme. Instead of simply turning the whole signal up or down, these units altered gain according to the incoming signal level. Louder passages could be made louder, while quieter passages could be reduced in level, giving the impression of increased dynamic range.
The dbx Model 1BX and Model 2BX both offered expansion up to a ratio of 1:1.5. At maximum expansion, a 10dB change at the input became a 15dB change at the output. This was not a dramatic special effect when used properly. It was intended as a subtle corrective process for records, tapes and broadcasts that had been compressed or limited before reaching the listener.
The dbx Model 128 went further by combining dynamic range enhancement with a full dbx noise reduction system. Its compressor and expander section could operate from full limiting, through normal linear operation, into expansion as steep as 2.0. This meant it could be used not only to expand playback material, but also as part of a tape recording system where compression before recording and expansion on playback could provide a substantial reduction in tape noise. We look at a few models from the late 1970s to evaluate their performance.
The dbx Model 1BX
The dbx Model 1BX was the simplest of the three units, but it was also one of the most convincing examples of the concept. It was a single-band stereo dynamic range expander designed to be inserted into a hi-fi system through the tape monitor circuit. It included its own tape recording inputs and outputs, effectively replacing the amplifier’s normal tape monitor switching when the unit was in use.
The Model 1BX used two main slider controls. The Expansion control adjusted the slope from 1.0, meaning no expansion, to 1.5, meaning that a 10dB input change became a 15dB output change. The Transition Level control set the operating point of the unit so the expander could be matched to the level of the programme material.
A row of ten LEDs showed the unit’s activity. Yellow LEDs indicated gain below unity, while red LEDs indicated gain above unity. In normal use, the Transition Level slider was set so average programme material sat around the centre of the display, with peaks moving into the red LEDs and quieter passages lighting the yellow LEDs.
One of the strengths of the Model 1BX was its constant-slope operation. Rather than using a fixed threshold above which expansion suddenly began, the unit applied its expansion continuously. This made its action much less obvious and helped avoid the abrupt side effects sometimes associated with simpler expanders.
The Model 1BX also included PRE and POST operation for tape use. In PRE mode, expansion could be applied before recording, although this needed care because too much expansion could exceed the dynamic limits of the recorder, particularly with cassette decks. In POST mode, the unit processed playback from the recorder.
For stereo operation, the Model 1BX used common control sensing for both channels, helping to prevent the stereo image from shifting as the gain changed. A Quad Coupler jack also allowed two units to be linked for quadraphonic systems so all four channels could track together.
The dbx Model 2BX
The dbx Model 2BX developed the same idea into a two-band stereo expander. Instead of applying one control process across the entire audio spectrum, it divided the signal into two frequency bands and expanded each band independently. This gave it an important practical advantage over a single-band expander.
With a single-band unit, a strong bass note or low drum hit can affect the gain of the entire signal. That means vocals, guitars, keyboards or reverberation in the mid and high frequencies may rise or fall in response to low-frequency energy. The two-band design of the Model 2BX reduced that problem by allowing the lower frequencies and the rest of the spectrum to be treated separately.
The front panel retained the same basic operating logic, with power switching, an Expansion slider and a Transition Level slider. The Model 2BX added two rows of LEDs to show gain change in each of its two frequency bands. Red LEDs showed upward expansion and yellow LEDs showed downward expansion.
The Model 2BX used RMS detection, meaning it responded to the root-mean-square value of the input signal rather than simply reacting to peaks. This was intended to follow musical level changes accurately without overreacting to brief transients or noise spikes. Like the Model 1BX, it used a common control voltage derived from the summed left and right inputs so stereo image stability was preserved.
Bench testing confirmed the intended expansion behaviour. With the unit set to maximum expansion, 10dB input steps became 15dB output steps. The tests also showed that upward and downward changes did not need to have identical attack and release behaviour. This was important because musical signals do not behave like test tones. The Model 2BX allowed attack and release times to follow the rate of change of the programme envelope, with different scaling in each of its two frequency bands.
In practice, this made the Model 2BX more natural than a single-band expander on complex music. Material with strong percussion, bass movement, vocals and mid-range instrumentation could be expanded without one part of the spectrum constantly driving the behaviour of the whole signal.
The dbx Model 128
The dbx Model 128 was a broader and more versatile processor. It combined a dynamic range enhancer with a dbx II noise reduction system, allowing it to be used for playback enhancement, tape recording noise reduction and dbx-encoded disc playback.
Its dynamic range enhancer used an adjustable compressor and expander with a single slope control. The control could move from heavy compression or limiting, through neutral 1.0 operation, to expansion as steep as 2.0. At the 2.0 expansion setting, a 10dB input increase produced a 20dB output increase. In compression, the process worked in the opposite direction.
The Model 128 could operate in linear mode across the full dynamic range of the programme or in threshold mode, where processing occurred in relation to a preset level. In threshold mode, LEDs showed whether the signal was above or below the selected threshold. Linear expansion could restore some dynamic movement to compressed programme material and reduce the audibility of hiss and hum by lowering gain during quiet passages.
The Model 128’s second major function was tape noise reduction. In this mode, the unit compressed the signal with a 2.0 slope before it reached the recorder, then applied complementary expansion on playback. The aim was to restore the original dynamics while reducing noise added by the recording and playback process.
This was especially relevant for cassette recording. The source material describes cassette recorders as often measuring flat at low recording levels but losing high-frequency response at higher levels. Recording at lower levels could preserve high-frequency transient detail, but tape hiss normally made this impractical. With the dbx process adding around 30dB of effective dynamic range, recording at much lower levels became workable while maintaining a very low hiss level on playback.
The Model 128 also included internal frequency shaping in the signal and control paths. The processed signal sent to the recorder had a shaped response, with complementary shaping applied on playback to restore flat response. Because of this, a dbx-encoded signal could not be decoded correctly by a different type of expander, even if the expansion slope appeared similar.
How they sounded in use
The strongest praise for these dbx units was that, when set correctly, their action could be difficult to hear directly. That was the point. The purpose was not to create an obvious effect, but to restore movement, reduce noise and make the programme sound less constrained.
The Model 1BX was especially noted for being easy to use and difficult to make sound obviously wrong in normal operation. Full expansion was rarely needed, because most recorded music did not require that much correction. Used more moderately, the Model 1BX could reduce background noise during quiet passages without obvious pumping, breathing or chopped reverberation tails.
The Model 2BX improved on that behaviour by separating the spectrum into two bands. Its advantage became clear with music containing strong low-frequency energy. By preventing bass content from controlling the expansion of the full signal, the Model 2BX could handle complex arrangements more naturally than a single-band design.
The Model 128 required more care when used as a dynamic range enhancer. With too much expansion, modulation noise and unnatural effects could appear, particularly on transient sounds such as piano or solo instruments against quiet backgrounds. The practical answer was to use less expansion. Moderate settings around 1.2 or 1.3 were described as more useful than extreme settings.
As a tape noise reduction system, however, the Model 128 was much more than a simple expander. Its record and playback compression-expansion process could remove much of the audible limitation of cassette recording by allowing lower recording levels without the usual penalty of hiss.
Why multi-band expansion mattered
The move from the Model 1BX to the Model 2BX shows why multi-band expansion was such an important development. A full-range expander has to make decisions from the total signal. A loud bass event, a kick drum or a low-frequency roll can make the entire programme change gain, even when the mid-range and treble content should not be treated in the same way.
By splitting the signal into separate bands, the Model 2BX made expansion more selective. Bass could be expanded according to bass content, while the rest of the programme could respond to its own level changes. This made the process more natural, particularly on music with wide dynamics and dense arrangements.
The source material also references the dbx Model 3BX as a three-band unit, with the view that multiple-band expanders offered the best way to gain the benefits of dynamic expansion while reducing intrusive pumping and breathing. The Model 2BX was presented as a lower-cost route to much of that behaviour.
What they were good for
These dbx units were most useful when the source material already had some real dynamic range and a relatively low noise floor. A good record, a clean FM broadcast or a well-made tape could be made to sound more open and less compressed. Quiet background hiss could be reduced or even disappear during low-level passages.
They were less effective on already heavily compressed material. If a rock record or FM broadcast had only a narrow dynamic range to begin with, expanding it by 50 percent could not restore dynamics that were not present in the source. A 10dB programme range becoming 15dB was technically real, but not necessarily transformative.
They were also limited by noise. If a source had obvious hiss or hum, the listener could hear that noise rise and fall as the expander changed gain. On clean material, the result could be impressive. On noisy material, expansion had to be used carefully or not at all.