dbx 4BX Dynamic-range Expander Review

The dbx Model 4BX three-band dynamic-range expander features an “impact restoration” circuit and a wireless (infrared) remote control that duplicates all of its expansion controls and most of the front-panel function switches. The 4BX is normally connected to the tape input and output jacks of the system amplifier, and it duplicates their functions on its rear apron. It also has front-panel controls to expand a signal either before it is recorded or after it is played back by a tape deck connected to its jacks.

A dynamic expander is essentially a variable-gain amplifier whose amplification is proportional to and controlled by the amplitude of the incoming signal. Since virtually every recorded or broadcast signal has undergone some compression (quieting the loud parts while making the quiet parts louder), a well-designed expander can help restore a more natural and open quality to the reproduced sound. But since there is no way for the listener to know what compression characteristic was used, an expander must be adjusted by ear for the most pleasing results.

A single-band expander, in which the gain varies uniformly for all frequencies in accordance with the total signal level, often creates audible side effects. A strong bass signal, for instance, can unnaturally vary the levels of higher-frequency signals, and the dynamic-gain variations sometimes modulate the background noise and create audible swishing sounds or pumping effects. In the 4BX, the gain in each of its three frequency bands (below 150 Hz, from 150 to 6,000 Hz, and above 6,000 Hz) is separately determined by the signal level in that band. Combined with a careful selection of the operating time constants (which control the rates at which expansion is applied and released), the use of multiple bands essentially eliminates the usual audible indications that an expander is being used.

The dbx 4BX is turned on by a momentary pressure on its power on/mute button. Initially, the volume setting at turnon is —40 dB (nearly off), and nothing will be heard unless the 4BX’s power on/mute button is pressed again. This increases the volume to — 15 dB or whatever other level it was at before the unit was last turned off. The volume is smoothly adjustable thereafter by two buttons (identified by arrows), one to increase and the other to decrease; a column of red LED’s above these buttons indicates volume levels from —30 to +10 at 10-dB intervals. After the unit is turned on, the power on /mute button functions as a muting switch; pressing it alternately changes the gain between the set listening level and the — 40-dB mute level. There is a separate front-panel button for turning the unit off.

Another pair of up/down buttons controls the degree of expansion, which ranges from 10 per cent (an output/input slope of 1.1:1) to 50 per cent (1.5:1). The selected setting is shown by another column of LED’s above the buttons. Similar controls and indicators set the transition level (at which the expander gain is 0 dB) and the amount of impact restoration. The latter feature, unique to the 4BX, is intended to give an additional sonic emphasis, by means of additional expansion, to sudden level changes. This extra expansion is independent of the unit’s normal expansion settings and can be set as high as 12 dB. The effect diminishes, at an adjustable rate, to the steady-state expansion condition.

Three horizontal rows of LED’s monitor the expansion action in the 4BX’s three frequency bands. The gain change indicators are red in the upper part of their range and yellow in the lower part. The red lights show the amount of increase in gain, to a maximum of 12 dB, as the signal rises above the transition level, and the yellow lights show decreases in gain, to a maximum of 20 dB, as the signal level falls below the transition level. When none of the lights in a row is glowing, the expansion gain in that frequency band is unity. A fourth row of red LED’s shows the effects of the impact-restoration circuit.

The remaining buttons are for source and tape selection, pre and post equalization of recorded signals, and bypass. The last routes signals around all the expansion circuits; pressing it lights a red LED and simultaneously extinguishes all of the gain change indicators. Finally, there is a vertical slider control marked display that dims all the LED level indicators, extinguishing them entirely at its lower limit.

The dbx 4BX is about 18 inches wide, 12-1/4 inches deep, and 3-1/2 inches high. It is finished in black with silver panel markings. The remote control, also black, is about 5-1/2 inches long, 2 inches wide, and 1 inch thick; it is powered by a 9-volt battery. Price: $799.

Laboratory Measurements

We were able to confirm most of the specifications of the dbx 4BX. Only the noise level (specified as 90 dB below 1 volt, A-weighted) was beyond the range of our instruments. We measured over the full range of the volume and expansion settings; it was typically — 78 unweighted and well below our —80-dB measurement limit with A-weighting. The amount of expansion was essentially as rated in each band and for the impact-restoration circuit. The transition level could be set well beyond the rated limits of 30 to 300 millivolts in the middle frequency band (we measured a range of 20 to 1,600 millivolts at 1,000 Hz).

The 4BX was tested with a standard IHF load of 10,000 ohms in parallel with a capacitance of 1,000 picofarads. The 1,000-Hz output clipped at 7.2 volts, and the harmonic distortion varied from 0.007 per cent at 0.1 volt output to 0.08 per cent at 5 volts (the rated maximum is 0.15 per cent). The frequency response at the minimum expansion setting (10 per cent) varied less than 0.5 dB overall from 20 to 20,000 Hz (considerably better than the rated ±0.5 dB). As expansion was increased, the response varied only slightly up to about 5,000 Hz (about ±2 to 3 dB at 50 per cent expansion) but rose at higher frequencies to a plateau of +5 to + 10 dB, relative to the lower frequency level, in the octave from 10,000 to 20,000 Hz. The overall level also increased by as much as 10 dB at maximum expansion. The high-frequency transition level is adjustable with a rear-panel control in order to vary the balance between the very high frequencies and the rest of the spectrum. We used the factory setting for our measurements and listening.

To determine the degree of isolation of the three operating bands of the 4BX, we measured the output-vs.-input characteristic at 100 Hz, using maximum expansion, over a 70-dB input range. At each input level we also measured the output-level changes at 3,000 and 10,000 Hz. In the maximum signal condition (0 dB), all the red lights were on in the low-frequency display, indicating a full 12-dB expansion of the 100-Hz signal. From the maximum down to —30 dB (the transition level), the expansion slope was approximately the indicated 1.5. Below —30 dB the slope was 1.0 (a linear transfer characteristic). Neither of the high-frequency signals changed by more than a decibel or so over the full measurement range, which encompassed a change of about 80 dB in the 100-Hz output signal.

The expansion time constant was checked with a 1,000-Hz tone-burst signal. The output burst reached its full amplitude in about 4 milliseconds at the maximum 1.5:1 expansion. Switching in the impact-restoration circuit initially gave about 12.7 dB additional expansion, which was maintained for about 100 milliseconds and then dropped back to its steady-state value over a period of about 0.8 second (the rate of this decay can be adjusted on the rear of the unit).

Comment

Measurements of a dynamic signal processor can verify the manufacturer’s statements concerning its operation parameters but tell us little about its subjective listening effects. To determine this we must use the device in listening tests. We have used a number of dynamic expanders over the years, most of them single-band units. Their overall success in achieving a worthwhile degree of expansion without obvious and undesirable side effects has varied widely, depending largely on the skill with which the designers selected the operating thresholds and time constants. At best, some of these devices were quite good, but usually it was necessary to use relatively little expansion (1.2 or 1.3 at most) to retain a natural quality in the music.

Despite its seeming complexity, the 4BX proved to be very simple to use and adjust. The three-band design seems to be about optimum for minimizing audible hints of expansion; we never detected any pumping or noise modulation even at a full 1.5 expansion. The LED indicators provide a graphic and convincing display of the expander’s operation at all times (and our measurements confirmed that their indications are valid), yet we were never audibly reminded that expansion was being applied. You might ask, why bother with expansion if you cannot hear it? To answer that question, simply press the 4BX’s bypass button. As with virtually every other worthwhile signal-processing system (time delay, image restoration, etc.), if an expander’s audible effect is blatantly obvious in use, then its degree of processing is excessive. Yet the difference in listening pleasure between using it and not using it should be obvious— and with the 4BX it certainly is. When the 4BX is properly adjusted, switching it out presents an immediate and unmistakable loss in musical naturalness.

The impact-restoration system of the 4BX also sets it apart from any of the other expanders we have used. Only rarely did we find even its maximum setting to be excessive, and it did add more “life” to most program material. In common with other expanders, the 4BX is at its best when used in moderation, but it is much more forgiving than most when used to its fullest. Like other expanders, the 4BX is effective both for reducing background noise at low program levels and for increasing peak levels.

Although in most respects the 4BX is easy to use in a music system, there is one annoying aspect of its design. Because the unit “forgets” all previous control settings whenever its power input is completely disconnected, it should not be powered from the switched “convenience” outlets on a preamplifier, integrated amplifier, or receiver. The complete and well-written manual explains the reason for this restriction: sudden level changes of 30 dB or more could be encountered with certain control settings if the expander were turned on and off by a switched outlet. One can, of course, simply use an unswitched outlet or connect the unit separately to a wall outlet, leaving it “powered” at all times and activating or de-activating its functions with the front-panel power on/mute and off buttons.

The dbx 4BX is the most sophisticated dynamic expander for home music systems that we have used. It is also, by a comfortable margin, the best—and the most expensive. Clearly, it is not for everyone, since many users with modest systems would be better advised to invest the same money in better speakers or other components. But for those who already have top-quality components and who have no prejudice against manipulating a program’s dynamics, the 4BX could be an ideal choice for making one more step toward sonic perfection.

Read all