Apogee Stage & Mini-Grand loudspeakers 1994 Measurements

Sidebar 2: 1994 Measurements

On the test bench, the dedicated active crossover (DAX) had a gain of 3.4dB ±0.05dB in the unbalanced mode for either channel on both the high- and low-pass legs (taken at 1kHz and 40Hz, respectively). Its gain in the balanced mode was virtually the same (about 0.1dB higher). The balanced and unbalanced output impedance also measured the same: 50-51 ohms. The input impedance measured just under 7.4k ohms, unbalanced, and between 8k ohms and 8.2k ohms, balanced, for either channel, high- or low-pass. The latter is moderately low, but should only be a problem with those few preamps (generally tube designs, but not all tube designs) that have high output impedances.

The high- and low-pass level controls on the DAX altered the gain the specified amounts (±3dB in 1dB increments) with a variation of no more than 0.02dB.

Fig.1 shows the frequency response of both the high- and low-pass sections of the DAX, with the low-pass reference level adjusted so that the curves cross at the specified 80Hz crossover frequency. Note that the low-pass section provides a boost to the extreme bottom end of the range. This is an accepted technique for one particular type of ported woofer alignment (a so-called sixth-order alignment), but it's normally accompanied by some sort of infrasonic filtration. Here the boost continues well below 20Hz, reaching a maximum somewhere below our test equipment's 10Hz lower limit. This would explain the overload problems I encountered, particularly with the extremely powerful Krell amplifier. It should be noted that a level boost of 6dB, which does not appear extreme, requires four times the power. The Krell can accommodate this requirement.

Fig.1 Apogee DAX crossover, high- and low-pass output responses (right channel dashed, 5dB/vertical div.).

The crosstalk of the DAX's high-pass section is shown in fig.2. The unbalanced mode is good, the balanced mode excellent, with only the slightest increase at the highest frequencies. The THD+noise percentage for the high-pass section (fig.3) is very low in either mode, even at the 2V input used for the measurements. This level was chosen after I plotted out the THD+noise vs level at 1kHz (high-pass) and 50Hz (low-pass). This result (not shown) indicates an extremely low distortion up to 12V output on both sections (below 0.02% at 0.1V, below 0.003% at 1V, and below 0.002% at 12V). Distortion increases rapidly after 12V, but no amplifier I know of requires much more than 2V for its maximum output.

Fig.2 Apogee DAX crossover, crosstalk in unbalanced mode (top) and balanced mode (bottom) (10dB/vertical div.).

Fig.3 Apogee DAX crossover, high-pass THD+noise vs frequency at 2V input level in unbalanced and balanced modes (right channel dashed).

John Atkinson measured the Apogee Mini-Grand—the Stage and subwoofer—after I completed my listening. The B-weighted sensitivity of the Stage measured approximately 81dB/W/m, using the 82.5dB/W/m sensitivity of the LS3/5A as a reference point. While the Stage's sensitivity is quite low, it's important to note that with its low impedance, it actually draws more than 2W at the standard input used for sensitivity measurements (2.828V), rather than the 1W normally drawn by a hypothetical, standard 8 ohm impedance.

These measurements also indicated, interestingly, that doubling the distance from the Stage resulted in an spl drop of 5.3dB. Normally, the expected drop for a line source would be 3dB—see the Audiostatic ES-100 review in this issue—6dB for a point source. Despite its use of a ribbon, the Stage appears to behave more like a conventional speaker.

The impedance of the Stage (fig.4) is very uniform, and also very low—the minimum magnitude is 2.9 ohms. Some care should be taken in selecting an appropriate amplifier—it should be one comfortable driving a 3 ohm load. The small ripples at 37Hz and 44Hz indicate the fundamental woofer panel resonances. The electrical crossover between the woofer-midrange panel and the tweeter ribbon is reflected in the increase in impedance magnitude centered at 350Hz. The impedance changes caused by the "Normal" and "High" tweeter-level switch positions, which are centered at about 10kHz, are quite small.

Fig.4 Apogee Stage, electrical impedance (solid) and phase (dashed) with HF switch set to "Normal" (top at 10kHz) and "High" (2 ohms/vertical div.).

The subwoofer impedance (fig.5) shows a very low port tuning frequency—the "saddle" at 23.5Hz between the two spikes at about 13Hz and 37Hz—and a minimum magnitude of 2.8 ohms. A small ripple at 350Hz indicates a resonance—either in the port or the enclosure—but it's well above the crossover frequency in normal operation when the DAX is in use, and should be of no audible significance.

Fig.5 Apogee Stereo Subwoofer, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

Fig.6 shows the nearfield responses of the Stage subwoofer drivers and port, driven without the DAX in-circuit. Despite the low port tuning, the actual maximum acoustic output of the port and minimum output of the woofers occur about an octave apart—at 33Hz and 17Hz, respectively. The output of the port peaks again at about 540Hz (corresponding to a dip in the driver response)—likely due to a resonance of some kind. This should be audibly innocuous when the DAX is used. Fig.7, which shows the subwoofer drive-unit and port outputs with the DAX in the circuit, illustrates that this is so. The boost in LF output provided by the DAX at the lowest frequencies is evident when comparing figs.6 and 7.

Fig.6 Apogee Stereo Subwoofer, nearfield response of woofer and port without DAX low-pass filter.

Fig.7 Apogee Stereo Subwoofer, nearfield response of woofer and port with DAX low-pass filter.

To the left of fig.8 is shown the vector sum of the responses of the DAX-equalized subwoofer drivers and port (weighted in the proportion of the square roots of their areas). The center curve shows the response of the Stage's woofer-midrange panel with the DAX engaged, the right curve shows the response of the ribbon tweeter. (The subwoofer curve is a nearfield measurement; the Stage measurements are both a combination of a nearfield measurement for the lower frequencies and the response 45" from the vertical midpoint of the tweeter, directly on the tweeter axis.) The LF response holds up well to below 20Hz; normal room reinforcement should enhance the bottom-end response even further.

Fig.8 Apogee Mini-Grand, summed nearfield responses of Stereo Subwoofer woofers and port, with individual responses of Stage LF and HF ribbons on-axis midway up panel at 45", corrected for microphone response, with nearfield LF and HF ribbon responses plotted below 300Hz and 600Hz, respectively.

The two panel resonances of the Stage woofer-midrange at 37Hz and 44Hz, noted from the impedance plot, are visible. Note that the subwoofer begins reinforcing the response of the Stage's woofer ribbon at almost exactly the point where the latter drops off like a rock. The response ripple in the Stage's woofer-mid panel, visible in its top-end rolloff from about 2-3kHz, appears to be an acoustical anomaly—likely due to interference—rather than a resonance (it does not affect the impedance plot, for example). The acoustical crossover for the Stage itself lies just above 600Hz. The tweeter response, taken with the "High" setting of the tweeter-level control, shows some irregularity and moderate peaks in the low to mid-treble, reaching a maximum at 10kHz.

The overall response of the Stage alone, with the tweeter setting on "High" and the DAX out of the circuit, is shown in fig.9. Without the moderating effect of the DAX's high-pass rolloff, the Stage's rise below 50Hz is more pronounced. Some of the gradual downtilt in the overall response with increasing frequency is due to the proximity effect; when measuring a large panel, the microphone distance is not much larger than the overall driver dimensions, which adds a slight clockwise tilt to the measured response.

Fig.9 Apogee Stage, anechoic response on-axis midway up panel at 45", corrected for microphone response, with nearfield LF ribbon response below 300Hz.

Fig.10 shows the effect of the tweeter-level control in its Normal setting on the overall response. (The curve shows only the changes due to the control.) Note that the control is actually a contour control, not just a simple level control. The response is very close to the inverse of the low-mid treble peaks seen in the previous curves, effectively compensating for them (though, of course, it cannot smooth the response completely). Note that I did most of my listening in this Normal mode, and while I still noted some brightness, it was clearly less obvious than in the High setting, which I felt to be too "hot." The added brightness in the High setting might, however, be of help in a very dead listening room.

Fig.10 Apogee Stage, effect of HF control set to "Normal," normalized to response with it set to "High" (5dB/vertical div.).

The lateral response plot in fig.11 was taken toward the tweeter side of the Stage. That is, it shows the change in response as the listener moves off-axis toward center "stage" in a normal stereo setup. Remember, Apogee recommends, at maximum, a very small toe-in; the listener in this configuration will always be displaced slightly inward of the direct axis. The HF response rolls off as we move off-axis in this direction, but the rolloff is generally smooth up to and somewhat beyond 30 degrees (though with a small peak cropping up above 15kHz). Note the progressively deeper dip at 800Hz at greater off-axis angles—the result of lateral interference between the woofer-midrange panel and the tweeter. The vertical-response family of curves is not shown, but indicates what is obvious from listening: The Mini-Grand, like the Stage itself, is a "sit-down" loudspeaker. Above the top of the ribbon, the high-frequency response drops drastically in level.

Fig.11 Apogee Stage, horizontal response family at 45", normalized to response on-axis midway up panel, from back to front: reference response; differences 5 degrees through 90 degrees off-axis.

The impulse response of the Stage in fig.12 is quite clean, with a fast rise-time and little ringing. The step response calculated from the impulse response (fig.13) shows the initial rise of the tweeter, positive in polarity, followed by the negative-going response of the woofer-midrange panel. The latter is connected out-of-phase with the tweeter—the only way (without using digital signal processing) to obtain a relatively flat frequency response in the crossover region with even-order filters.

Fig.12 Apogee Stage, impulse response on-axis midway up panel at 45" (5ms time window, 30kHz bandwidth).

Fig.13 Apogee Stage, step response on-axis midway up panel at 45" (5ms time window, 30kHz bandwidth).

Though the impulse response for the subwoofer is not shown, it indicates that, as observed in the listening tests, the subwoofer is acoustically out of phase with the Stage woofer-midrange panel when hooked up as specified. This may well be intentional. Note that, in fig.8, the maximum output of the subwoofer falls near the region where the Stage's woofer-midrange panel has its resonance peaks. Because the Stage's response drops off very rapidly below this, an acoustically out-of-phase hookup with the subwoofer might help cancel out this peak, at little apparent sacrifice to the extreme bottom end of the Mini-Grand as a whole.

Note that when I connected the subwoofers in inverse electrical polarity in my listening tests—resulting in their being acoustically in-phase with the Stages—my rudimentary acoustical measurements at the listening position did indicate a peak at about 50Hz. And while I ultimately felt that the overall bass response was best in my room using this hookup, the elevated 50Hz response I encountered might be more of a problem in a smaller room, making Apogee's recommended connection more appropriate. Experimentation here is mandatory—each room will respond differently.

The waterfall plot in fig.14 shows a clean decay, with only a minor HF resonance at about 6.6kHz. There's notably less hash here in the decay of the tweeter response than we've often seen in other planar loudspeakers, both electrostatic and electromagnetic.

Fig.14 Apogee Stage, cumulative spectral-decay plot at 45".

Panel loudspeakers are generally difficult to measure, and the results sometimes differ from listening impressions. This is less true of the Mini-Grand—and, by implication, of the Stage, which is a major part of the system. In many respects the system measures better than a number of pricier panels we've seen here in the past. The subwoofer, Stage woofer-midrange panel, and Stage ribbon tweeter—with the possible exception of the sometimes problematical extreme low-frequency boost applied by the DAX to the subwoofer—integrate effectively into a coherent whole.—Thomas J. Norton