Gradient SW-57 subwoofer Measurements

Sidebar 3: Measurements

With its two drive-units in parallel, the SW-57's impedance (fig.1) drops to 3.75 ohms in the upper bass. However, this shouldn't present the amplifier with any problems, since the phase angle is benign in this frequency region. The 16 ohm peak at 32Hz indicates the drive-units' resonant frequency, below which their output will roll off. The wrinkle in both magnitude and phase traces at 320Hz indicates some sort of resonant problem at this frequency.

Fig.1 Gradient SW-57, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).

The single-ended crossover is of basic build quality: outboard 15V AC supply, an onboard '7815-type regulator chip, carbon-film resistors, and LF353 op-amps. Its input impedance was up to spec at 23k ohms; and while its output impedance was also up to spec, this was relatively high, measuring 1100 ohms (1kHz, high-pass ESL outputs) and 960 ohms (50Hz, low-pass woofer outputs, level control at maximum). Aside from the right woofer output, which offered +14mV, DC offsets were below 3mV. Channel separation was moderate, with minima of 45dB (low-pass) and 55dB (high-pass). The unit's distortion remained well below 0.1% across each output's passband, though the high-pass outputs overloaded (defined as 1% THD) at 2.285V RMS input at 1kHz. While this may not seem like much headroom, remember that the crossover is inserted in the signal chain after the volume control. Any typical amplifier that's fed more than 2V will be either heavily into clipping or on the verge of overload.

The ESL outputs suffered a slight (1dB) insertion loss at 1kHz, while the maximum gain of the subwoofer outputs was 5.35dB. This allows for a degree of flexibility—using amplifiers of differing sensitivities—in matching the woofer level to the panels. The crossover's shaped, high-pass output responses are shown in fig.2. With the Mid-Adjust switch set to Off, the passband is basically flat, with a slight boost (1.5dB) at 300Hz. The $*–3dB point lies at 190Hz, with an ultimate second-order, 12dB/octave rollout slope. Position I of the Mid-Adjust switch gives a 2dB valley through the mid-treble; position II halves this depth.

Fig.2 Gradient SW-57 crossover, high-pass drive-signal to electrostatic panel with Mid-Adjust switch set to (from top to bottom): Off, II, and I (0dB = 200mV, right channel dashed, 2dB/vertical div.).

Fig.3 shows the low-pass drive signal supplied to the amplifier used with the Gradients (be wary of the different vertical and horizontal scales of this graph compared with fig.2). The rolloff slope is third-order, 18dB/octave, but there's some additional response shaping apparent in the woofer's passband. This peaks at 28Hz, with an 18dB/octave rollout.

Fig.3 Gradient SW-57 crossover, low-pass drive-signal to woofer with level control set to maximum (0dB = 200mV, right channel dashed, 5dB/vertical div.).

The effect of this shaping can be seen in fig.4, which shows the SW-57's nearfield acoustic response when driven both full-range and with the signal conditioned by the crossover. In the full-range trace, the small peak at 320Hz correlates with the wrinkle in the impedance traces; the crossover's low-pass response suppresses this by 25dB. The free-air loading of the drive-units is intrinsically overdamped. The unequalized bass response starts to roll off gradually below 125Hz, reaching $*–6dB at 56Hz. But when the crossover is in circuit, the passband boost results in an overall woofer response that's flat down to 28Hz, with then a fourth-order, 24dB/octave rollout slope that protects the unit against infrasonic overload.

Fig.4 Gradient SW-57, nearfield response of woofer with and without crossover (10dB/vertical div.).

Remember, however, that this is the unit's small-signal response. Applying 10dB of boost to a free-air–loaded woofer's low bass will drastically curtail its dynamic range. Luckily, many forms of music don't have high energy in this frequency region, so this won't be a limiting factor. Remember, too, that the Quad ESL has a limited dynamic range, accepting a maximum of 50 amplifier watts.

However, even though the lows will be reinforced by the drive-units' proximity to the floor and the additional baffling provided by the electrostatic panels, the SW-57 is still, as SS found in his auditioning, a woofer—not a subwoofer. Fans of organ and heavy rock music who want thunderous bass from their old Quads will still have to use a separate subwoofer, crossing over from the Gradient SW-57 below 50Hz or so. (This is not such a silly idea. In my experience, the loudspeakers which have performed best with subwoofers are those which already extend quite low in the bass.)—John Atkinson

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