Integrated Amp Reviews

I measured the Exposure 3010S using Audio Precision's top-model SYS2722 system (see www.ap.com) for most of the tests, with some use made of the older AP System One. As usual, I preconditioned the amplifier by running both channels at one-third power into 8 ohms, which maximally stresses an amplifier with a class-B or -AB output stage. The normal preconditioning period is one hour, but the Exposure turned itself off after just 20 minutes. While the chassis was warm, the area of the top cover above the internal heatsink was almost too hot to keep my hand on. After the amplifier had cooled down, I turned it on again, and all seemed well. But this does suggest that the Exposure's heatsinking is inadequate for sustained use at levels around 40Wpc, which could well happen if the amplifier is used to provide music at a party.

The maximum voltage gain at the 3010S's speaker output jacks was 41.15dB into 8 ohms, which is typical for an integrated amplifier. As the 3010S also has preamplifier output jacks, I could measure the maximum gain at these: it was 17.25dB, which implies a power-amplifier gain of 23.9dB. The preamplifier outputs were non-inverting and had a source impedance of 48 ohms at high and middle frequencies, rising to 410 ohms in the low bass. The amplifier as a whole, and the phono stage, also preserved absolute polarity.

The input impedance of the line-stage jacks was moderately high at 21k ohms at low frequencies, dropping slightly to 18k ohms at the top of the audioband. The phono-stage input impedance was fixed at 47k ohms at 20Hz, dropping slightly to 45.6k ohms at 1kHz and 37k ohms at 20kHz. Measured at the tape output jacks, the phono stage had a gain of 40.6dB, which is appropriate for use with moving-magnet phono cartridges or moving-coils with a step-up transformer, which was how AD auditioned the unit. The phono section's signal/noise ratio (unweighted, ref. 5mV at 1kHz input signal) was good at 68.6dB, and the overload margin, again ref. 5mV at 1kHz, was superb across the audioband, at 26dB or greater. The only disappointing aspect of the Exposure's phono stage was its unflat RIAA correction (fig.1), which showed a 1–1.5dB boost in the midrange. I suspect that it is this error that gave rise to AD's feeling that the 3010S sounded "overly dark, overly heavy" via its phono input.

Fig.1 Exposure 3010S, phono-stage RIAA error (0.5dB/vertical div.).

The Exposure's output impedance at the speaker jacks was a low 0.1 ohm across the audioband. As a result, the modification of its frequency response driving our standard simulated loudspeaker (see www.stereophile.com/reference/60) remained within ±0.1dB limits (fig.2, red trace). Channel matching was excellent. The amplifier starts to roll off at the top of the audioband, with the response at 20kHz down by 0.75dB, which is inconsequential, though it does slow a little the leading edges of a 10kHz squarewave (fig.3). The 1kHz squarewave response (not shown) was perfect, with flat tops confirming the excellent low-frequency extension seen in fig.2. Channel separation via the line stage (not shown) was very good, at >90dB R–L and >80dB L–R below 2kHz.

Fig.2 Exposure 3010S, frequency response at 2.83V into: simulated loudspeaker load (red), 8 ohms (magenta), 4 ohms (green), 2 ohms (red). (1dB/vertical div.)

Fig.3 Exposure 3010S, small-signal 10kHz squarewave into 8 ohms.

The S/N ratio for the line input, measured with the input shorted but the volume control wide open, was 73dB ref. 1V (wideband, unweighted), this improving slightly to 78.6dB when A-weighted. Claimed to deliver 110Wpc into 8 ohms (20.4dBW), the Exposure easily exceeded that maximum power rating, with 140Wpc (21.5dBW) available at clipping into that load (clipping defined as 1% THD+noise). The maximum power into 4 ohms was 200Wpc (20dBW), again with both channels driven. Fig.4 shows how the THD+N percentage present in the amplifier's output varies with power into 8 and 4 ohms; the shape of the traces in this graph indicates that, below a few tens of watts, the distortion present is actually below the amplifier's noise floor with this 1kHz signal.

Fig.4 Exposure 3010S, distortion (%) vs 1kHz continuous output power into (from bottom to top at 100W): 8, 4 ohms.

I therefore measured how the amplifier's THD+N varied with frequency and load impedance at a fairly high output level of 10V. The results are shown in fig.5: both channels show a rise in THD in the treble, but the left channel was less linear than the right. For example, the right channel was more linear into 4 ohms (magenta trace) than the left channel into 8 ohms (blue). Similarly, the right channel into 2 ohms (green) was more linear than the left into 4 (cyan). I have no way of knowing whether this channel mismatch is due to a lapse in quality control or to a problem in design, circuit layout, or component selection, but fortunately, the spectral content of the distortion is heavily low-order in nature (figs. 6 and 7). Fig.7 was taken 6dB below clipping into 4 ohms; the amplifier is definitely working hard under these conditions, as revealed by the presence of power-supply components at 120 and 240Hz. Despite its use of a massive toroidal power transformer, which coupled hum into Art's moving-coil step-up transformer, the 3010S's output is commendably free of magnetically induced spuriae at 60Hz and its harmonics. The internal shielding works.

Fig.5 Exposure 3010S, THD+N (%) vs frequency at 13.15V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green).

Fig.6 Exposure 3010S, 1kHz waveform at 23.3W into 4 ohms (top), 0.0111% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

Fig.7 Exposure 3010S, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 4 ohms (linear frequency scale; left channel blue, right red).

Finally, with the left channel's reduced linearity at high frequencies, it was not surprising that it performed less well than the right channel on the demanding high-power, high-frequency intermodulation test (fig.8). The 1kHz difference component reached an okay –80dB in the left channel (red trace), but an excellent –97dB in the right (blue).

Fig.8 Exposure 3010S, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 120W peak into 4 ohms (linear frequency scale; left channel red, right blue).

Other than the unflat RIAA equalization and the difference in channel linearity, the Exposure 3010S's measured performance was excellent.—John Atkinson