Integrated Amp Reviews

Despite the InPoL Legacy operating partly if not wholly in class-A—which means that increasing the current leaving those 4mm speaker outputs causes the amplifier to cool down rather than heat up—I preconditioned the unit by running it at 10% of the rated 100W/8 ohm output for one hour. Over that period, distortion (fig.1) increased slightly, from ~0.047% to a steady 0.06%; this uplift with temperature is a feature of most low-feedback designs. The distortion is second- and third-order in roughly equal measure.

The maximum spot temperature registered across the heatsinks was 59.9°C at a room temperature of 20°C, so this expanse of ornate metalwork requires careful siting and plenty of airflow. It draws 855W from the wall at idle.

Such is the physical scale of this amplifier that it's easy to forget it's an integrated rather than a power amp; its 31dB of overall gain is closer to typical for the latter than the former. The 630mV input required to raise the output to the rated 100Wpc at 8 ohms is well within the compass of all modern (digital) line-level sources. The stepwise attenuator, meanwhile, covers just 1dB over the top six steps and 3dB over the top 10 steps (99 to 89). The top 20 steps cover 7.8dB, the top 30 steps 12.5dB; 40 steps, 18.9dB; 50 steps, 23.9dB; 60 steps (99 down to 39), 28.9dB; 70 steps, 34.8dB; 80 steps, 40.3dB; 90 steps (99 to 9), 51.4dB; with the last nine steps operating over 2–4dB each. The maximum useable volume range is 70dB.

In practice, the InPoL Legacy achieves higher power than its rated 100Wpc into 8 ohms (20dBW), offering 2×160W (22dBW) with that load before falling back to 2×105W into 4 ohms (17.2dBW) (ref. 1kHz at <1%THD). Increasing the THD limit to 2% realizes a substantially higher 2×220W into 4 ohms (20.4dBW), and under dynamic conditions, to 182W and 225W into 8 and 4 ohms, respectively (fig.2). Current output is limited to just 7.5A; although this current is sufficient to maintain the rated class-A condition into 8 ohms (where draw from the wall increases slightly to 900W), this limitation is at least partially responsible for the collapse to just 70W and 33W into 2 and 1 ohm loads, respectively.

Distortion increases gently with level over the first 10W, from 0.016% at 1W to 0.032% at 10W, then increases more steeply to 0.046% at 20W, 0.12% at 50W, and 0.3% at the rated 100W (all ref. 1kHz into 8 ohms)—all consistent with Pathos's published specifications. As a function of frequency, distortion increases at higher frequencies—another feature of the reduced compensation—and also at lower frequencies, especially at the lower power levels (fig.3). At 1W into 8 ohms, the range is 0.009–0.05%; at 10W, the range increases to 0.018–0.45%, and at 100W, to 0.20–3.4% (all ref. 20Hz–20kHz).

Noise is low, and the A-weighted S/N ratio is above average at 88.9dB (ref. 0dBW), while the amplifier's output impedance (dotted trace, fig.4) is a usefully flat 0.218–0.25 ohms from 20Hz–20kHz, rising thereafter to 0.74 ohms at 100kHz. This is still sufficient to modify the InPoL Legacy/speaker system's frequency response due to swings in loudspeaker impedance, but the amplifier's "native" response is largely, and uncommonly, unaffected by changes in nonreactive test loads between 8 and 1 ohms. The response reaches from 1Hz to 30kHz into 8 ohms (–0.5dB), and is –4.9dB at 100kHz. Into 1 ohm, the rolloff is only slightly higher: –0.5dB at 20kHz and –6.6dB at 100kHz (fig.4, black and green traces, respectively).

In short, this amplifier is powerful enough for medium sensitivity loudspeakers of, preferably, 6 ohm nominal impedance and above. The unity-gain solid state output is (probably) a single-ended, class-A MOSFET stage in a fairly typical common source configuration but with an inductor rather than series resistor between the FET source and ground. This would increase the efficiency of the class-A circuit as claimed but also limit the current available into lower impedances, as seen here.—Paul Miller