Tube Power Amp Reviews

To take these measurements, I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com). The sample shipped to me for measurement had its output transformer set for an 8 ohm load. Before performing any tests, I made sure the output tubes were all correctly biased, using the top-panel LEDs. The LP125M's voltage gain into 8 ohms was higher than the norm, at 31.2dB, and the amplifier preserved absolute polarity (ie, was non-inverting). The input impedance was to specification, at close to 100k ohms across the audioband.

The source impedance from the single pair of output terminals was a high 2 ohms at low and middle frequencies, rising slightly to 2.2 ohms at 20kHz. This gave rise to a ±1.2dB variation in response with the magazine's standard simulated loudspeaker (fig.1, gray trace). The response is flat within the audioband into all but the lowest impedances, but a slight discontinuity at 30kHz becomes increasingly accentuated as the load impedance drops. This discontinuity is not to any significant degree, but may be associated with the very small amount of overshoot in the amplifier's 1kHz squarewave response (fig.2). Note, however, the superbly square nature of this response, which suggests that Conrad-Johnson uses a well-specified output transformer that performs well at low frequencies. The overshoot can also be seen in the 10kHz squarewave (fig.3), with one cycle of well-damped ringing visible.

Bob Deutsch reports on the problem he had with ground loops with the LP125M. I didn't find the amplifier particularly susceptible to grounding problems, though its signal/noise ratio did improve by about 8dB, due to the suppression of hum, when I floated the single-ended output ground of the Audio Precision measurement system, the opposite to what usually happens. In that condition, the LP125M's wideband, unweighted S/N, ref. 1W into 8 ohms, was an okay 75.1dB. This improved to 80.1dB when the measurement bandwidth was restricted to the audioband, and to 93dB with an A-weighting filter in circuit.

Fig.4 shows how the THD+noise percentage in the amplifier's output varies with output power and load impedance. The distortion remains well below 0.1% for powers below 10W into 8 or 16 ohms, but rises more rapidly into lower impedances. With clipping defined as 1% THD+N, the Conrad-Johnson amplifier clips at the specified 125W into 8 ohms (21dBW), but offers less power into both lower and higher impedances, confirming the 8 ohm setting for the output transformer. The LP125M clips at 85W into 16 ohms (22.3dBW), 68W into 4 ohms (15.3dBW), and 28W into 2 ohms (8.5dBW), though the last two figures are somewhat misleading, given the rise in distortion below the 1% point.

That the LP125M is clearly happier when set to a load impedance of 8 ohms or higher is also shown in fig.5, which plots the THD+N percentage against frequency at a moderate level, 4V. The audioband distortion is mainly very low into 16 and 8 ohms, with the rise in the low bass due to the limited amount of iron in the output transformer. At the other end of the spectrum, that the distortion begins to significantly rise above the midband level only above 8kHz or so is, again, another testament to the design of the output transformer.

Also commendable is the fact that, into higher impedances and at moderate output powers, the distortion is almost entirely the subjectively innocuous second harmonic (fig.6). The distortion signature remains strongly second harmonic at higher powers into higher impedances, though with some higher-order harmonics now appearing, albeit at low levels (fig.7, blue trace). However, at the same voltage level into 4 ohms (red trace), the third, fourth, and fifth harmonics are now at almost the same levels as the second, and these components are also now surrounded by power-supply–related sidebands at ±60Hz and ±120Hz. With the 8 ohm load, these sidebands are present to a noticeable degree only around the fundamental tone. These sidebands can be more clearly made out in the 8 ohm spectrum plotted up to 1kHz rather than 10kHz (fig.8). Even though the actual supply-related hum components at 60Hz and 180Hz lie respectably low in level at –97dB and –90dB, respectively, the related sidebands are almost 20dB higher.

For a tube amplifier, the C-J LP125M did surprisingly well on the high-frequency intermodulation test into 8 ohms (fig.9, blue trace), its second-order difference product at 1kHz lying at –63dB (0.07%), and its higher-order products at –80dB (0.01%) and below. At the same voltage level into 4 ohms (red trace), all higher-order products increased a little, but the difference product remained at –60dB (0.1%).

Conrad-Johnson's LP125M measures well for a traditional tube amplifier.—John Atkinson