PrimaLuna ProLogue Premium integrated amplifier Measurements
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 unit shipped to me for measurementserial no.10115729, a different sample from the one auditioned by Robert Deutschwas fitted with EL34 output tubes; I made sure the bias switch on the amplifier's left side panel was set to the appropriate bias position for that tube.
Maximum voltage gain into 8 ohms was on the low side for an integrated amplifier, at 36dB from the 8 ohm output transformer tap and 34dB from the 4 ohm tap, but this shouldn't be an issue with modern line-level source components. The ProLogue Premium preserved absolute polarity and the input impedance was 35k ohms across the audioband, dropping to 30k ohms at 20kHz. Though these figures are around half the specified 68k ohms, they will result in no practical consequences.
I assess an amplifier's output impedance by comparing the output voltages when it is loaded down with 8 and 4 ohms with the voltage when it is loaded only by the Audio Precision's 100k ohms. Things went smoothly with the 4 ohm tap, which measured 1.7 ohms across the audioband. However, when I tried to measure the impedance of the 8 ohm tap, the amplifier went into runaway ultrasonic oscillation when not loaded down, causing the ProLogue to mute its output and illuminate one or more of the Bad Tube Indicator (BTI) LEDs. Normal service was restored when I left the amplifier turned off for a few minutes, then turned it back on, but ProLogue Premium owners must take care never to disconnect their speakers when the amplifier is powered up.
The impedance of the 8 ohm tap appeared to be around 2 ohms, which gave rise to audible ±1.8dB variations in the amplifier's response when connected to our standard simulated loudspeaker (fig.1, gray trace). There were some slight variations in response even into purely resistive loads, but more important, at the 200kHz upper limit of this graph can be seen a large resonant peak whose height and frequency increased with increasing load impedance. With the 100k ohm input impedance of the Audio Precision test system, the amplifier became unstable and went into oscillation at the frequency of this peak. This ultrasonic peak was better-damped from the 4 ohm tap (fig.2), when the variations in response with our simulated loudspeaker were a less-audible ±1dB (gray trace). But the ultrasonic resonance resulted in visible overshoot and ringing with the PrimaLuna driving a 10kHz squarewave into 8 ohms from the 8 ohm tap (fig.3). Even from the 4 ohm tap, some overshoot can be seen with a 1kHz squarewave (fig.4), though the flat tops of this waveform are a testament to the ProLogue Premium's extended low frequencies. These four graphs were taken with the volume control set to its maximum. Commendably, there was no change in the response with the volume control set to lower positions and the channel balance remained excellent.
Channel separation (not shown) was good, at >70dB in both direction below 1kHz, but just 50dB at 20kHz. I tried all possible ways of connecting the ground of the PrimaLuna amplifier to that of the Audio Precision test system, including running a separate wire from the ProLogue Premium's grounding post on its rear terminal to the AP's chassis, but whatever I tried, the wideband, unweighted signal/noise ratio, taken with the input shorted and the volume control at its maximum and ref. 1W into 8 ohms from the 8 ohm tap, was a disappointing 57.3dB in the left channel, 63.1dB in the right. As shown by fig.5, this was because some low-level hum was present at the AC supply frequency and its harmonics. Switching an A-weighting filter into circuit improved the measured ratios to 76.8 and 77.7dB, left/right, respectively, which gives a closer estimation of the audibility of this noise. Even so, I note that RD had no noise problems with his sensitive Avantgarde horn speakers, so perhaps his sample of the ProLogue Premium was quieter than mine. What I did find with my sample was that it was mildly microphonic. If I banged the top of the transformer covereven with the input shortedI could generate a ringing tone with a frequency of around 250Hz and a level of 30mV RMS. I doubt that this will be a problem in normal use.
With the amplifier's output transformer tap matched to the load, the PrimaLuna ProLogue Premium just met its specified power of 35Wpc (15.6dBW). Fig.6 shows how the THD+noise percentage present in the 8 ohm output changes with power into 8 ohms. The THD+N reaches 1%, our standard definition of clipping, at 35Wpc. Figs. 7 and 8 show how the THD+N percentage changes with the 4 ohm tap driving 8 and 4 ohms, respectively. The ProLogue Premium clips at 21Wpc into 8 ohms (13.2dBW) and 32W into 4 ohms (12dBW), but the amplifier does meet its specified power from this tap at 1.4% THD. I haven't shown the clipping behavior into 2 ohms from either tap, as the amplifier switched itself off and lit one or more BTI LEDs as the power reached 40Wpc into this load.
As figs. 68 make clear, the ProLogue Premium is not a low-distortion design. I plotted how its THD+N percentage changes with frequency at 4.8V, a level equivalent to 2.9W into 8 ohms, where I could be sure that the distortion was higher than the noise floor. The results are shown in figs. 9 (8 ohm tap) and 10 (4 ohm tap). The amplifier is more linear from its 4 ohm tap, but there are significant rises in distortion at the frequency extremes and into lower impedances. This amplifier really does need to be used with speakers with the same nominal impedance as the transformer tap or higher.
Fortunately, at low powers into high impedances, the distortion is strongly second-harmonic in nature (fig.11), which is relatively innocuous. At higher output currents, the third starts to develop (fig.12), and even at moderate powers at low frequencies, a regular picket fence of higher-order harmonics appears (fig.13). At the other end of the spectrum, despite the amplifier's decreasing linearity at high frequencies, the PrimaLuna did quite well on the high-frequency intermodulation test. At a level about 3dB below visible waveform clipping on an oscilloscope, the second-order difference product at 1kHz resulting from an equal mix of 19 and 20kHz tones was at 56dB (0.15%), with the higher-order products at 18 and 21kHz lying at 60dB (0.1%) (fig.14).
PrimaLuna's ProLogue Premium measures about as well as I expect from a classic tube design that uses a push-pull pair of EL34 output tubes. Both the hum and the amplifier's instability when unloaded probably stem from the compromises necessary to manufacture a transformer-coupled amplifier of this sort that sells for a relatively affordable price.John Atkinson