Sonic Euphoria PLC passive line stage Measurements
When I removed the top of the Sonic Euphoria PLC so that its innards could be photographed, I couldn't help whistling. The considerable amount of hand-soldered, point-to-point wiring mandated my respect for the skill of the person who'd assembled it. Then it was back to business as I wrangled the preamp onto my test bench for measurement.
As BD explains in his review, because the PLC is based on transformer circuitry, it can actually have gain, unlike a conventional passive preamp. I measured a maximum of 3.83dB gain into 100k ohms with both balanced and unbalanced operation. The Volume control operated in accurate 2dB steps, the half-step switch offering an additional –1dB for each step. The PLC preserved absolute polarity for balanced and unbalanced operation, the XLRs apparently wired with pin 2 hot.
The PLC's input impedance varied with both frequency and the Volume-control setting. At 1kHz the unbalanced impedance ranged from a low of 35k ohms with the Volume control at its maximum to a high of 193k ohms with it set to –12dB. The 20Hz and 20kHz figures were a little lower, at 7.4–10.4k ohms and 10.5–124k ohms, respectively. The balanced figures were approximately twice the unbalanced impedances. The PLC is best used with source components having an output impedance less than 1k ohm if the frequency balance is not to be skewed.
The PLC's output impedance also varied with frequency and Volume-control setting. What was most interesting, however, was that at the preamplifier's unity-gain setting, 2 clicks below maximum on the Volume control, the preamp's circuitry appeared to be bypassed altogether, the source impedance being just that of the signal generator! At the maximum setting of the Volume control, the unbalanced output impedance ranged from 586 ohms at 20Hz to 600 ohms at 1kHz and 1.7k ohms at 20kHz. With the control set to –12dB, the output impedance ranged from 163 ohms at 20Hz and 1kHz to 235 ohms at 20kHz, these figures sufficiently low not to give matching problems with typical power amplifiers.
With the Volume control set to its maximum, the PLC's high source impedance at 20kHz gives rise to a 2dB rolloff at 20kHz into a 1k ohm load (fig.1, bottom trace). What I was not expecting was the ultrasonic peaking into higher load impedances that also can be seen in this graph. The output into 10k ohms, this the typical input impedance of many solid-state power amps, starts to rise above 10kHz and reaches +6dB at 39kHz, while into 100k ohms (fig.1, top trace), the output peaks by 12.8dB at 48kHz. The response rise in the audioband is still negligible, but the amplification of ultrasonic noise from, for example, SACD players or low-output moving-coil cartridges might lead to unpredictable results with some combinations of power amplifier and speakers.
Fig.1 Sonic Euphoria PLC, Volume control at maximum, frequency response at 1V into (from top to bottom at 40kHz): 100k ohms, 10k ohms, 5k ohms, 1k ohm (5dB/vertical div.).
The response measurements in fig.1 were taken with the PLC's Volume control at its maximum. Fig.2 shows what happens to the preamp's frequency response at successively lower Volume-control settings. Basically, the peak moves higher in frequency. The exception is the unity-gain setting (third trace from top), where, as noted above, the transformer appears to be out of the circuit.
Fig.2 Sonic Euphoria PLC, frequency response at 1V with Volume control at (from top to bottom at 1kHz): maximum, –2dB, unity gain, –6dB, –8dB, –10dB, –12dB, –14dB, –16dB, –18dB, –20dB, –22dB, –24dB (5dB/vertical div.).
The channel separation was excellent (fig.3), with the balanced crosstalk a little lower than the unbalanced. Distortion at 1V (fig.4) was close to the residual level of the signal generator in the midrange and treble, but rose below 300Hz, reaching a maximum of 0.08% at 20Hz, presumably due to the influence of the transformer core.
Fig.3 Sonic Euphoria PLC, balanced (bottom) and unbalanced (top) channel separation (R–L dashed, 10dB/vertical div.).
Fig.4 Sonic Euphoria PLC, THD+N (%)vs frequency at 1V into 100k ohms.
With its idiosyncrasies of input and output impedance, the PLC needs to be more carefully matched to the source components and the power amplifier than a conventional active preamplifier. Other than that, it appears to be a well-engineered piece of kit.—John Atkinson