Jeff Rowland Design Group Synergy line preamplifier 2002 Measurements

Sidebar 2: 2002 Measurements

The Rowland Synergy IIi's maximum gain was 19.95dB, but this can be adjusted in accurate 0.5dB steps down to -20dB. The volume control also operated in accurate 0.5dB steps, with "63.5" the maximum indicated setting. The input impedance at 1kHz was 28k ohms or 641 ohms with the optional input termination selected. The balanced output impedance measured 51 ohms across the band, and the preamp was noninverting, with pin 2 of the XLRs connected as "hot" and the front-panel polarity-inversion LED unlit.

The frequency response (fig.1) was identical into all loads and at all gain settings. It's flat within the audioband and sensibly rolled-off at ultrasonic frequencies, reaching -3dB at 160kHz. Channel separation (not shown) was superb, at better than 115dB below 3kHz, with very slight capacitive coupling increasing the crosstalk to a still-excellent 100dB at 50kHz.

Fig.1 Rowland Synergy IIi, frequency response at 1V into 100k ohms (0.5dB/vertical div., right channel dashed).

Looking at how the measured percentage of distortion and noise varied with frequency, fig.2 appears to show that this varies with the gain setting. However, what is being shown in this graph is that when the Synergy is set to give 20dB of gain, the measurement is dominated by noise above 60Hz (upper pair of traces). With the gain set to 0dB (lower traces), the measured distortion+noise percentage drops dramatically, with the rises at the frequency extremes indicating slight but real increases in THD compared to the midband level.

Fig.2 Rowland Synergy IIi, THD+noise vs frequency at (from top to bottom at 1kHz): 2V into 600 ohms with Maximum Gain, 0dB Gain (right channel dashed).

I was a little puzzled by the low-frequency rise in THD, until I remembered that the Synergy uses input-coupling transformers. These will have finite headroom at low frequencies, leading to the rise in distortion. Fig.3 compares the THD at a 2V RMS output level—about the maximum that the Synergy will be asked to give in real-life situations—and at 7V RMS, which is just below the clipping point into 600 ohms. While the low-frequency distortion does rise dramatically at the higher output level, the Synergy's bass will be clean at typical output levels.

Fig.3 Rowland Synergy IIi, Maximum Gain, THD+noise vs frequency at (from top to bottom at 1kHz): 2V into 100k ohms, 7V into 100k ohms (right channel dashed).

Fig.4 shows that the low levels of distortion present at low frequencies are predominantly the third harmonic, presumably due to the onset of saturation in the transformer core, though a trace of second harmonic can be see at -96dB (0.0015%). Both will be subjectively innocuous. The Rowland preamp is also very linear when it comes to intermodulation distortion, all the products visible in fig.5 being due to the DAC I use to produce this test signal.

Fig.4 Rowland Synergy IIi, spectrum of 50Hz sinewave, DC-1kHz, at 2V into 600 ohms (linear frequency scale).

Fig.5 Rowland Synergy IIi, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 2V into 600 ohms (linear frequency scale).

Finally, fig.6 shows how the Synergy's measured THD+N percentage changes with output voltage into 100k and 600 ohm loads. The linear rise in the percentage with decreasing level reveals that, below 4V or so output, the preamp's THD figure is actually dominated by noise. Regardless of the gain setting, the Synergy clips at 7.5V into 100k ohms and 7.05V into 600 ohms, which is more than enough dynamic range for any real-world system.

Fig.6 Rowland Synergy IIi, distortion (%) vs output voltage into 100k ohms (bottom and right) and 600 ohms (top and left).

As with all the products from Jeff Rowland that we have examined, the Synergy IIi's objective performance is beyond reproach.—John Atkinson

Jeff Rowland Design Group
2911 N. Prospect Street
Colorado Springs, CO 80907
(719) 473-1181