Jeff Rowland Design Group Consummate preamplifier Measurements

Sidebar 3: Measurements

With a variety of gains, input and output impedances, and either balanced or unbalanced inputs and outputs, some selectivity was needed to keep the measurements of the Consummate from getting out of hand. The line stage was measured at its highest available input impedance and lowest available output impedance, and at 18dB gain except where otherwise noted. Measurements were made both with balanced inputs and outputs and unbalanced inputs and outputs. The phono stage was measured with the standard 47k ohm input impedance (MM) and at 200 ohms input impedance (MC, 200 ohms chosen to minimize interaction with the minimum 25/50 ohms output impedance (unbalanced/balanced) of the Audio Precision test set). The output impedance of the phono stage was left as delivered—300 ohms unbalanced, 600 ohms balanced. Note that the balanced input from the Audio Precision System One to the Consummate phono stage differs slightly from the normal hookup used with a balanced phono input in that pin 1—the ground pin—is connected at both ends with the Audio Precision.

With an unbalanced input and output, unity gain for the Consummate line stage was reached at a control setting of "101" (again, with an 18dB internal gain setting). The voltage gain actually measured 17.8dB. Output impedance varied between 150.3 and 151.1 ohms depending on the channel and the setting of the level control. Input impedance varied between 96.8k ohms and 102.9k ohms for the same conditions. The resolution of the Audio Precision for such high impedances makes accurate measurement difficult, but essentially the specified input impedance was verified. The output impedance of the tape monitor measured 299 ohms regardless of the output impedance of the source (25 ohms or 600 ohms) feeding the input—indicating a buffered tape output.

With a balanced input and output, unity gain for line stage was reached with "67" set on the front-panel level control. (Gain measured 23.8dB.) Output impedance varied between 299 and 300.5 ohms, depending again on the channel and level control setting. Input impedance measured 197k ohms at full volume in both channels and, for the left channel only, 186k ohms at unity gain and 175k ohms at a level setting of "50." The output impedance of the tape monitor measured 599 ohms with a source output impedance of either 50 or 600 ohms. The gain was also checked in the balanced configuration with the internal gain switches set to the 6dB (measured 11.9dB) and 12dB (measured 17.9dB) settings. The internal gain switches are clearly referenced to an unbalanced configuration, with the balanced hookup giving the expected additional 6dB of gain.

The phono stage output impedance measured 597.4 ohms (L) and 600.9 ohms (R) in balanced mode, 299 ohms (L) and 301.7 ohms (R) unbalanced. Phono gain measured 45.8dB in moving-magnet mode and 65dB in moving-coil, the former slightly below spec (47.5dB) and the latter slightly above (62.5dB). The Consummate is non-inverting through both its phono and line stages, and DC offset at the outputs measured a negligible 1.5mV (left) and 1.6mV (right).

The 1% distortion point for the line-level stages was reached at an input of 1.212V (1kHz) (with a resultant output of 9.4V) for the unbalanced configuration, 1.24V for the balanced (output of 19.22V). With an unequalized input signal, the moving-magnet phono input overloaded at 119mV in (23.17V out) at 1kHz, 12.68mV (22.03V out) at 20Hz, and 1.15V (23.27V out) at 20kHz. The same figures for moving-coil were 11.7mV (23.14V out) at 1kHz, 1.3mV (22.96V out) at 20Hz, and 113.5mV (23.28V out) at 20kHz. Note that the reduced overload margin at low frequencies and increased margin at high frequencies is a natural result of the requirements of the RIAA curve.

Fig.1 shows the Consummate's frequency response. The top two curves (left is solid, right is dotted in all cases), which are practically a straight line with a slight rolloff above 30kHz, are the responses through the line-level stages, from the input to the main output. (The unbalanced input/output curves are shown; the balanced were essentially the same.) The lower two curves are for the RIAA response from the input of the phono stage to its output, taken for the low gain (MM) setting with an unbalanced input (other conditions were measured and were not essentially different).

Fig.1 Jeff Rowland Design Group Consummate, line-stage frequency response (top) and phono stage RIAA error, set to MM and low gain (right channel dashed, 0.5dB/vertical div.)

Note the shape of the curve. The absolute deviations are small, though closer to +/-0.2dB on the worst channel than the +/-0.1dB spec (from 20Hz to 50kHz). Note also that while the deviations are minor compared with those of a phono cartridge, they do occur over several octaves, enhancing their probable audibility, and might, in comparison with an absolutely flat curve, give the Consummate a slightly lean character through its phono stage, perhaps marginally enhancing the openness and clarity of the sound. If you refer to the Consonance review in Vol.14 No.3, you'll note a curve which is also +/-0.2dB (approximately), but with a rather opposite slope, tending gently downward toward the top end rather than upward as in the case of the Consummate.

Fig.2 indicates the line-level separation, showing the usual decrease at higher frequencies due (generally) to capacitive interchannel coupling. The curves virtually overlay each other above 2kHz. The two bottom-most curves, referenced at the 300Hz level, are for the balanced (in/out) mode. The topmost curve is unbalanced mode (in/out) separation from the left channel to the right (output from the right channel with a signal into the left only), and the second from the top is the unbalanced mode from right to left.

Fig.2 Jeff Rowland Design Group Consummate, line-stage channel separation: unbalanced (top), balanced (bottom) (5dB/vertical div.).

The separation for the phono stage is shown in fig.3. Referenced at the 10kHz level, these curves, all for the low-gain (moving-magnet) setting, were, from top to bottom, unbalanced input from left to right channel, balanced input from left to right channel, balanced input from right to left channel, and unbalanced input from right to left channel. The curves for the high-gain (moving-coil) setting are not shown as the data there were largely buried in noise. Note that the phono separation was actually superior to the line-stage separation, although both were excellent.

Fig.3 Jeff Rowland Design Group Consummate, phono-stage channel separation (from top to bottom): L-R, unbalanced; L-R, balanced; R-L, balanced; R-L, unbalanced (5dB/vertical div.).

The THD+noise readings are shown in fig.4 (line stage) and fig.5 (phono stage). The line-stage curves are for the unbalanced configuration (top curves) and balanced configuration (bottom). Referenced to 10kHz, the curves in fig.5 are (top to bottom) moving-coil unbalanced, moving-coil balanced, and moving-magnet unbalanced. All are composed more of noise than distortion. In all cases the input signals to the phono stages used for the separation and distortion measurements were far higher in level than would normally be encountered in practice, in an attempt to counteract the fact that the distortion was so low as to be buried in noise. For separation measurements, 80mV was used into the high-level (moving-magnet) input; for distortion readings, 100mV was used into the high level, and 10mV into the low level—all at 1kHz.

Fig.4 Jeff Rowland Design Group Consummate, line-stage THD+noise (%) vs frequency, unbalanced (top), and balanced (bottom) (right channel dashed).

Fig.5 Jeff Rowland Design Group Consummate, phono-stage THD+noise (%) vs frequency (from top to bottom): MC unbalanced, MC balanced, and MM, unbalanced (right channel dashed).

To provide some feel for how the Vendetta phono preamp—used in one of the comparisons—measured in comparison with the Consummate, I ran the Vendetta through two tests: frequency response (fig.6) and THD+noise (fig.7, taken with a 5mV input and largely noise). Notice that the RIAA response of the Vendetta, while not flatter than that of the Consummate phono stage on an absolute basis, has a slightly different curve which could account at least to a degree for their sonic differences.—Thomas J. Norton

Fig.6 Vendetta Research SCP-2, RIAA error (right channel dashed, 0.5dB/vertical div.)

Fig.7 Vendetta Research SCP-2, THD+noise (%) vs frequency at 5mV input at 1kHz.

Jeff Rowland Design Group
P.O. Box 7231
Colorado Springs, CO 80933
(719) 473-1181