Sidebar 3: Measurements
As increasingly appears to be the case with high-end phono preamplifiers, the Gold Note PH-1000 is a complicated product. It has single-ended and balanced inputs, and one of the latter is capable of accepting a line-level signal. It has balanced, single-ended, and headphone outputs, with the option of either taking the output from the phono stage (Stage) or a preamplifier stage with a volume control (Preamp). (The latter appears to be the default for the headphone output.) Each phono input can be set to MC or MM, with gain adjustable from –9dB to +9dB in 3dB steps relative to the default 0dB setting. There are multiple choices for input impedance and adjustable input capacitance for both MC and MM modes.
I performed a complete set of measurements with my Audio Precision SYS2722 system using one of the single-ended phono inputs and both the phono stage's single-ended and balanced outputs, then repeated some of the tests with the preamplifier switched into the circuit. Most of the measurements were performed with the input set to MM, the gain set to its central, 0dB level, and the input capacitance set to 100pF. The lowest noise was obtained with the Audio Precision's signal generator ground floating and a separate grounding wire running from the PH-1000 input's ground terminal to the analyzer's chassis. I repeated some tests with the input set to MC mode and with the balanced phono and line inputs.
All the PH-1000's inputs and outputs preserved absolute polarity in both MM and MC modes through both the single-ended and balanced outputs. In MM mode with impedance set to 47k ohms, I measured 44k ohms at 20Hz and 1kHz for both balanced and unbalanced inputs, dropping to 20k ohms at 20kHz. In MC mode, the single-ended input impedance with the control set to "1000" was 1021 ohms at 20Hz, 998 ohms at 1kHz, and 992 ohms at 20kHz. With the control set to "100," the impedance was 104 ohms at 20Hz, 102 ohms at 1kHz, and 106 ohms at 20kHz. Set to "10," the impedance ranged from 13 ohms at 20Hz to 11.6 ohms at 1kHz and 20kHz. The balanced phono input supported only MC mode. Its input impedance defaults to 470 ohms and has a maximum setting of 4700 ohms. The measured impedances were close to the single-ended values, with the exception of 4700 ohms, where I measured 2400 ohms (1200 ohms per phase).
The PH-1000's output impedance is specified as 100 ohms unbalanced and 150 ohms balanced. I measured the balanced output impedance as 248 ohms in both Stage and Preamp modes from 20Hz to 20kHz. The unbalanced output impedance in both modes was 103 ohms at 1kHz and 20kHz and a still-low 208 ohms at 20Hz. The headphone output impedance was 40 ohms at 20Hz, 3.4 ohms at 1kHz and 20kHz.
The voltage gain is specified as 40–65dB, depending on the mode. In MM mode, with the output set to Stage, the measured gain was 42.2dB at both the balanced and unbalanced outputs with the gain set to –9dB, 51.2dB with the gain set to 0dB, and 60.2dB with the gain set to +9dB. The gain in MC mode set to 0dB was 65.4dB for both single-ended and balanced inputs. Switching to Preamp mode with the "Low" setting reduced the maximum gain by 6dB. With the Preamp "Hi" setting, the maximum gain was 8dB higher than in Stage mode.
RIAA correction was superbly accurate, with excellent channel matching (fig.1, blue and red traces), rolling off at ultrasonic frequencies, reaching –3dB at 41kHz. With the subsonic filter engaged, the response rolled off sharply below 35Hz, reaching –3dB at 19Hz (cyan and magenta traces). Turning on Enhanced mode increased the level in the midrange by 0.7dB (green and gray traces) and boosted the output above 10kHz by a maximum of +6dB between 40kHz and 50kHz. This would appear to be implementing the so-called Neumann 4th pole modification of the original RIAA curve.
Channel separation was superb at around 90dB in both directions across the audioband. Spectral analysis of the Gold Note's low-frequency noisefloor with it set to MM and 0dB gain (fig.3) indicated that both random noise components and power supply–related spuriae were very low in level. The PH-1000's unweighted, wideband S/N ratio, measured in MM mode with the unbalanced input shorted to ground and the gain set to 0dB, was an excellent 83.8dB in both channels, referred to an input signal of 1kHz at 5mV. Restricting the measurement bandwidth to 22Hz–22kHz increased the ratio to 87.2dB, while switching an A-weighting filter into circuit gave a further increase to an astonishing 91.6dB. Increasing the gain by 6dB reduced the S/N ratios by the same 6dB, while reducing the gain by 6dB increased the ratios by 3–5dB. In MC mode with 0dB gain, the S/N ratios were lower but still good, at 67.9dB (unweighted, wideband), 70.5dB (22Hz–22kHz), and 74.9dB (A-weighted), these all referred to an input signal of 1kHz at 500µV.
Distortion was very low. In MM mode set to 0dB gain and with a 1kHz signal 6dB higher than the nominal MM reference level (fig.4), the highest-level harmonic was the second, at –76dB (0.015%). Though other harmonics can be seen, these all lie at or below –90dB (0.003%). Distortion was even lower in MC mode, with the second harmonic lying at –82dB (0.009%), this with a 1kHz signal 6dB higher than the nominal MC level of 500µV. These spectral analyses were taken with the PH-1000 driving the high 100k ohm load. Commendably, reducing the load impedance to the current-demanding 600 ohms didn't increase the levels of the distortion harmonics.
The Gold Note PH-1000's performance on the test bench reveals that it is a well-engineered phono preamplifier, especially considering the enormous range of adjustments it offers. Both distortion and noise are very low in level. While its overload margin at the top of the audioband is relatively low, this can be increased by increasing the phono stage's gain without incurring any significant noise penalty.—John Atkinson
Fig.1 Gold Note PH-1000, MM, Stage mode, response with RIAA correction into 100k ohms (left channel blue, right red), with subsonic filter (left cyan, right magenta), and in Enhanced mode (left green, right gray) (1dB/vertical div.).
Compared with RIAA (fig.2, red traces), the various equalization options offer a wide range of modified responses (blue and green traces). The most extreme of these is "Telefunken," which peaks at 15dB at 11kHz compared with RIAA, "Columbia," which is –8dB at 10Hz and –3dB at 20kHz, and "Old RCA," which is +6dB at 10Hz and –2.5kHz at 1kHz.
Fig.2 Gold Note PH-1000, MM, Stage mode, response with RIAA correction into 100k ohms (left channel gray, right red) and with 8 of the 18 alternate EQ settings (left green, right blue) (2.5dB/vertical div.).
Fig.3 Gold Note PH-1000, MM, 0dB gain, Stage mode, spectrum of 1kHz sinewave, DC–1kHz, for 5mV input (left channel blue, right red; linear frequency scale).
The Gold Note's overload margins were affected by the gain setting. In MM mode with the gain set to 0dB, the margin ref. 1kHz at 5mV was a good 15.1dB at 20Hz and 1kHz but dropped to 3dB at 20kHz. Setting the gain to –6dB reduced the margins by 6.5dB, while setting the gain to +6dB increased the margins by 6dB. In MC mode, the overload margins were 20.8dB at 20Hz and 1kHz and 8.6dB at 20kHz, all ref. 1kHz at 500µV. These margins were calculated from when the THD+noise percentage reached 1% and were equivalent to the PH-1000's output level in Stage mode reaching just over 10V RMS.
Fig.4 Gold Note PH-1000, MM, maximum gain, spectrum of 1kHz sinewave, DC–10kHz, into 100k ohms for 10mV input (left channel blue, right red; linear frequency scale).
Intermodulation distortion with an equal mix of 19kHz and 20kHz tones, with the gain set to 0dB and at a peak input level equivalent to 6dB lower than the overload margin in this region, resulted in the second-order difference product at 1kHz lying at –47dB (0.5%). Reducing the input level by 6dB dropped the 1kHz product to –54dB (0.2%, fig.5).
Fig.5 Gold Note PH-1000, MM, 0dB gain, Stage mode, HF intermodulation spectrum, DC–30kHz, 19+20kHz into 100k ohms for 25mV peak input (left channel blue, right red; linear frequency scale).
Turning to the balanced line input, which has no EQ, gain, or input impedance settings: The gain in Stage mode was 0.76dB at the balanced outputs and 0.43dB at the single-ended outputs. Preamp mode with the volume control set to its maximum increased the gain by 8dB. The Line input preserved absolute polarity, and its input impedance was a usefully high 52k ohms. The line input's distortion was very low, with the second harmonic the highest in level at –96dB (0.0015%, fig.6).
Fig.6 Gold Note PH-1000, Line input, Stage mode, spectrum of 1kHz sinewave, DC–10kHz, at 1V into 100k ohms (left channel blue, right red; linear frequency scale).
