Herron Audio VTSP-1 preamplifier & VTPH-1MC phono preamplifier Measurements, Herron VTPH-1MC
The Herron phono stage's voltage gain, driven by the Audio Precision's 25 ohm source impedance, was a high 67.7dB. Despite this, the VTPH-1MC's noise floor was very low in level (fig.6), with power-supply-related spuriae more than 86dB below a nominal 2.5V output. The A-weighted S/N ratio, ref. 1V output, was 87dB, this decreasing to 70dB unweighted from 10Hz to 500kHz---in other words, excellent.
Fig.6 Herron VTPH-1MC, spectrum of noise floor ref. 2.5V output, input physically shorted.
The phono stage inverted absolute polarity. Its input impedance measured 7570 ohms at 1kHz, lower than the 47k ohms I was expecting. The source impedance was 284 ohms at 1kHz, rising to 333 ohms at 20kHz and 872 ohms at 20Hz. This will be fine with line stages having input impedances of at least 20k ohms. The matching VTSP-1 will hardly load the phono stage's output at all.
RIAA error (fig.7) was a little higher than specified, with a -0.2dB depression in the mid-treble and a 0.2dB rise above 30kHz. The latter is inconsequential, but the former might just be audible as a slightly laid-back quality, as it covers around three treble octaves. You can also see from this graph that there is a channel imbalance of about a quarter of a dB, this easily corrected with the line stage's balance control.
Fig.7 Herron VTPH-1MC, RIAA error into 100k ohms at 750mV into 100k ohms output, driven by Audio Precision System One with 25 ohm source impedance. (Right channel dashed, 0.5dB/vertical div.)
As well as low noise, the Herron phono preamp has low distortion. This can be seen in fig.8, which shows how the THD+noise percentage changes with increasing output voltage at three frequencies: 20Hz, 1kHz, and 20kHz. At all three frequencies, the graphed figure below 1V output is dominated by the very low-frequency noise seen in fig.6. The increase in distortion above 1V is as well controlled as in the Herron line stage, though, as is usual, the 20kHz distortion increases more rapidly than at lower frequencies. The 1% clipping points were 23mV at 20kHz, 8mV at 1kHz, and 685µV at 20Hz, these equivalent to overload margins of 13.25dB, 24.1dB, and 22.7dB, respectively, referenced to a standard MC level of 500µV at 1kHz. The lowish margin at 20kHz suggests that the MC version of this preamp should not be used with a MM cartridge.
Fig.8 Herron VTPH-1MC, distortion (%) vs output voltage (V) into 100k ohms with (L-R) 20kHz, 20Hz, and 1kHz signals.
Finally, fig.9 shows that, like the Herron line stage, the VTPH-1MC's distortion is primarily the innocuous second harmonic, at just under -60dB (0.1%). The highest-level power-supply component is at 180Hz, but this is way down in level, at around -83dB (0.007%).
Fig.9 Herron VTPH-1MC, spectrum of 50Hz at 2.5V RMS into 100k ohms.
It was a pleasure to measure such a well-engineered phono stage.---John Atkinson