Sidebar 3: Measurements
I measured the Leema Acoustics Essentials phono preamplifier (serial number ES140704B) using my Audio Precision SYS2722 system (see www.ap.com, and the January 2008 "As We See It"). As always with phono stages, I experimented with the grounding between the Essentials and the Audio Precision system to get the lowest level of noise.
The voltage gain at 1kHz was 35.7dB with the Leema set to MM (moving-magnet), and 61.2dB when set to MC (moving-coil). Both values are very close to their specifications, and the preamp preserved absolute polarity (ie, was non-inverting) at both settings. The input impedances for both the MM and MC settings were also very close to spec. The MM impedance was 46k ohms at 20Hz and 1kHz, dropping inconsequentially to 40k ohms at 20kHz; the MC impedance was 92 ohms at all audio frequencies. The output impedance at 20kHz and 1kHz was very low, at 60 ohms, rising to a still low 432 ohms at 20Hz due to the presence of a coupling capacitor.
The Leema's RIAA equalization (fig.1) was well matched between the channels, and flat from 30Hz to 1kHz. Above 1kHz the response sloped gently up, reaching +0.25dB left and +0.4dB right at 20kHz. This should not be audible. At the other end of the audioband, the output was down by 1dB at 10Hz without the Bass Cut switched into circuit (blue and red traces), –1dB at 43Hz and –3dB at 22Hz with Bass Cut active (cyan, magenta).
With low RIAA error, noise, and crosstalk, the Leema's distortion was also low. Fig.2 shows the spectrum of the Essentials in MM mode, driving a 1kHz tone at 1V into 100k ohms (equivalent to an input level of 17mV). The only harmonics visible above the noise floor are the second, at –76dB (0.015%), and the third, at –99dB (0.001%). The picture wasn't significantly different in MC mode. While reducing the analyzer's load impedance to a very low 600 ohms raised the level of the second harmonic, it still lay at –60dB (0.1%), which, given the innocuous nature of second-harmonic distortion, is low enough. The Essentials also had low intermodulation distortion (fig.3), the second-order difference product resulting from an equal mix of 19 and 20kHz tones lying at –81dB (0.01%). However, there is a peculiar rise in the noise floor visible in this graph between 25kHz and 30kHz.
It was a pleasure to measure a product as well sorted as Leema's Essentials phono preamplifier, especially considering that it sells for less than $1000.—John Atkinson
Fig.1 Leema Acoustics Essentials, response with RIAA correction (left channel blue, right red) (1dB/vertical div.).
The Leema's channel separation was very good, at 80dB in both directions above 1kHz. The Essentials was very quiet, its wideband, unweighted, MM signal/noise ratio (ref. 1kHz at 5mV, taken with the input shorted) measuring 70dB, and improving to 81dB when A-weighted. The MC ratios (ref. 500µV at 1kHz) were almost as good, at 66dB and 76dBA.
With its lower-than-usual gain in MM mode, it came as no surprise to find that the Essentials had a superb overload margin: about 25dB throughout the audioband. It was almost as good in MC mode, at 22dB across the band.
Fig.2 Leema Acoustics Essentials, spectrum of 1kHz sinewave, DC–10kHz, at 1V into 100k ohms (linear frequency scale).
Fig.3 Leema Acoustics Essentials, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 1V peak into 100k ohms (linear frequency scale).
