Liberty Audio B2B-1 phono preamplifier Measurements

Sidebar 2: Measurements

I measured a second sample of the Liberty B2B-1 (serial no. 1004), using Stereophile's loan sample of the Audio Precision SYS2722 system (see the January 2008 "As We See It" and The voltage gain of the Liberty set to its moving-magnet mode with its internal jumpers was 44.5dB; set to moving-coil, it was 64.2dB—both a little higher than usual, but still appropriate. The B2B-1 preserved absolute polarity (ie, was non-inverting) in both modes. The output impedance was a low 47 ohms across the audioband; the input impedance depended on the settings of the internal jumpers. With no impedance jumpers, the input impedance was 45k ohms at low and middle frequencies, dropping inconsequentially to 40k ohms at 20kHz. With the jumpers set to 500 ohms, the input impedance was 495 ohms at all audio frequencies; set to 330 ohms, it was 332 ohms; set to 220 ohms, it was 221 ohms; and set to 100 ohms, it was 101 ohms.

Specified as being within 0.1%, the RIAA accuracy was the best I have encountered, with excellent matching between the channels (fig.1). Right–left channel separation was excellent, at >90dB below 16kHz; though L–R crosstalk was about 12dB higher, this is still good separation. As MF noted, the Liberty is a very quiet preamp. The unweighted, wideband signal/noise ratio in MM mode, taken with the input shorted and ref. 5mV at 1kHz, was an excellent 76dB in both channels, improving to 90.5dB left and 92.1dB right when an A-weighting filter was switched into circuit. With the B2B-1 in MC mode, the wideband, unweighted ratios were an excellent 60.8dB left and 71dB right, these improving to, respectively, 75 and 83.8dB when A-weighted.

Fig.1 Liberty B2B-1, response with RIAA correction (left channel blue, right red; 0.25dB/vertical div.).

Not only had the Liberty's noise not been compromised by the slightly higher-than-usual gain, neither had its overload margins. In both MM and MC modes, the margin at all frequencies was close to 25dB (ref. 1kHz at 5mV and 500µV, respectively), which is excellent, considering that when the THD reaches 1%—our standard definition of clipping—the B2B-1 is outputting 14V RMS, which is well above the level that the partnering preamp would like to see.

Fig.2 Liberty B2B-1, MC mode, spectrum of 1kHz sinewave, DC–10kHz, at 2V into 600 ohms (left channel blue, right red; linear frequency scale).

The Liberty phono preamplifier was also a low-distortion design, at typical levels of around 0.04% across the audioband in both modes. As you can see in fig.2, which was taken into the demanding 600 ohm load, the only harmonic visible above the noise floor is the subjectively innocuous third. However, some power-supply–related spuriae are visible, albeit at a very low level. The presence of these spuriae was not affected by experimenting with the grounding between the Liberty and the Audio Precision or by floating the Liberty's ground; they can also be seen in the spectra of the B2B-1's reproduction of an equal mix of high-level, high-frequency tones (fig.3), though actual intermodulation distortion is vanishingly low.

Fig.3 Liberty B2B-1, MC mode, spectrum of 19+20kHz sinewaves, DC–24kHz, at 2V into 100k ohms (linear frequency scale).

Made in America but inspired by the work of legendary Hungarian/Norewegian electronics engineer Erno Borbely, the Liberty Audio B2B-1 is one of the best-measuring phono preamplifiers I have encountered. Nice. Very nice.—John Atkinson

Liberty Audio
380 Vernon Way, Suites I–J
El Cajon, CA 92020
(619) 579-6450