Zesto Audio Andros PS1

Zesto Audio Andros PS1

Lab evaluation of Andros PS1 included measurements of both the MM and MC inputs. In the second case, we chose the single ended input adjusted for a 400 Ohm load.
The preamplifier requires 1.6mVrms in the MM input and 0.290mVrms in the MC input for an output of 240mVrms. Accordingly, the total gain is 43.5dB and 58.4dB respectively, close to the published specifications. There is a good overload margin, since the preamplifier requires up to 22mVrms in the MM input and up to 4.1mVrms in the MC input to clip the output signal, which happens at nearly 3.3Vrms. Good signal rooting practices are paying back through a very good crosstalk figure, at -83dBr, which is among the best we have seen lately in a phono preamplifier.
The de-emphasis curve implies a circuit with good accuracy with a maximum deviation in the range of 0.2dB (0.23dB/318mS), a much better performance than the official spec value of +/- 0.5dB given by the manufacturer. The same diagram shows a difference in gain between the two channels, about 1.4dB across the entire range of the measurement. Zesto indicate that, indeed, there can be such a difference and prompts the user to make some tube interchanging to reduce this to a minimum.
Output spectrum includes a strong second harmonic at about -55dBr and from that point up, things appear to be very quiet. Harmonics disappear very quickly under-100dBr, since the de-emphasis curve dominates the high frequency behavior. Given that the input signal includes both 1kHz and 1.5kHz tones (in the measured channel and the opposite channel respectively), an 1.5kHz residual is expected and confirms the static crosstalk measurement, with a level around -80dBr.

RIAA curve de-emphasis. 0dBr: 240mVrms/1kHz.

Output signal spectrum. Input 1kHz/1.5kHz. 0dBr: 240mVrms/1kHz.

Noise spectrum includes components at 50Hz and 100Hz from the power supply and a number of intermodulation products. Overall, the curve lies below-60dBr.
Generally, the Andros PS1 seems to be a rather quiet preamp. The static noise measurement was-58dBr (A) for MM and -43dBr (A) for MC with a reference level of 240mVrms. Those figures are equivalent to 0.3mVrms and 1.6mVrms of noise for MM and MC respectively, with the first value being in the vicinity to that given by the manufacturer (-75dBu representing approximately 0.13mVrms), given that no tube interchange has taken place.
The diagram of harmonic distortion (THD+N) versus frequency appears to be between 0.5% and 0.9% in the lower midrange and never exceeds 1% for most of the spectrum, above 1kHz. The static distortion measurements was 0.185% (THD, at 1kHz with an output level of 240mVrms) and 0.012% (IMD/SMPTE, same output level). Both these figures could be considered very good for the type of the device. Note that there is an expected difference between the diagram and the static measurement in harmonic distortion, since the latter does not include the noise of the preamplifier.

Output noise spectrum. 0dBr: 240mVrms/1kHz, inputs shorted.

Distortion (THD + N) versus frequency. Output level 240mVrms.

Finally, the graph of THD+N versus output level indicates that the output level for optimum behavior is between 300mVrms and 900mVrms. Above this point the distortion increases smoothly up to the limit of 3% (which is located just over 3Vrms) while, at lower levels, the noise begins to play its expected dominant role.

Zesto Audio Andros PS1, Lab Evaluation

Distortion (THD + N) versus level. Signal frequency 1kHz.

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