Tube Preamp Reviews

Editor: Thank you for the thoughtful review of the VAC Signature SE preamplifier. We are grateful that Robert J. Reina found the Signature to be "the most significant audio product I've ever hooked up to my reference system."

With regard to John Atkinson's concern about the low-frequency measurements of the moving-coil phono stage, with the Signature SE's MC input, the selected load resistance is reflected through a transformer designed specifically for the particular traits of MC cartridges. Many active electronic signal generators bog down when confronted with this sort of load at low frequencies, such that the test equipment may operate outside of the assumptions of its designers and yield incorrect readings. However, using an external known resistance (10 ohms, for instance) to make the measurement and monitoring both sides of the resistance, the generator's distress can be observed, and one can confirm that the input impedance of the Signature MC phono stage does not fall dramatically and is responsive to the load switch at 20Hz.

If, as one sweeps frequency during test, one does not watch the generator's output but watches only the voltage at the output of the known test resistor, one will conclude that the preamp's input impedance is falling, when in fact the generator's output is falling. If the test system has its own internal "known" resistance (as does the AP SYS2722 at 20 ohms minimum), this problem may go unnoticed.

The reported measurement of 36 ohms input impedance at 20Hz likely represents the signal generator's difficulty coupling to the preamplifier, possibly due to interactions between the reflected load and the AP's own output transformers. These problems do not occur with actual MC cartridges.

JA also notes a slight rise in moving-magnet input impedance at low frequencies. We are unable to duplicate this result with our Agilent test system or with the external resistance method, and there is nothing in the circuit design to account for a rise. The small impedance drop at 20kHz is, of course, due to the 100pF MM termination (loading) capacitance.

Three other measurement notes:

1) JA is correct: The traces of 60 and 180Hz signal are due to stacking the audio and power-supply chassis on the test bench; we recommend greater spacing in normal use.

2) A slight channel imbalance can occur over time due to tube drift; incorporating feedback would tend to suppress this, but in our experience would limit the sound quality.

3) The difference in nominal MC load vs the chassis markings is due to the SE's input transformer having a slightly different turns ratio than the regular Mk IIa's transformer (1:8 vs 1:10). We will address this, but in practical terms, JA is right: Use the setting that works best with your cartridge.

Thank you again for the interesting and thorough review. And Robert? You're right: You really should hear the Statement instruments!—Kevin Hayes, VAC/Valve Amplification Company

Unfortunately, the review sample of the VAC preamplifier had long since left my lab when I received Kevin's Comment, otherwise I could repeated MC-stage measurements along the lines he suggests.—John Atkinson