Tube Preamp Reviews

Sidebar 1: Measurements

The output impedance of the Jadis JPL at its line output measured 990 ohms left, 1000 ohms right, with the level control at maximum; lower settings of the control changed this measurement by only a few ohms. The input impedance at the CD input was very high—this is generally desirable, as it minimizes the effect of the source's output impedance—measuring 750k ohms in the left channel at maximum gain, somewhat higher in the right channel and at lower settings of the level control. At such high readings, the absolute accuracy of the measurement is rather unreliable; a very small change in the output voltage readings on which the calculation is based can result in a large change in the result. The important point here is that the input impedance of the Jadis at the CD input is unlikely to be a factor in matching to other components. The input impedance at the auxiliary inputs was far lower: 37.5k ohms in the right channel, slightly higher in the left. The latter is more typical, and still unlikely to cause problems with most sources. The output impedance at the tape output was 1.4k ohms regardless of the source impedance feeding the preamp, indicating some kind of buffering at the tape outputs.

The JPL is non-inverting at both its tape and line outputs; its DC offset measured an inconsequential 0.1mV on both channels from all outputs. The gain (input to line output) measured lower than specification at 16.3dB from its CD inputs and 19.3dB from its aux inputs.

The JPL's frequency response (fig.1) was taken at the CD input with the level control at maximum gain. At unity gain (approximately 11 o'clock on the level control through the CD input) the response dipped slightly at higher frequencies (not shown), but the change was small: less than –0.1dB at 20kHz and –0.2dB (L) and –0.35dB (R) at 50kHz. The response through the aux inputs (not shown) was even flatter, with zero rolloff visible at 10Hz and even less rolloff above 20kHz at unity gain (10 o'clock level setting with the aux inputs) than that observed under the same conditions from the CD input. Measurements taken at several different gain settings showed excellent tracking of the level control, never worse than 0.1dB difference between channels down to a level of 10dB below unity gain.

The JPL's crosstalk (fig.2) at unity gain is just slightly less than at full gain, much of the difference due to noise. Crosstalk through the aux inputs is a bit better than that through the CD inputs. Even the latter, however, is more than acceptable, if somewhat greater than typically seen with a solid-state preamp.

THD+noise vs frequency is shown in fig.3. The distortion for the CD input is slightly higher, but still very good. Observing the 'scope trace of the distortion product showed the latter to be primarily second harmonic plus noise from the CD input, and primarily noise from the aux input.

Finally, the Jadis would accept a signal level of 1.47V at the CD input before reaching 1% THD+noise (with an output of 9.606V). At the aux input, 5.6V could be input to the JPL—with an astonishing 51.6V output—before a reading of 1% THD+noise was reached.

As with many tube preamps, the JPL's slightly high output impedance, though not high enough to be a serious concern, suggests that some care should be taken in matching it with a power amp. The Jadis JPL's overall measured performance is first-rate.—Thomas J. Norton

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Footnote 1: Some readers were concerned by Mike Moffat's dissing of the 12AX7 last October. Rest assured, gentle Stereophiles, that a designer of talent can wring magical sound out of anything that conducts electricity.—John Atkinson