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Equipment Measurements

April 2006

Einstein The Tube Preamplifier: Measurements

All  preamplifier measurements are performed independently by BHK Labs. Please click to learn more about how we test preamplifiers there. All measurement data and graphical information displayed below are the property of SoundStage! and Schneider Publishing Inc. Reproduction in any format is not permitted.

Additional Data
  • Measurements were made at 120V AC line voltage. Measurements made on the right channel with balanced input and output connections unless otherwise noted.
  • This preamplifier does not invert polarity.
  • AC line current draw at idle: 1.57A
  • Input impedance at 1kHz:
    • Unbalanced input = 22k ohms
    • Balanced input = 44k ohms
  • Output impedance at 1kHz: 105 ohms
  • Gain,  volume at maximum:
    • Instrument loading, Lch/Rch = 3.53X, 11.0dB / 3.67X, 11.3dB
    • IHF loading, Lch/Rch = 3.50X, 10.9dB / 3.63X, 11.2dB
  • IHF Sensitivity, input volts for standard IHF output of 0.5V, IHF loading:
    • Lch/Rch = 143.0mV / 137.8mV
  • Output noise versus bandwidth and volume-control position, wideband/A weighted:
    • at maximum = 36.3uV / 10.2uV
    • at unity gain = 15.0uV / 3.3uV
    • at typical listening level (20dB below unity gain) = 6.8uV / 1.2uV
Measurements Summary

General

Einstein's The Tube line-level preamplifier is definitely a departure from the normal tube preamp. This unit doesn’t have a selector switch as such, but utilizes a separate tube circuit for each input. The outputs of these input tubes are all summed together and the selected input is chosen by enabling the tubes’ heater supplies. Ten of the 19 tubes of the unit are used for this purpose. The remaining nine tubes are for the main line section. The outputs of the test unit were balanced XLR only.

Chart 1 shows the frequency response of The Tube with the volume control set for unity gain for 0.5V input with instrument and IHF loading. In this chart, only the left channel is shown to show the effect of the IHF loading, which is pretty negligible. The two channels are tracking within 0.05dB at this point on the volume control. The bandwidth of this unit (as defined by the -3dB points) is less than 10Hz to over 200kHz. With a 600-ohm load, the frequency response was the same and dropped about 1.4dB, yielding an output impedance of about 105 ohms. This is pretty amazing performance for a tube circuit. After studying the circuitry, it turns out that the output stage utilizes four triode sections of two 6922 tubes paralleled per output phase. The number of tubes and the circuit topology explain the 600-ohm driving ability.

In charts 2A and 2B, the frequency response of the preamp is shown for both channels with the volume at maximum and at a typical listening position (20dB below unity gain). The high-frequency response does change a bit with volume-control position (not unusual), having the most high-frequency roll-off at the full volume position, which would never be used in normal circumstances. At the lower level of -20dB below unity gain, the high-frequency response is quite flat and remains so for further volume attenuation. Volume-control tracking is very good over the working range, generally being better than 0.5dB.

Chart 3 shows how total harmonic distortion varies with input level and frequency for both IHF and instrument loading. Again, we have a design in which the distortion vs. level is virtually the same for frequencies from 20Hz-20kHz and with instrument and IHF loading. For clarity, the data shown is for the right channel for instrument loading. Further, to my amazement, the distortion with a 600-ohm load was just a little higher than with the instrument load! This would definitely break the back of a usual tube circuit.

A spectrum of the distortion and noise residue of a 1kHz test tone at 0.5V output with IHF loading is plotted in Chart 4. AC-line hum harmonics here are very low. The signal frequency harmonics consist of a dominant second harmonic and some much lower-level higher-order components.

If I can put in my two cents’ worth, I listened to The Tube and found it to be one of the best-sounding preamps that I have had the pleasure of putting in my system.

Chart 1 - Frequency Response at Unity Gain with IHF and Instrument Loading


Red line = left channel with instrument loading
Magenta line = left channel with IHF loading

Chart 2 - Frequency Response as a Function of Volume Control Setting

Chart 2A - gain at maximum

Instrument loading
Red line = left channel
Magenta line = right channel

Chart 2B - gain at -20dB

Instrument loading
Red line = left channel
Magenta line = right channel

Chart 3 - Distortion as a Function of Output Voltage and Frequency


Instrument loading
Red line = 20Hz
Magenta line = 1kHz
Blue line = 20kHz

Chart 4 - Distortion and Noise Spectrum


IHF loading
Red line = spectrum of 1kHz test-signal distortion and AC-line harmonics at 0.5V output at unity gain.

 

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