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12AX7 Tube Preamplifier Kit

Mark Houston    Australia Flag     To email Mark, type out the email address.

Page 1: Introduction, 12AX7 Valve Preamp, Kit Assembly
Page 2: Enclosure, Sound
Page 3: Measurements
Page 4: More Photographs


Measurements - 12AX7 Tube Preamp Kit

The frequency response of the tube preamplifier kit as measured by Silicon Chip Online is shown in Figure 03. As you can see, the frequency response shows a very slight rise in the low bass region and is just 1dB down at 180kHz.

12AX7 Preamp Measured Frequency Response
Figure 03 - 12AX7 Preamp Measured Frequency Response
(Figure courtesy of Silicon Chip Online)

The measured Total Harmonic Distorsion (THD) is very low for a tube amp. Figures 04 and 05 show the measured THD of the 12AX7 Tube Preamp Kit as measured by Silicon Chip Online.

Measured THD+N (%) vs Frequency
Figure 04 - Measured THD+N (%) vs Frequency
(Figure courtesy of Silicon Chip Online)

Measured THD+N (%) vs Output (V)
Figure 05 - Measured THD+N (%) vs Output (V)
(Figure courtesy of Silicon Chip Online)

The valve preamp is dead quite (absolutely no hum or buzz or anything at all) and my measurements of frequency response are similar to those measured by the manufacturer. Photographs 06 and 07 show the 100 kHz and 200 kHz sine wave response.

100kHz Sine Wave Response
Photograph 06 - 100kHz Sine Wave Response

200 kHz Sine Wave Response
Photograph 07 - 200 kHz Sine Wave Response

I know that there will be a lot of people who will be skeptical of a switch mode power supply in a tube preamp (as was I), so I took some measurements with my scope. The B+ measures a dead flat 261 V. The B+ is shown below in Photograph 08 at a resolution of 1 µV / cm and as you can see, there is no ripple!

Power Supply B+ (Resolution: 1 microV / cm)
Photograph 08 - Power Supply B+ (Resolution: 1 µV / cm)


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