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12AX7 Tube Preamplifier Kit

 Mark Houston  Australian 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


Introduction - 12AX7 Tube Preamp Kit

Using a K-12 tube amp with a NAD C542 CD player, there is no need for a buffer or line level preamplifier. The K-12 is a very polite and sweet amp with all the warmth of it's "hot bottle" heritage. However, when I compare the K-12 with my monoblock chip amps (LM3875), I was never convinced that the chips amps really delivered the passion of the music which the K-12 does so easily. There was always something missing when it came to a convincing delivery. I felt a little of that beautiful valve warmth was required, so I tried a very affordable Valve Preamp kit from Silicon Chip Online.


12AX7 Valve Preamp Kit

The preamp kit is based around a common 12AX7 dual triode. The line level (Hi-Fi) version of the kit has the gain set at 4 (6 dB). What makes this kit very inexpensive is that power comes a switch mode power supply, fed from a 17 volt DC wall wart! Photograph 01 shows a stock build of the kit.

Stock Build of 12AX7 Line Level Preamplifier Kit
Photograph 01 - Stock Build of 12AX7 Line Level Preamplifier Kit
(Image courtesy of Silicon Chip Online)

The schematic of the preamplifier section is shown in Figure 01. (Please note that this circuit is © Silicon Chip Online and permission to host the schematic on this site has been provided by Silicon Chip Online.

12AX7 Preamp Schematic
Figure 01 - 12AX7 Line Level Preamplifier Schematic
(Schematic © Silicon Chip Online)

The schematic of the switch mode power supply section is shown below in Figure 02. (Please note that this circuit is © Silicon Chip Online and permission to host the schematic on this site has been provided by Silicon Chip Online.

Power Supply Schematic for 12AX7 Preamp Kit
Figure 02 - Power Supply Schematic for 12AX7 Preamplifier
(Schematic © Silicon Chip Online)


12AX7 Preamp Kit Assembly

Prior to building the preamplifier kits, I purchased some nice Sprague capacitors in order to upgrade the signal path capacitors. Photograph 02 shows the completed power supply and preamp module with the upgraded capacitors.

Power Supply and Preamp Modules
Photograph 02 - Power Supply and Preamp Modules

Once I had put together the boards, I proceeded to add snubbers to the preamp and power supply sections. Photograph 03 shows the snubbers added to the underside of the PCBs.

Snubber Capacitors
Photograph 03 - Snubber Capacitors


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Enclosure, Sound