Tube preamplifiers such as the 12AX7 series are currently very rarely used, because of the many new preamplifier products that rely more on solid state systems.

Between Tube And Solid State

The balanced sound generated from the solis state / op-amp preamplifier may already feel good, sound clean, calm and accurate. But sometimes I feel the results feel too simple. Because of that, I think it’s time to practice making a preamplifier device that has several nuances such as sweetness, warmth and have “taste of music.

“A tube preamplifier using a 12AX7 tube with a single ended system I m selected, and my plan for this preamplifier is:

  • 1. Can do some second harmonic distortion, less strange high harmonic distortion.
  • 2. Have sweet voice, too thin, fast (ie contemporary tube sound)
  • 3. Simple plan, several components
Tube Charactersistic

Planning And Progress

The first plan is to consider the reactive current and tube anode voltage. From the 12AX7 uniform plate characteristic curve, I chose the idle running point in a relatively linear region. From the red dot in the image is the selected control point, you can see that the network voltage is -1V, the board voltage is around 130V, and the board current is at 0.85 mA. I chose the loading line and highlighted it in blue. The preamplifier’s advantage is 51. In a normal listening configuration, you don’t need to go up 5V higher to your amplifier

Therefore, the input needed is only about 0.1V. This highlights the green in the image. So the green channel is based on where my preamplifier will run. Because it is a short way the distortion becomes low. In the case of high acquisition, low distortion occurs in reality.

Selection of components

from Telefunken 12AX7 tubes.

Jensen sterling silver line.

DACT volume control, which provides a very slippery switch.

dact volume preamp 12ax7

Tube amp, because this is relatively simple, the wiring event will be on the prototype board. I drew a piece of paper and finished it before the actual cabling at the planning agent.

prototypr board preamp 12ax7

Prototype board for mounting poles and headers.

Tube strength is a member of the collection forum, and the power amplifier provides a constant of 300V and 12.6V

Testing The Sound

Frequency response –

frequency test preamp 12ax7
Frequency responses

Harmonic distortion is exactly what I want: some 2nd harmonic distortion is 70 dB, with a third harmonic very low at -100 dB. RMAA measured by total harmonic distortion is 0.033%.

thd test preamp 12ax7

There is a simple plan to change space: the preamplifier gain is 51 “(34 dB), which is rather high. Output impedance can be reduced to 625 ohms, and a higher output current is needed. Therefore, I am aware of the plan. With 10,000 hectares of media others to submit the weaknesses above, see my 12AU7 and 6922 tube preamps.


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