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12AX7 tube pre-amp (rev. 2)




 

My Opamp based balanced preamplifier sounds good, it sounds very clean, quiet, accurate and very high resolution. But at the same time I found it a bit plain. So I think it is time for me to try something sweet, warm, and "musical".

 

Even order harmonics sound as musical chords (notably octaves), which subjectively makes the sound "richer". Odd order harmonics sound less pleasant. Aharmonic distortion is discordant and is often implicated in designs that sound "brash", "harsh", "brittle", etc. Push Pull amplifiers use two nominally identical gain devices "back to back". One consequence of this is that all even order harmonic products cancel, leaving the - subjectively less musical, less "rich" - odd order products to dominate. The total (measured) harmonic distortion content is lowered, but subjectively the design may sound worse. A push-pull amplifier is said to have a symmetric (odd symmetry) transfer characteristic, and accordingly produces only odd harmonics.

(Reference - Wikipedia - Harmonic_content_and_distortion)

 

A tube run in a single-ended configuration is chosen for this project, my design objective for this pre amplifier is to have :

  • Some 2nd harmonic distortion, with less odd or high order harmonic distortion
  • Sweet, detailed, and fast sound (i.e. modern tube sound)
  • Simple design with less components

My design is a single gain stage triode tube with cathode follower. Schematics :

 

The first design consideration is the tube's idle current and plate voltage. From the 12AX7's average plate characteristics graph below, I have picked a relatively linear region as the idle operation point. From the graph, the red dot is the picked operation point, you can see the grid voltage is at -1V, plate voltage is about 130V, plate current is at 0.85mA. I choose the loadline and highlight it in blue. The gain of the preamp is 51. In a normal listening setting you won't need more than 5V into your power amp. So the required input need is only about 0.1V. This is highlighted in green in the graph. So the green path is basically what my preamp will operate at. Since it is a short path the distortion is low. The fact that having a high gain actually helped to produce a low distortion.

With these numbers I can calculate the resistors values in my schematics by using ohm's law.

 

Let's start with the plate resistor. Assume there are enough current flow, we could drop all the available voltage across the plate resistor and none across the valve. Using V=IR, we have 300V=1.5mA * Plate R. So the plate resistor = 300V/1.5mA = 200k ohm.

 

Now for the cathode resistor. Since the grid voltage need to be at -1V compared to the cathode, and the grid is grounded via a 1M resistor, so the cathode voltage need to be at 1V above ground. At idle current of 0.85mA, the cathode resistor value = 1V/0.85mA = 1176 ohm.

 

The gain for this stage can be calculated by this formula :

Gain = (Tube Gain x Plate load resistor)/(plate load resistor + plate resistance + ((tube gain +1) * cathode resistor)).

 

With 12AX7, the plate resistance = 70k, tube gain = 100. Using these value in the above formula, the calculated gain is 51.

 

The output impedance of the 1st gain stage is equal to the plate load resistor paralleled with the plate resistance, so in my case it is about 51k, which is a bit high to drive a real load. So a cathode follower is needed to reduce the output impedance. With the cathode follower, the output impedance would roughly equal to 1/(tube's transconductance). For 12AX7, its transconductance is about 1600 micromhos. So the cathode follower's output impedance would be at about 625ohms

 

The design of the cathode follower is similar to the gain stage. Although the load is at the cathode, a loadline can still be drawn the same way as in the gain stage :

I have chosen a steeper loadline as I want to have the cathode to run at higher output current.

Cathode load resistor = 300V / 3mA = 100k ohm.

Cathode bias resistor = Bias voltage / Idle current = 1V/1.35mA = 740 ohm

 

At idle point, the tube will drop about 163V as seen from the graph. So the cathode idle voltage is equal to 300V - 163V = 137V. The grid will be at 1 V below the cathode so it will be running at 136V.

 

Components selection : 12AX7 tube from Telefunken.

 

Jensen pure silver wires.

 

DACT volume control, it provides very smooth switching action.

 

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