Spice and the art of preamplifier design

Posted on Feb 4, 2014

One of the principal choices in phono preamplifier design is between active (feedback) and passive equalization. The ideal feedforward preamplifier for a moving magnet cartridge would have about 40dB gain prior to the equalizer (more for low-output moving coil cartridges) and 20dB gain following it This isn`t really practical with the three 12AX7 sections available for the phono preamplifier.

Spice and the art of preamplifier design
Click here to download the full size of the above Circuit.

We could use two sections in a mu-follower configuration to get the 40dB gain, but since no extra gain is available for feedback, the gain of this stage is entirely dependent on the amplification factor of the lower 12AX7. This could result in an imbalance between channels if tubes are not absolutely identical. With the help of SPICE, we have been able to design a feedback phono preamplifier with flawless performance (above). Except for a few small changes, the circuit topology is similar to the `94 mod. We now use dc-coupling in the negative feedback circuit. Since there is little feedback at low frequencies, this makes less difference than it does for the line amplifier, but eliminating an unnecessary capacitor never hurts. We have eliminated the positive feedback resistor (RPFB in Fig. 1) between the gain stage cathodes. We have added resistor RPL at the output to prevent switching pops that can result from charging of output capacitor C5C. Most importantly, we have added 33 pF capacitor C1M between the grid (node 1G) and plate (node 1P) of input stage TU1. A capacitor connected between a grid and plate is called a Miller capacitor after the Miller effect: Its capacitance must be multiplied by the closed-loop gain of the tube to calculate the correct RC time constant. This capacitor stabilizes the feedback loop (very important because of the extremely high loop gain at high frequencies), reduces sensitivity to RF...

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