Balanced Phono Stage

The Balanced Phono Cartridge

three, but four output terminals, they lend themselves to the use of a balanced phono preamp. The referencing of ground is very flexible in this case, but would not have been if phono cartridges came with only three terminals, as the placement of the grounding point would have been fixed.
   Four output terminals means that each

Last month we covered the first half of reader Yoon's request; this month, the second half.



  Always appreciate your circuits with comprehensive explanations! I'd like to request a balanced line stage preamplifier that uses:
   1. Both (+) AND (-) B supply voltage.
   2. Subminiature tubes like 6111, 6112, or 5719 which
       are the last tube generation.
   3. Would be even more interesting with subminiature
       phono equalizer.

Thank you again for a great site.

Yoon
 

Phono outputs

channel's winding or piezoelectric element or strain gauge wire is left floating relative to the grounded case, the fifth terminal. We can attach one output terminal to ground or the other or we can reference ground to the midpoint of the two output terminals by using a two resistor voltage divider. If we reference ground to the midpoint, then a balanced  circuit can be fed a plus and minus input signal from the phono cartridge, as the phase of the output signal is effectively split in two by the midpoint ground referencing.

Why have a balanced phono preamp?
      Using a balanced preamp with a phono cartridge makes a lot of sense for the same reason it makes a lot of sense to use it with a microphone: immunity from noise. The output signal from a cartridge is miniscule compared to the signal levels coming from a tuner or CD player. Consequently, the gain must be great in a phono preamp, which means that in an unbalanced preamp the noise signal (along with the desired signal) that is superimposed on the leads connecting the cartridge to the preamp will be greatly amplified. But not so in a balanced system. Here the noise, which is common to both the plus and the minus leads, drops out of the amplification process because of the inability of a balanced preamp to amplify what is common to both of its inputs, its common mode rejection ratio (CMRR).
   An additional advantage is the inability of a balanced preamp to amplify much of its own power supply noise. As one stage cascades into the next, its portion of power supply noise is treated as a common mode signal and ignored. Even the last stage's noise contribution will tend to cancel in the line amplifier, if that stage is also balanced in design.
   A further advantage of all balanced circuits is the ease with which the output phase can be switched from non-inverting to inverting, as both phases are already present. A nice touch would be to use a three position toggle or rotary switch to allow going from inverting to mute to inverting, as the ear requires a small pause in between switching signal phase to reorient itself. The switch could be wired to the outputs, with ground as the center position option.
     Because all (well, almost all) phono cartridges have not

Three places to reference the ground with a phono cartridge

Balanced Moving Magnet Preamp
  The first issue to tackle is gain. How much? Most moving magnet and variable flux cartridges need at least 40 dB of amplification to be brought up to line level, i.e. 1 : 100. Now 40 dB is a good amount of gain and it is achievable with tubes, but as a phono preamp must also equalize the output signal to conform to the RIAA equalization curve, more gain is needed. In fact, about ten times more gain is needed. This puts our goal at 60 dB of gain. Getting 60 dB of gain, i.e.  1 : 1000, from one stage is almost impossible with tubes, but quite doable with two stages, as each stage need only provide 30 dB of gain, i.e. 1 : 31.6.
   The long tail Differential amplifier is both the most obvious and best circuit for the first stage of amplification. Gain of this circuit is set by:
         Gain = ÁRa / (rp + Ra).
    A negative power supply could easily be used, but as many readers will be tempted to build this circuit in to an existing preamp case, it is unlikely that they will find a negative power supply to use. Consequently, this circuit was designed to work with a single power supply voltage. If a negative is used, then the tricks used last month's article on the balanced line stage could be summoned for use in this phono preamp.
  Passive RIAA equalization is easy to implement. All that

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