Why tube amps don’t have balanced inputs

June 9, 2021
 by Paul McGowan

7 comments on “Why tube amps don’t have balanced inputs”

  1. I can tell you about four companies that are making true balanced tube amplifiers.
    They are, Mcintosh Rogue Audio Cary Audio, and finally, VTL.
    I would name a fifth and a sixth one.
    But the reason why I won’t, is because I don’t really know if Manley and ARC are doing that as of today.
    ARC as far as I’m aware of, they once made truly balanced tube amps.
    I have seen quite a few of their tube amps in other people’s recording studios.
    And on top of that, they were very happy with them.
    One of them, God rest him, was Prince.
    He had 6 Audio Research 600watt mono block tube amps in his recording studio, at his house.
    His exact words to me were, “tubes are in deed the way to go!”
    I found out, that he was right too!

  2. I haven’t seen any classic amplifier schematics where the designer has used a differential pair to create a balanced input. A simpler approach is to use an input transformer. If the design already uses an input transformer for impedance matching, then it’s essentially zero additional cost to connect a floating input to the two ends of the primary.

    I’m somewhat puzzled by the definition of “fully balanced”. It seems that there isn’t enough information. Some designs use a balanced input with a differential amplifier or transformer, and a single internal amplifier chain. That’s surely not “fully balanced”? It would be possible in principle to connect each side of a balanced input to an independent amplifier chain, but it’s not obvious if this would be beneficial; for example, if the input wasn’t balanced, one amplifier chain would be wasted.

    1. Well, what do I know? Not much, obviously. The Audio Research Reference 600 (thanks John Price) uses both halves of a 6922 dual triode as a differential input stage and phase splitter.

      1. Good morning Mark!
        Check out the Cary Audio CAD120-SMKII all vacuum tube power amp.
        It uses a pare of 6SL7 tubes to do the same thing as the Audio Research Reference 600 does.
        The same thing is also true about the Mcintosh MC-275 tube amp.
        Except only, it uses a pare of 12AX7 tubes to do that.
        The same thing is also true about the large 650watt VTL mono block amps too as well.

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