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Can all active preamps be modified so they can be made switchable to passive?

Discussion in 'd.i.y.' started by Rosewind, May 22, 2020.

  1. Rosewind

    Rosewind Lost in Translation

    In the past I have seen a few preamps that have a switch where they can be "passified" (used as a passive pre) or activated (used as an active pre) at the flick of a switch. Schiit Saga is just one example of this.

    So the question is whether all preamps - including my unused Michell Argo HR - can be modified with a switch to be used as a passive pre with the active preamp section bypassed?

    I wonder about this as it would be extremely useful to have that versatility in one box given the fact that some power amps, like the Leak Stereo 20, work the best when driven by a passive preamp, whereas other power amps seem to need the gain or simply sound that little bit better when driven by an active preamp.

    I am going to have a new volume pot fitted to my Michell Argo HR and wonder if it would be possible to have a passive mode option made at the same time.
     
  2. david ellwood

    david ellwood Kirabosi Kognoscente

    In much the same way as any car can be converted to run on steam.
     
    glenn jarrett and Rosewind like this.
  3. Rosewind

    Rosewind Lost in Translation

    Thanks for that comment, David! So leave well alone and use two separate preamps is the message.
     
  4. JensenHealey

    JensenHealey pfm Member

    No

    A passive 'preamp' is entirely a misnomer. It is really just a simple potentiometer (the volume control) perhaps with some added input selection switches and maybe the pot is electrically powered so it can be controlled remotely.

    Active preamps are littered with any number of active stages and the potentiometer volume control is (usually) buried in the middle somewhere. There might be the odd exception where the volume is right at the start of things and with appropriate modification (perhaps needing some relays) could be turned into a passive version.

    If most designs were easy to switch, you would expect to have seen more versions with an active/passive switch.
     
  5. awkwardbydesign

    awkwardbydesign Officially Awesome

    If it's not designed for it, it would probably be less work to build a passive! I have a Concordant pre where the volume control is on the output, not the input, so that would be tricky.
     
  6. awkwardbydesign

    awkwardbydesign Officially Awesome

    Not quite. There are (and I have) TVCs and AVCs. Plus, of course, switched attenuators.
    One of the preamps I have built recently has a pot on the input, and the other has a TVC on the output.
     
  7. Rosewind

    Rosewind Lost in Translation

    JensenHealey - I know that part. I own a single input passive unit in a plastic box, a diy buffer pre that is actually quite active and the active Michell Argo HR.

    EDITED.
     
  8. Rosewind

    Rosewind Lost in Translation

    So a resounding "no" - not all preamps can be made passive (inputs-wires-volume pot + selector switch-wires-output). Better then to have two preamps, which is what I will make sure I have then.
     
  9. JensenHealey

    JensenHealey pfm Member

    True, I had forgotten about TVC types.
     
  10. mandryka

    mandryka pfm Member

    This makes me realise that I don’t understand what a preamp is. What sorts of things do these active stages do? Is it a matter of selectable boosts and filters like on old quad preamps, or what?
     
  11. JensenHealey

    JensenHealey pfm Member

    Nowadays, CD players and DACs chuck out a big signal that strictly does not really need amplifying before the power amplifier. It just needs a volume control - so passives can work here.

    But Phono signals from your turntable cartridge are very small signals. Amplification is necessary. And you have to apply the RIAA response correction, which itself can be any combination of active/passive.

    All other Hifi sources (tape players both cassette and reel-to-reel, tuners, graphic equalisers for instance) also only output a quite a low voltage which needed amplifying before the power amplifier.

    Within the preamp other active stages also buffer signals, a sort of isolation function, to supply the output that goes to the power amplifier. When we all had cassette players, they were driven from other outputs of the preamp (sometimes buffered too) and switching was in place to provide replay and/or monitoring, also with active buffer stages.

    Then you might have tone control circuits, or loudness controls too of many variations. These things are mostly 'active' too, even if it is just more buffering circuitry.

    A comprehensive preamp then ends up with a several-link chain of active stages, sometimes with passive bits along the way. Each stage is optimised for the bit it does.
     
    mandryka likes this.
  12. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yes many/most (not all!) active pres can be made switchable to passive. The Musical Fidelity Preamp 3a has passive and active output sockets as standard FWIW and is a very good sounding pre.
    It involves drilling holes etc to add a switch and often further holes in the rear panel for extra output sockets. The physical size and layout of the pre will then often determine the practicality of it.
    A full function valve hybrid pre amp (with MM phono stage) I'm building for someone ATM has both passive and active outputs from its inception, with Seiden selector switch and Khozmo switched attenuator.
     

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