Next build...01A preamp with center channel

Discussion in 'Tube Audio' started by analog addict, Aug 10, 2018.

  1. analog addict

    analog addict Glory or Death!

    Central NC - RDU
    Starting to look with designs for a new preamp with a real center channel. I don't have a tube designated for the mixer, but am leaning towards a 6SN7. HV PS is just roughed in so far, but will feature a VR tube regulating at 150V. The basic pre amp doesn't get much simpler than this. Thoughts?

    [​IMG]01A preamp by Analog Addict, on Flickr


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  2. kward

    kward AK Subscriber Subscriber

    In my opinion, the 01A has wimpy electrical characteristics for topology you depict. I'm not sure how well it is going to work as an outboard preamp driving a power amp through interconnect cables, but it depends on specifically what it will be driving. The 01A has rp of about 10K, and gm of only 0.8 mA per volt. By comparison, a 6SN7 has rp of 7.7K and gm of 2.6 mA per volt (3x as much transconductance). If the cathode/filament is fully bypassed, output impedance will be about 7K, which is high by any standard of an outboard preamp driving interconnect cables. Depending on how much output voltage swing you need, you could be creeping into slew limited territory with the 0.47 uF output coupling cap. How fast can the tube change the voltage across the output coupling cap? Under slew limited conditions, the preamp could sound sluggish and anemic. To minimize this, you would typically want to do one or more of the following: a) use a higher transconductance tube, b) idle the tube at a higher quiescent current, c) reduce required output voltage swing, d) make the output coupling cap smaller.

    Additionally, the other typical cautions are in order here: drive power amps with at least 10x greater input impedance than the output impedance of the preamp, and don't drive high capacitive loads.

    On the other hand, if you wanted to augment the basic preamp design by adding a direct coupled output buffer, it would eliminate all of these concerns in my mind. There might be other approaches to consider that might lessen the negative effects of some of these concerns, such as utilizing an active plate load.

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