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Megatron Electrostatic Amplifier

Discussion in 'High-end Audio Forum' started by nopants, Jul 7, 2015.
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  1. JimL11
    I just found this thread.  Appreciate NoPants subjective impressions.  
     
    If I was to design/build a Megatron-type amp I would probably use different front end tubes, like a 6SL7 for the input diff amp/LTP and 6SN7 for the driver tube.  To me, the advantage of a high gain tube in the first stage is it makes the gain similar for the two phases regardless of whether it is driven balanced or unbalanced.  Since my system is single-ended that makes a difference.  The 6SN7 is a better driver tube than a 12AX7, which is kinda wimpy.  Furthermore the 6SL7 and 6SN7 are more linear than their 12AX7/12AU7 counterparts.  Not sure why Kevin decided to do 12AU7 for the input and 12AX7 for the driver rather than the other way around, although to be fair, the Jadis power amps use a similar input topology with the same tubes in the same order and have a good reputation for sonics.  If I had to guess, I would think that the reason for using the 12AU7 first is to keep the gain between the two phases close, since there is no overall feedback to the first stage.  There is overall feedback between the second stage and the output stage, and the 12AX7 provides high overall gain to allow the closed loop gain to be identical (within the tolerance of the feedback resistors) between phases.
     
    I also agree with Frank that I'd use solid-state current sources, e.g. cascoded MOSFETs for the output current  loads as they are much closer to the ideal constant current than a single EL34 power tube.  However, that would mean there would have to be some additional circuitry to adjust the balance and output voltages, whereas with the Megatron because the top and bottom are identical there is a self-balancing to keep the output voltage near zero.
     
  2. JimL11
    Actually the ES-X (which is based on the Stax SRX circuit from the 1970s) is a quite different topology from the Megatron.
     
    The SRX/ES-X is a two stage all differential design, a cascode differential (or LTP if you use it single ended) input stage AC coupled to a differential output stage, with overall feedback from output to input.
     
    The Megatron is a three stage design with a differential first stage AC coupled to a current source loaded second stage which is AC coupled to the output stage. The output stage is self balancing because the output tube and the current source tube are configured exactly the same, so if the tubes are identical the output voltage should sit 1/2 way between B+ and B-.  The second and third stages form two separate single-ended amps driven with opposite phases, rather than differential pairs.  Thus, unlike the SRX, especially with current source tails, the overall current draw is not necessarily constant.  Also, although there is feedback from the output stage to the second stage, there is no overall feedback between output and input.
     
  3. Pale Rider Contributor
    Just began listening to my Mjölnir Megatron today. Beautiful construction, a pleasure to look at, and even more so to hear. Haven't done any critical listening yet, because it is a bit of pain to swap the amps back and forth on the MSB. Simon [@isquirrel ] told me to get two outputs on the MSB; probably should have done so. Anyway, on first listen, I cannot say it is better than the Carbon CC, or worse, but it is different. Will have to listen some more. Damn.
    IMG_7871.jpg
     
    IMG_7872.jpg
     
    HemiSam and Pokemonn like this.
  4. Rossliew

    So you are the one who grabbed the Megatron :wink: Was sold pretty quickly once it was put up for sale ...
     
  5. Pale Rider Contributor
    Yeah, that was me. Been running her all day, and man does she get warm.
     
  6. Rossliew
    Can't wait for your impressions, Greg!
     
  7. GarageBoy
    What power supply schemes are you guys running? I see there's pcbs on eBay for cheap, but are they to spec (any mistakes etc?)

    The schematic doesn't look to crazy to point to point
     
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