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    Direct Compare of my VTA ST-70 and my VTA Mark III

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    edgobb

    Posts : 60
    Join date : 2010-11-09
    Location : Texas

    Direct Compare of my VTA ST-70 and my VTA Mark III

    Post by edgobb on Wed Apr 29, 2015 11:37 pm

    I recently finished one of my VTA Octal Mk III mono blocks.  All my voltages check out as expected with my 122V mains through my Sencore Powerite bringing it down to 117V.

    I decided to A-B compare it to my ST-70 with the older high gain driver board.  I plugged the left channel from my SP-12 into the Mk III and ran the right through the ST-70 and used the preamp's separate gain controls to listen alternately to each channel to compare.

    The sound characteristics were immediately apparent, but my question is about volume. To my surprise, the volume levels of each amp were the same as I switched back and forth. I expected the Mk III channel to be louder at the same preamp volume level than the ST-70.

    Can you fine gentlemen help me to understand where my logic is flawed in this matter?  35WPC vs 60WPC...how does the power difference translate into tangible, real world differences?  

    Thanks for whatever insight you may supply.

    Best to all,
    Ed

    Kentley

    Posts : 300
    Join date : 2015-03-06
    Age : 64
    Location : Worcester, MA

    Re: Direct Compare of my VTA ST-70 and my VTA Mark III

    Post by Kentley on Thu Apr 30, 2015 1:20 am

    Your experiment was well-planned, and shows graphically what I believe is a common mis-conception about "watts".
    A 60 WPC amp doesn't use much of its potential until you either drive it very loud OR on punishing transients. Or if you are driving inefficient speakers. Most of our listening is done with 10 WPC or less. In other words, "WPC" describes the upper potential of the amp before clipping. Even then, sometimes the symptoms of clipping only become clearly audible when the output stages are grossly overdriven.
    In short, what seems confusing is not at all - given a relatively equal driver board, the amps should sound the same UP TO A POINT.
    Hope this makes a tad of sense. It is an excellent question.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Direct Compare of my VTA ST-70 and my VTA Mark III

    Post by Bob Latino on Thu Apr 30, 2015 7:45 am

    Hi Ed,

    The VTA ST-70 although rated at 35 WPC like the original Dynaco ST-70, is actually a 45 WPC amp. The difference between 45 watts and 60 watts is maybe 1 dB which is almost a negligible difference. Also > One amp may have a different amount of gain than the other which may further complicate any comparison between the two amps. See independent test link (done by a customer) of a VTA ST-70 amp > 45 WPC in ultralinear mode and 21 WPC in triode mode.

    VTA ST-70 independent test link

    Bob

    edgobb

    Posts : 60
    Join date : 2010-11-09
    Location : Texas

    Re: Direct Compare of my VTA ST-70 and my VTA Mark III

    Post by edgobb on Thu Apr 30, 2015 10:59 am

    Thank you for the discussion.  That all makes perfect sense.  I knew that things were were functioning as designed, but the initial reconciliation of my--albeit flawed--expectations vs reality came as a surprise.

    I have a pair of RBH studio monitors in the system while I'm finishing my new speaker build and wondered how they will affect the overall output when I can repeat the experiment.  Link to speaker design I'm working on if interested... https://sites.google.com/site/undefinition/diy-sunflowers

    As to the overall sound differences, the Mk III did away with some of the harsh highs that I experience in my particular system and listening environment.  A very welcome change for me.  With essentially the same topography in the amps but with different tube sets it is an interesting exercise to directly compare the characteristics of the tube types in each.  It allows the opportunity to compare EL-34/6L6/KT-66 group and the 6550/KT-88/KT-120 family to each other.

    An interesting exercise overall with more fun to be had.

    Maybe somewhere in the back of my head I imagined myself being blasted backwards like Michael J. Fox in Back to the Future when I turned up the volume on the Mark III.



    Thanks again and I look forward to anyone else's input.

    Ed

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