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    Opinions For Using Two VTA ST-70 As Monoblocks


    Posts : 15
    Join date : 2019-12-28
    Location : Rochester, NY

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    Post by knotscott on Fri Feb 14, 2020 10:51 am

    Hi All - I recently completed a VTA ST-70 upgrade to one of my two Dyna 70s.  Now I'm beginning the same upgrades to the 2nd unit, so I plan on using both amps eventually.  The question is how best to use them together.  

    I've read up on horizontal vs vertical bi-amping modes, but wondered if anyone has tried removing the output tubes from one side of each amp, and just running them as single channel mono blocks using tri-wiring to the speakers?  I ran them in this mode for years before taking them out of service for the VTA upgrades.  The original logic was that the power transformer wasn't overly robust on the ST70, and using only half the outputs would reduce the load on the PT.  As with any mono blocks, there's always some improvement to left-right separation as well.

    The speakers are my own design, and were the culmination of many years of making and selling high end speakers as a local business.  They're average efficiency, use a SEAS 1" aluminum dome tweeter, Focal 5K013L midbass in an aperiodic chamber, and a Focal 8K412 in a transmission line.  Each driver has a separate input jack.  Crossovers are passive and proprietary using top grade poly caps, heptalitz inductors, and are wired with Kimber Kable.  They're darn transparent IMHO.  I don't play them very loud, so they seem plenty content from the 20 watts or so of the ST70 in triode mode.

    I'd appreciate your feedback on the pros and cons of these different approaches.


    Posts : 9
    Join date : 2018-02-11

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    Post by Wotan on Fri Feb 14, 2020 11:31 am

    Personally, my inclination would be just strap each one together.  You could parallel them--easiest to to--or drive them out of phase and series them--easy to do only if you have a balanced pre.  The main failure point in each amp is going to be the tubes, not the OT or PT.  With this setup the tubes will be working less hard than each amp only single channel tubed.  Also your plate voltages will tend to run higher with only one channel tubed, which may decrease tube life.   

    You could vertically bi-wire, but the amps on the bass end would be working much harder.  Or you could horizontally bi-wire, but you lose channel separation.  The performance advantages to bi-wiring (as opposed to bi-amping, which involves a low level crossover) are in my opinion minimal.

    With my suggestion you have immediate redundancy when you lose output tubes on one channel of one amp.  Though I'd want to pull the tubes out of the other amp for balance, and disconnect the non-functioning channel from both speakers.  Then you end up with your original setup until you can get more tubes, and you don't necessarily need to keep them in stock until you need them.

      Current date/time is Thu Feb 27, 2020 11:06 pm