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    Rewiring the ST-35 as a "mono-block"



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    Rewiring the ST-35 as a "mono-block"

    Post by Swede52 on Sun Aug 29, 2010 6:46 am

    Hi all you Dyna addicts!
    I'm Bo from Sweden and I own, among lots of other units, three (3) ST-35's of which I have plans of trying to rewire two (2) of them as "mono-blocks". Having been in contact with Kevin @ Dynakit Parts as well as Bob Latino before I start up this project I still have to ask you all if anybody has any experience whatsoever on this? As I see it there is no actual difference - electronically - between rewiring an ST-35 this way from doing it with a ST-70. WHAT would the difference be - in your opinion??? I appreciate ALL replies that will guide me onto the correct track - before I try making something work that evidentially won't - anyway...
    Nice to hear from you!


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    Re: Rewiring the ST-35 as a "mono-block"

    Post by GP49 on Mon Aug 30, 2010 12:19 pm

    There is no difference between paralleling the channels of an ST-35 and doing the same on an ST-70. On both, it works just as poorly. You do not get double the power because you are doubling the load on an already marginal power supply, and you invariably get higher distortion because the distortion-reducing negative feedback, which MUST be separate to each channel in order to function properly, gets mixed and combined so that it actually INCREASES distortion.

    If you want double the power of an ST-35, buy an ST-70. You may even be able to SELL an ST-35 in fine condition for more than a good used ST-70 would cost.

    If it's the sound of the EL84 tube that you like, but you want more power: with substantial effort and with a proper output transformer you can use the four EL84 output tubes of the ST-35 as a push-pull/parallel four tube output stage, preceded by a single, redesigned driver stage that can drive four output tubes. That would double the output power, if the power supply limitations are also addressed. If you do this, you should also add provisions for individual biasing and balancing of the four tubes. But it's a lot of work, probably with plenty of trial and error, and would completely devalue the ST-35 in the used equipment market.

    Last edited by GP49 on Tue Aug 31, 2010 10:12 am; edited 1 time in total

    Bob Latino

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    Re: Rewiring the ST-35 as a "mono-block"

    Post by Bob Latino on Mon Aug 30, 2010 12:56 pm


    I could not have said it better than GP49. Everything he said is on the money. When you parallel two channels the two feedback lines mix and you will usually get negative interactions that will reduce sound quality. Also, when you parallel two output transformers that do not have EXACTLY similar characteristics the same thing will happen (negative interactions) even if you remove the feedback line and run the amp without feedback (not a good idea by the way, but possible with some speaker loads). I stopped recommending that the VTA ST-70's and ST-120's could be used as monoblocks some time ago. If you want to use an ST-35 on ONE channel then I recommend that you BIAMP. Run one ST-35 on the left stereo channel with one of the amp's channels on the woofer and the other on the tweeter. Do the same thing with another ST-35 on the other stereo channel. Most newer speakers will allow this because they have separate woofer and tweeter inputs. If you have older speakers with only one input then you would have to get inside the speakers and get access to the crossover to allow you to feed each driver separately.

    If you want higher power and you still want "monoblocks" get a pair of Dynaco Mark IV's. You still get the 35 watts per channel and still get separate amps.


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