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Thread: Eico ST70 OT feature or flaw?

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    Eico ST70 OT feature or flaw?

    As you can see on the schem. below, the Eico ST70 has it's OT wired so that the 4ohm tap is grounded, not the com. tap. There are lots of online posts, web sites, etc. that say that is a mistake and should be changed. I'd like to know what the consensus here is. Mistake or design feature? Anyone else ever do it that way?
    Thanks
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    Lifetime Member Enzo's Avatar
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    I don't know who else might have done it, but I don't see it as a mistake. Note the OT is wired with C (common), 4 8, and 16 ohm taps. The speakers themselves do not reference to ground. You are used to seeing one side of a speaker grounded, but they only do that for circuit convenience.

    The voltage between the 4 and 16 ohm taps is what they use as NFB. So they grounded the 4 ohm tap to reference the 16 tap signal to ground for NFB. You could pick any tap you want to ground or ground none at all. They chose 4 ohm.

    It also seems that they used the arrangement to create a center speaker signal.

    But the first most essential question to ask yourself is this: what problem are you having that rewiring this thing would solve?

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    Old Timer Leo_Gnardo's Avatar
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    As Enzo mentioned, some hifi manufacturers did it this way in order to provide a feedback signal of the preferred polarity off the "0" winding. I've seen it done in some Scott amps. But no such thing in the Eico. If you insisted, you could put the 0 winding at ground, take the fb from the 4 ohm winding, and wind up in the same place electrically. OTOH, as Enzo notes: the first most essential question to ask yourself is this: what problem are you having that rewiring this thing would solve? If none then leave 'er be. Save time & don't bother solving problems that don't exist - especially when tempted to by some so-called experts on the interwebs. Lots of dodgy ideas floating around out there.

    FWIW ARC offered a mod to the Dyna Stereo 70 in an article in Audio Amateur way back when, about 40 years ago. In their redesign, one of the features is changing from having the 0 tap at ground. 4 ohm winding goes to ground instead and 0 tap to a feedback circuit. Well, they revamped the drive circuit entirely and this scheme fits well so no complaints here. Just to show, in a certain mod the move was made in the opposite direction. That's not to say it's universally applicable, it isn't.

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    Thanks guys. That's what I was thinking. It sounded pretty good to me the last time I used it, about 10 years ago. I'm re-capping it now. and found lots of advice online that I wasn't sure about.

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    Enzo, your mention of a center speaker reminded me of the Fischer 500c that I owned as a teenager. I just looked up it's schematic, and it has the 4 ohm tap grounded too. Thanks again. I'll be leaving all that stock.

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