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    ST-70 Input Signal

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    mazeeff

    Posts : 122
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    ST-70 Input Signal

    Post by mazeeff on Tue Jan 28, 2014 6:37 am

    By way of introduction, my name is Mike, and I recently acquired a 1967 ST-70 with original transformers, and the following mods.

    diytube driver board
    SDS cap board w/ SS rectification (B+ close to 450v)
    Svetlana EL34's

    I have been trying all manner of different input options, searching for the perfect sound for me. It is quite surprising to me that different DAC/speaker combinations can sound so different.

    My question concerns the maximum input signal level for the ST-70. If I were to run a CD with 3V output into the input with a volume pot, what happens if the input is driven to the full 3V?

    Will the amp clip or distort, or will it somehow regulate itself through NFB or other circuit?

    Thanks,

    Mike

    Bob Latino
    Admin

    Posts : 2414
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 Input Signal

    Post by Bob Latino on Tue Jan 28, 2014 10:01 am

    Hi Mike,

    The Diytube driver board for the Dynaco ST-70 only needs a little over 1 volt for full output. If an ST-70 with that Diytube board looks at 3 volts, the amp will be overdriven and the output will be quite loud and severly distorted. You should never overdrive a tube amp like that.

    If you want to drive any ST-70 directly from a CD player, you should use some type of passive preamp or a stepped attenuator like the VTA stepped attenuator. The use of either will "knock down" the voltage coming from the CD player to a reasonable level that the input of the ST-70 can use without being driven into distortion.

    Bob

    mazeeff

    Posts : 122
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: ST-70 Input Signal

    Post by mazeeff on Tue Jan 28, 2014 10:32 am

    Thank you Bob. I have a old Onkyo 787 that I will use as a pre-amp. I tried it the other day, and found that by using a powered Veledyne sub with it, my ST-70 sounds much better to me. The preamp routes everything below 80hz to the sub, and the ST-70 gets everything above that. I am using BOSE 301's with the ST-70, until I can afford something better!

    Mike

    sKiZo

    Posts : 1312
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST-70 Input Signal

    Post by sKiZo on Tue Jan 28, 2014 4:36 pm

    The ability to adjust the input either via pots on the amp or a pre is a good thing for matching your equipment.

    I added volume pots to my ST120 build to attenuate input and it's been real handy. Not the only adjustment available either. I drop the db out from my HTPC by about 2.5, then adjust the DAC three clicks down, then run the ST120 at around 2 0'clock on the dial. Slight tweaks here and there does make a noticeable difference.

    I also like dual controls with large knobs and scales for micro-tweaks. The dual controls also give you channel balance. I used good quality old school Mallory pots instead of stepped attenuators for infinite adjustability. You'd be surprised how small a turn can make a substantial difference.

    If you don't want to clutter things up with actual controls, you can also use a VR and a meter to figure out what's coming into the amp, then substitute a resistor for that value, or just leave the VR in place. That does the job, but you do lose the ability to tweak as needed without digging around inside the box.

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