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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    SP12 preamp revisited

    tubes4hifi
    tubes4hifi
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    SP12 preamp revisited Empty SP12 preamp revisited

    Post by tubes4hifi Sat Apr 13, 2019 7:40 pm

    Six years ago the SP12 was the top selling preamp in the Tubes4HiFi (VTA) product line, almost every person buying a VTA amp was buying the SP12.
    But 7 years ago the SP12 got a big brother, the SP14, and in the past 5 years or so the SP12 has almost been forgotten by buyers, as buyers were
    no longer looking for a "budget" tube preamp, but the very best tube preamp kit available, the SP14.   Maybe 100 SP12s were sold in 4-5 years.
    During the past 4-5 years well over 400 SP14s have been sold, more than 4x as many.

    But let's NOT forget the SP12.   The SP12 uses the EXACT same circuit design as the SP14.   But it uses the popular 12AU7 tube instead of the larger 6SN7 tube.
    The SP12 has a single power power instead of dual power supplies.   It only needs one power transformer.  The SP12 kit is $250 less than an SP14.
    ALSO, the SP12 is the ONLY VTA preamp that you can add an INTERNAL phono stage to.  The older PH12 phono stage has been replaced by a much superior PH14 phono stage,
    the PH14 can fit into the same chassis.  The improved PH14 phono stage also has it's own power supply, and it has an output buffer.
    The SP12 is definitely NOT a "budget" tube preamp, it's the smaller brother of the HI-END SP14, 3/4 the size and 3/4 the price, and when you add the PH14
    phono stage you can have a COMPLETE tube preamp with integrated phono stage, for LESS money than the cost of the SP14 preamp.

    Here's a link to a recent SP12 build . . . SP12 - 2019 build photos

    Here's a link to a customer review (back in 2011) SP12 review
    tubes4hifi
    tubes4hifi
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    SP12 preamp revisited Empty Re: SP12 preamp revisited

    Post by tubes4hifi Fri Apr 26, 2019 5:07 pm

    RJPJNK,
    yes, you are about 90% correct. Forms of power supply noise are 60Hz line, 120Hz rectification, and SS diode hash.
    In most amps or preamps, some of this noise is introduced into the amplifier stage thru either the filament supply or B+ HV supply.
    However, in my preamps, any power supply noise is sampled, inversely adjusted to the gain of the amp, and injected (summed) into the amplification stage,
    thus eliminating 99% of any power supply noise. However, other power supply noise can be from EMI radiated from the transformers, leaking into non-shielded audio paths,
    or inducted/transmitted by other wires (transformer leads, etc) and possibly picked up by internal elements inside the tube (plates, grids, filaments).
    Most of the time this doesn't happen in a well-constructed amp or preamp, using short wires, shielded wires, twisted pairs, and shielding where necessary.
    And yes, you're correct about amplification. So an amp or preamp with more gain is more likely to make that noise more apparent by the time it gets to the speakers.
    Super high efficiency speakers work better with very low gain amps/preamps
    tubes4hifi
    tubes4hifi
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    SP12 preamp revisited Empty Re: SP12 preamp revisited

    Post by tubes4hifi Mon Dec 30, 2019 3:12 pm

    Hi Rich,
    the SP12 board is 7.2 x 5.2" and uses an Antek AS-05T240 power transformer.
    I've had a few customers actually gut PAS3 or PAT4 preamps and put it in there if you're on a tight budget,
    although $160 for the PCB kit isn't really a huge deal. It's the custom chassis that costs $$
    www.tubes4hifi.com/pre11.htm
    DavidR
    DavidR

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    Post by DavidR Thu Mar 26, 2020 6:52 pm

    Looking forward to my SP12 (with PH14). I have a number of 12AU7 and it's variants to choose from for tubes. I know it may be subjective but if I have a pair of 5963 that I think have a better tone than a regular 12AU7 are positions V3 and V4 the place to put those 'preferred' tubes?
    tubes4hifi
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    Post by tubes4hifi Thu Mar 26, 2020 7:22 pm

    it's the input tubes that will make the greatest difference in sound quality (V1 & V2). V3 & V4 are simply ouput buffers.
    solderblob
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    Post by solderblob Wed Jun 16, 2021 1:09 pm

    I built my SP12 last year and love it! Thanks Roy!

    Dave
    tubes4hifi
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    Post by tubes4hifi Sat Sep 04, 2021 5:42 pm

    as of Sept 1 2021 SP12 is discontinued due to lack of interest, everyone wants the SP14
    DavidR
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    Post by DavidR Sun Oct 17, 2021 9:28 pm

    Can you run (2) 12BH7 tubes in the two V1 positions?
    tubes4hifi
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    Post by tubes4hifi Yesterday at 12:41 pm

    12BH7 requires double the filament current of 12AU7, so it's possible it would either cause hum or overheat the power transformer, but you can try it just be aware of those facts
    DavidR
    DavidR

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    Post by DavidR Yesterday at 7:35 pm

    Would a bigger transformer solve those possible issues.

    The 12AU7 tubes don't have a lot of gain.
    tubes4hifi
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    Post by tubes4hifi Yesterday at 7:53 pm

    yes, of course. Use an Antek 1T250 instead of AS-05T240

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