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    SP12/PH12 Preamp Mods


    Posts : 57
    Join date : 2011-04-05
    Location : Boulder, CO

    SP12/PH12 Preamp Mods

    Post by jjones3318 on Sun May 08, 2011 5:26 am

    I thought I'd share some mods I did on my SP12/PH12 preamp today.

    I had been reading some rather rave reviews about the Salas HV shunt regulator on diyaudio, and had recently picked up some PCB's (from dvb-projekt on diyaudio) for the regulator for a dual TD1541A tube DAC I'm building (waiting on parts). So, I set out to change the B+ supply on my preamp to use the Salas HV shunt regulator.

    The shunt needs at least 20v difference between Vin and Vout, and uses around 25ma internally to operate. As the SP12/PH12 uses around 37ma, I configured the shunt for 80ma.

    I didn't want to buy a new transformer, so I needed to reconfigure my DC supply for more voltage. With the 12X4 rectifier I was getting 265v with 37ma load, and I needed 295v with 80ma load in order to supply 270v B+ to the preamp.

    Using PSUD2 I found that 6W4GT TV dampening diodes had a rather low voltage drop, were cheap, and also plentiful. The downside was that two would be required for full wave, but configured CLC with 33uF, 86Ohm choke, and 200uF would give me a very clean 298v at 80ma.

    The 6W4GT's need 1.2A of current for their heaters so another transformer was needed. It quickly became apparent that I was going to run out of realestate in my chassis as these tubes are quite large. I decided to change plans and use a solid state full wave bridge rectifier and feed the DC from that in to a single 6W4GT to retain the power on delay and (if you believe in such things) characteristic.

    While I was at it, I:

    1) Upgraded the critical resistors to PRP and Tak.
    2) Changed the coupling caps to Mundorf silver/gold/oil.
    3) Changed the larger PS bypass caps to Auricap
    4) Swapped in nylon standoffs for the SP12 and PH12. The PH12 ground plane is connected to the screw holes, so by using nylon I can float everything inside from the chassis. Chassis is still connected to earth, but this (I think) eliminates the chance of a gound loop forming through interconnects.

    The shunt was running a little hot at 80ma so I changed it to 60ma which brought the Vin up to 310v, so I need to change the DC filter to CLCRC to bring it back down. I also need to fine tune the current adjustment, and get a larger heatsink for the IRF840 as it's running hotter than I'd like.

    The results so far? Channel 1 is off the 6W4GT (10v/div) and channel 2 is off the shunt (50mv/div). 12v P-P and 0.99mv P-P (according to my DMM) respectively.

    The soundstage is wider and more holographic, and the resolution is greatly increased. We're talking drum sticks on drums and strings being plucked resolution. Voices, especially female, seem to have an added degree of realism as well. I've only a couple hours on it so far - can't wait until the caps break in.

    As I did all the upgrades at the same time I don't know what made the biggest difference to the sound. It's probably between the shunt regulator and the coupling caps, as I don't think the other mods would make that much of a difference.


    Posts : 53
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    Age : 68
    Location : Wa

    Re: SP12/PH12 Preamp Mods

    Post by Pooch on Sun May 08, 2011 3:53 pm

    I have a CJ PV8 pre that I've swapped literaly every cap back and forth at least once. I finally went back to all the stock caps because they were actually better than what I was trying. BUT, this last time I replaced the bypass caps on the final stage with Mundorf S/O and the results were much as you described, wider sounstage and a very real sense of everything being more alive.I'm guessing you're S/G/O are making the differance.CJ PV-8 original final stage caps were 2 uf bypassed with .15 uf. I replaced the .15 with .22 S/O. I have a couple more of these S/O and I'm thinking of trying them in an earlier stage or in the phono stage. By the way I bought these S/O to go in my VTA MK4s, but I just couldn't listen long term in that location. I'm using Russian K40 oils there now with Audiocap Theta for the .1uf coupling and bypass positions.

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    Join date : 2008-11-30

    Re: SP12/PH12 Preamp Mods

    Post by tubes4hifi on Sun May 08, 2011 8:02 pm

    the regulator and the caps both make a big difference, that's why all my preamps now have regulators, including the newer version of the SP12.
    You could have cheated on the full-wave bridge by using both the tube and two more SS diodes, and I don't think the original 12X4 has any more voltage drop than that power hungry big tube you used, and you saved about 35v bypassing R17, so I think you could have done the same thing without adding the extra transformer and tube. But I guess it was a fun learning experience!

    Posts : 57
    Join date : 2011-04-05
    Location : Boulder, CO

    Re: SP12/PH12 Preamp Mods

    Post by jjones3318 on Mon May 09, 2011 1:44 am

    I'll have to swap the old caps back in at some point to see if the improvement is purely from the s/g/o's or if some of it may be attributed to the reg.

    I'm running the K40's in my M-125's (they come with them) and am quite happy. I had considered trying the s/o's or s/g/o's, but thought better of it - having a hard time finding fault with the current configuration. I also read somewhere that s/o's, and probably the s/g/o's have a lower max operating temperature than most caps.

    Roy - every config I tried in PSUDII with the AS-1T250 and 12X4 would only give me around 274v at 80ma. As the shunt reg needs to drop 20-25v to be happy it didn't give me what I wanted. I tried most of the tube rectifiers I could find and I'd either fall short or have oscillation at start-up that'd overshoot 300v (max for the reg) by 20-30v.

    Adding the second transformer didn't bother me as I'll need it (or one similar) for my next mod - adding the LDR attenuator. When I'm done experimenting and settle on a final configuration for this unit I'll work on optimizing it. For this round, the game was to get the job done cheap.

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    Re: SP12/PH12 Preamp Mods

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