TC-750 Phono Pre-Amp (Continued)

Discussion in 'Vinyl Nutjob World: Turntable and Related Gear' started by purr1n, Oct 6, 2015.

  1. purr1n

    purr1n Finding his inner redneck

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    Latest tweaks to the TC-750.

    Input caps are bypassed with small WIMA film caps (on underside of board). Found some random film caps to replace the electrolytics on the output.Less grainy sound slightly warmer sound (these caps are bigger than the stock, so maybe rolloff is lower). Maybe a tiny bit more plankton. Also bypassed the single diode for the +12V (presumably there in case of user error / wrong polarity). If I get a chance, I will swap out the input caps. I already pulled the 220poof caps because the Ortofon Black doesn't need that kind of capacitive load.
    DSC_0935.JPG

    This one was significant. Replaced the Sigma 11 power supply with this huge honkin' SOLAHD SLS‑12‑034T power supply. Find them on eBay for well less than $20 surplus. Cut the cable from the stock POS power supply and wire accordingly.
    DSC_0936.JPG

    Power supplies do have a sound of their own. Sometimes more, sometimes less. It depends upon the circuit. The sonic gains with the SOLAHD were huge. A ton of plankton. I know this will sound cliche, but a ton of ambient and reverberant stuff I've never heard before. That doesn't happen often with me. I might actually prefer this Franken-phono-preamp to the EC Transcription Amplifier, but would need a direct comparison to say. I know many of you may find this difficult to believe, but really, the design of the TC-750 is and pretty solid and uses discrete parts.
    DSC_0937.JPG
    BTW, the SOLAHD power supplies were used as a development power supply for the BW amp. It was definitely one of the finalists (nicknamed Camaro) but lost out because of its impractical size and weight. (The BW would have required two of the even larger 24V units for bipolar power rails.) I'll measure the power supplies on a 'scope when I get a chance. I know OJ would be interested.
     
    Last edited: Oct 20, 2015
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  2. HitmanFluffy

    HitmanFluffy Hoping to see real genitals someday!

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    Was there an earlier thread about modding this on Changstar? The search function there isn't turning up a dedicated one for it.
     
  3. drfindley

    drfindley Secretly lives in the Analog Room - Friend

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    Maybe we can find a half-decent case for this and make a little mod guide.
     
  4. Mr.Sneis

    Mr.Sneis Friend

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    This is awesome and I would be up to doing it myself if I had a guide! I've gotten my feet wet on a Jolida so far, the changes were not subtle and definitely flavored the amp a lot more.
     
  5. HitmanFluffy

    HitmanFluffy Hoping to see real genitals someday!

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    I'm going to order the TC-750 and the SOLAHD, and I'll probably start work in May when my sister brings them back with her. I'm also fishing around for a cheap Chinese case that will fit the damn SOLAHD.
     
  6. OJneg

    OJneg The Most Insufferable

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    BTW, for anyone wondering, I did get the TC750 to work on a higher voltage rail. 24V to be exact. You will have to remove the internal power filtering I believe (I'd remove it anyway)

    Like Marv mentioned before, I'd also highly suggest getting rid of the 220p input cap unless your cartridge actually needs it.
     
  7. mtoc

    mtoc SBAF's Resident Shit-Stirrer

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    Waiting for updates. Seems pretty good.
     
    Last edited: Jan 4, 2016
  8. purr1n

    purr1n Finding his inner redneck

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    I do feel that this is franken-phono worthy of putting in a nice chassis. I have a few other transistor designs lying around, but not going to bother building from scratch since the TC-750 seems to do the job. I happen to have an extra 24V power supply lying around, so I will give this a shot. Next steps for me are to pop the electrolytic input caps and replace. OJ did this and his unit definitely had more clarity and resolution than mine. I may replace with alligator clips so I can roll caps.

    My plan is to get some nice RCA jacks and see what I do with Front Panel Express. Will keep you guys in the loop.
     
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  9. purr1n

    purr1n Finding his inner redneck

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    Quick notes on mods (also with knowledge gleaned from @OJneg):
    1. Replace output 'lytic caps
    2. Replace input 'lytic caps
    3. Use better external PS
    4. Rip out the 18V 'lytics power filtering caps on the board and run 24V (the transistors on the board can take much more voltage)
    5. Bypass / rip out diode on power input
    6. Rip out 220poof caps on the input (depends on your cart - the 2M Black I use does not need them)
    I'll provide some pictures later. I've only done 1,3,5, and 6. @OJneg has done everything.
     
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  10. HitmanFluffy

    HitmanFluffy Hoping to see real genitals someday!

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    How beneficial is switching to a 24V PS? For that purpose something like would a SLS-24-036T be appropriate?
     
  11. purr1n

    purr1n Finding his inner redneck

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    No idea on sonics, but probably get more headroom and lower distortion. Might need to play with bias.
     
  12. OJneg

    OJneg The Most Insufferable

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    Bias seems to be fine. Output emitter sits at around 14V (rather than 5V) so you should be able to take advantage of extra output swing (although you'll never get that much out of with a 5mV input). Bit more bias current and higher Vce is (almost) never a bad thing. You might be able to go even higher.

    :)

    Other thing I did was remove the 1k base stopper from the input, as it serves no purpose but to increase input noise. I also bypassed the attenuator (I have the variable output version) and replaced with 1Meg pulldown. I wanted to use a smaller (smaller caps are always more transparent) coupling cap on the output without losing any bass.

    Schematic again for reference.

    [​IMG]
     
    Last edited: Jan 5, 2016
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  13. HitmanFluffy

    HitmanFluffy Hoping to see real genitals someday!

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    This seems a bit long but seems like a cost-efficient and even attractive case option from Hammond. [​IMG]
     
  14. purr1n

    purr1n Finding his inner redneck

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    I tried an SLS-24-012 (at 24V and also adjusted down to 18V) which sucked. The SLS-12-034 (adjusted up to 13V) sounded better. I'm pretty sure the higher current SLS power supplies with the two big transistors sound better than the smaller ones. The SLS-24-036T did sound better than the SLS-24-012 on the Black Widow when we were testing though. I'd advise to play it safe and get either the higher current -034 or -036 models short of building your own shunt regulated power supplies.
     
    Last edited: Jan 5, 2016
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  15. JK47

    JK47 The Beer Houdini

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  16. purr1n

    purr1n Finding his inner redneck

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    IMG_20160105_193953698.jpg
    • Remove power filtering caps if you will be using a good power supply. The POS stock switcher probably needs those caps.
    • I pulled the 220 poof cart load caps. There should be about a 100 or so poofs in the TT and interconnect cables. I know my 2M Black cart does better without them.
    • Replace output caps with 4uf or smaller as long as you don't get any bass rolloff.
    • Replace input caps with 10uf (stock) or smaller as long as you don't get any bass rolloff or distortion.
    • Bypass diode.
    IMG_20160105_210316963.jpg

    I used what I had available in the used parts bin and rigged it up tonight. The inputs are Solens. They suck. Don't use them. It's possible they may need burn in though.
     
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  17. mtoc

    mtoc SBAF's Resident Shit-Stirrer

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    If we use OJ's schematic to make it from scratch, maybe we could no need to buy it first then mod it?
     
  18. purr1n

    purr1n Finding his inner redneck

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    The darn thing is $39. Cheaper to buy it and replace a few parts than to build one from scratch.
     
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  19. OJneg

    OJneg The Most Insufferable

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    Yeah the hardware (chassis, board, connectors) only would add up to more than $40. Besides, I think these transistors are OOP. TCC is probably sitting on a box of them.
     
  20. mtoc

    mtoc SBAF's Resident Shit-Stirrer

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    @OJneg, there's a professional model (added an output level contro) on Amazon, if we gonna mod this, isn't the non-professional model more preferable?
     
    Last edited: Jan 7, 2016

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