TC-750 Phono Pre-Amp (Continued)

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

  1. mrflibble

    mrflibble Friend

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    TCC TC-750 vs Dynavox TC-750

    In Europe we have the Dynavox TC-750, which has cosmetic similarities to the TCC TC-750 available in North America. I imported a TCC TC-750 to compare to my Dynavox. Both models are available in black or silver, both have the RIAA curve depicted on the top. The curve for the Dynavox is not as flat at the TCC curve. The Dynavox takes 15v input but works with 12v (I tried both and could hear no difference between the two), the TCC takes 12v. I have another Dynavox which I managed to blow up, I had a look inside and the circuit is somewhat different to the TCC, both use discrete parts however.

    The test was carried out with the following setup:

    Technics SL1210 --> Nagaoka MP110 Cartridge --> TC-750 --> Vali 2 (or Needle-drop with M-Audio 192) --> HD650

    I used the same power supply for both TC-750's, it is a ZeroZone R-Core LPS: http://www.ebay.co.uk/itm/Black-100...lay-L165-26-/111997651336?hash=item1a13948988 . I believe it is based on the sigma-11 circuit. The voltage is adjustable with a little brass screw on the precision resistor (little blue box). I adjusted the voltage to 15v for the Dynavox test.

    Both phonostages are unmodified and in stock form.

    To cut to the chase, the two phonostages are not really comparable. The TCC is significantly better than the Dynavox, it is not even close. The TCC sounds so clear and airy, it is very easy to hear all of the details of the recording (and all the deficiencies in the record and my setup!). The Dynavox, in comparison, sounds muffled and not very dynamic. I wish I had the ability and experience to describe the differences in more detail, but sadly I am still a beginner. Suffice to say, I do not think the Dynavox is worth bothering with. I have very few records in my collection, the comparison was done with the Dirty Dancing soundtrack (original pressing).

    I bought the TCC from the states and luckily it sneaked through customs, so I didn't have to pay the 20% VAT. But even if I'd had to pay import VAT, it would still be worth it over the Dynavox.

    The main expense, of course, is the linear PSU. But there are cheaper linear power supplies available from China (both R-Core and toroidal), I wager they will work just as well. I went overkill on the PSU (and for one that was adjustable) because I can always use it for something else down the road.

    Next step, will be to perform some minor mods to the TCC, but I need to practice my soldering skills first - probably by repairing the first Dynavox I destroyed :)
     
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  2. JayC

    JayC Resident Crash Test Dummy

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    Good to know, I found a website in the Netherlands that sold the TCC and bought it over the Dynavox even though it was 10€ more expensive and always wondered. I am curious though what the differences between the circuits are. Do you claim they are different because the layout is different or did you actually find a schematic (or manually trace some paths) to say this? I always assumed the basic circuit remained the same, and only some component quality/brand differences separated the two.
     
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  3. purr1n

    purr1n Finding his inner redneck

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    Photos of the boards?
     
  4. mrflibble

    mrflibble Friend

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    Yes, I can take photo of the Dynavox board. I may have one still on my computer from previously. I may not post it until Monday because I have exams this weekend. Will check tomorrow, going to bed now!

    The layout of the dynavox is different, can't remember about the components. Haven't seen a schematic. But it definitely doesn't sound as good and the advertised RIAA curve is not as flat.
     
  5. purr1n

    purr1n Finding his inner redneck

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    I'm curious what's different. I know the TCC TC-750 had different versions where some parts were slightly different and laid out differently.
     
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  6. mrflibble

    mrflibble Friend

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  7. Gaspasser

    Gaspasser Flatulence Maestro

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    A74DB9F6-E13B-453B-8723-A066F585A7B8.jpeg 8C09B53A-686D-4415-9BCE-0FCEC98D4ECD.jpeg

    I finally got around to a new TC-750 and soldered in the upgraded input and output caps. My previous attempt was a bust :(
    I used the regular cheaper Audyns since I’m going for max performance : price. This combined with the SOLA-HD power supply is really flipping nice. This the first time I’m spinning vinyl in several months (gasp) and I’m listening to vinyl for the first time on the S7, so I really can’t offer much in the way of comparison right now.
     
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  8. OldPioVinyl

    OldPioVinyl Rando

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    First I want to say a big thank you for this forum to collect all relevant information about this RIAA preamp. I modded this unit by myself also and share my experiences.

    Low noise but cheap PSU for TC-750

    The filter caps on the circuit board are 25V rated, that is why any voltage in the 16-18V region is safe. Using a 15V/500mA linear unregulated wall psu with an already assembled 317 board is a cheap but good solution. And it is possible to mod the 317 to low noise operation. The unregulated wall psu has higher output voltage without load - measure it with a voltmeter. It should be around 19V. The LM317 regulator needs 3V to operate so if you measure 19,2V than you can set the output with the little pot to 16,2V.

    The regulator board costs only 1 USD and looks like this:
    [​IMG]

    Modding the LM317 board to low noise.
    It is a simple trick, the datasheet suggests a cap (called Cadj) to shunt the noise of the voltage divider. I use a 4,7uF/35V electrolityc cap. Connect between pin1 of the 317 and GND. Take care of the polarity.

    More information and measured effect of the bypass capacitor can be found in this pdf:
    https://www.edn.com/Pdf/ViewPdf?contentItemId=4422750
     
    Last edited: May 27, 2019
  9. OldPioVinyl

    OldPioVinyl Rando

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    This is my modified version of the circuit itself:

    [​IMG]
    https://pasteboard.co/IgBlY3Z.jpg

    1. There is a book written in the 70s, Motchenbacher & Fitchen - Low noise electronic design. They realized that large value caps at the emitter of the transistor can reverse bias the B-E junction and slowly degrade transistor noise performance. It happens when the circuit is just switched off. Their solution is simple, put two little diodes series (it works like one diode with 1,2V opening voltage) and connect it to the base and emitter of the first stage. This diodes are closed at normal operation and open only temporarily at shutdown to defend the transistor. 1N4148 diode is cheap and does it well.
    2. The power filter was not taken away but redesigned. There is a 1nF foil cap soldered directly to the power connector, it does not let in radio frequency noise. Use short leads. RF noise is just increasing these days and any wire acts like an antenna. The electrolytic caps can be used as a filter, so I made some changes. The input diode is paralleled with a 22 ohm and the 470 ohm is paralleled with a 100 ohm resistor (result is 82 ohm). This way we got a two stage RC filter. I placed an 1A diode (1N4007) in opposite direction to protect the circuit from accidental reverse polarity connection.
    3. Cartridge load cap 68pF seems to be fine for me.

    And as the previous mods mentioned in this forum, you can change the input-output caps to audio grade foil caps and place some small foil caps parallel to C3 and C7.

    All the diodes, resistors etc. placed to the foil side of the PCB.
     
    Last edited: May 27, 2019
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  10. JayC

    JayC Resident Crash Test Dummy

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    Is there a more robust way that I can get rid of this loud RF (I think) hum i get after opening up this preamp and modding it? It makes my subwoofer go crazy

    At the moment I wrap it with layers of aluminium foil but it's really finicky and most importantly it doesn't block everything
     
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  11. LeaveItTubeBeaver

    LeaveItTubeBeaver Rando

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    Hey everyone! Hopefully those who have modified their 750 still hang around here..
    I bought the PA and the sola power supply. I’m hopeful that I have a cord around the house that fits this since the one I’m getting doesn’t come with it!
    But that leads me to my question.. can I just solder some 18 or 22 gauge wire to the board? I haven’t found a picture from under the board, hopefully it’s doable.
    Thanks
     
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  12. purr1n

    purr1n Finding his inner redneck

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    In what respect? I soldered 22-24 gauge wire to the board so I could roll caps. This after I removed the stock caps from under the board.
     
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  13. LeaveItTubeBeaver

    LeaveItTubeBeaver Rando

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    If I can’t find the correct power cord, can I just bypass the power input and solder some wire directly to the board for power? This would be my preferred method since I plan on using 2 enclosures to separate the pre from the power
     
  14. Wushuliu

    Wushuliu Rando

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    Did this mod to my TCC that I've had sitting around for years. Really opens things up. I did get bad interference but as mentioned earlier I soldered a connection from rca ground to chassis ground and now its silent. No foil needed.

    Eventually I'll compare it to my Hagerman Bugle once I get that up and running again.
     
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  15. LeaveItTubeBeaver

    LeaveItTubeBeaver Rando

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    So far all I’ve done is removed the 200 poof caps and used a better wall wart I had lying around (12v 2.8a).
    It sounds better than my behringer but I’m anxious to get the solahd PSU hooked up, it seems to be the bees knees around here!
    Do I need to remove the diode to fully utilize the upgraded power?
     
  16. Pogo

    Pogo Friend

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    I've used a 750 for years as a 2nd system and test bench phono-pre powered with a $35 U1 12v deep cycle battery. No need for a fancy power supply, just a monthly charge top up from an AC charger I already had for boat and car use.
    Sounds good enough to me to not want to frankenmod the 750 itself.
     
  17. LeaveItTubeBeaver

    LeaveItTubeBeaver Rando

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    [​IMG] [​IMG]

    Finally hooked up the Soland power supply tonight. I’m pretty impressed with this thing, especially for less than 40 so far..

    I think I’m going to get some cheap caps and try to find 2 enclosures so I can separate power from everything else.
    Then I’ll probably just get back to enjoying the music again!
     
  18. LeaveItTubeBeaver

    LeaveItTubeBeaver Rando

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    So I removed the 220 poof caps as well as bypassed the diode. I opened it up today to take a look at things and my 104k ceramic cap blew, but it was playing fine last night. Was I supposed to have removed this as well?
     
  19. LeaveItTubeBeaver

    LeaveItTubeBeaver Rando

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    [​IMG]
    That was before removing the 220 caps and diode

    And this morning..
    [​IMG]
     
  20. LeaveItTubeBeaver

    LeaveItTubeBeaver Rando

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    [​IMG]
    I think I've done all I can to this thing and I'm very impressed with the quality for the price!
    I may just run it like this until I decide how I want to enclose all of this. I'm either going to separate the psu and the PA or throw them together in the same chassis with everything mounted to a aluminum top plate (similar to the bottlehead crack)

    Anyways.. I should have ran it longer with the poof caps removed. I removed them and instantly added the caps. I threw on "time further out" and was in awe. I'm very clad I didn't buy the schiit Mani. While I'm sure it's fine, I pieced this together for less than $90, including the caps and the parts I'll need when I get around to housing it.

    I appreciate the thread! It helped a ton and let me finally upgrade my behringer
     
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