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Turntable w/ hacked in pre-amp

Discussion in 'General Electronics Discussion' started by oddjobmj, Jun 27, 2012.

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  1. oddjobmj


    Jun 27, 2012

    I am looking for some help finishing up the addition of an RIAA phono pre-amp into my old turntable. There are a few fundamentals that I believe I am missing and my intent is to further my understanding rather than simply finish the project although I really am looking forward to some classy music with my scotch.

    I have the pre-amp package built which came with the pre-amp, a 12vAC adapter, and a universal power supply (all from paia). As far as I can tell these are working fine.

    The problem I am having is that these circuits are going to be stored inside the turntable and I don't want three cords to plug in to the wall when it's all said and done. One for the pre-amp, one for the turntable, and one for the actual amplifier so I can hear the darn thing. Two is fine so I am hoping to power both the turntable and the pre-amp with the 12vAC and UPS from the pre-amp package from paia.

    The turntable has a plug plug directly to main power without an adapter until inside the turntable. The picture below shows this internal adapter and the plug. I circled in yellow the leads that I snipped and tested. Between the two blue leads my voltmeter on AC read 22 and between either blue and the black it read 11. I am a little confused about what that means. I set the plug next to the internal adapter for the picture; the black cables are tied directly into the red, then to the adapter, then to the blue/black leads.


    So, first off, I guess I need to understand if those readings suggest I can scrap the internal adapter and that the turntable will work with the 12vAC provided with the pre-amp.

    Assuming that's a go I will continue on to the next difficulty I am having. The blue leads from the internal adapter are not labeled +/- so I am assuming that means it's still AC. They run into the circuit pictured below which has a single capacitor and two diodes. That also connects the power switch, the turntable motor, and a server to lift/lower the arm. I have no idea what the circuit does. Is this converting to DC?


    Lastly, what I was hoping to do is connect the blue/black leads pictured above coming out of the small circuit and plug those into the UPS shown below that is already powering the pre-amp. At the top of the UPS in the picture below there are jumper locations. One source of my confusion is that they are labeled polarized whereas the blue/black cords are not.


    I apologize for the long post. Even if you can't tie the whole deal together in one post please let me know if you have any suggestions and if I can provide any more details.

    Thank you for your time,

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    Last edited by a moderator: Jun 27, 2012
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Firstly -- Huge images. They take forever to download. Smaller may be better :)

    Second, one approach is to remove the separate power supplies from their enclosures and wire them all to a single mains connection. Clearly there are risks here in mains wiring etc.

    If you can find a single power supply that does the job, that would be a better solution that may not require any mains wiring.

    Placing a power board inside the unit and plugging the various supplies into it is another option, but there may not be sufficient room.

    The transformer sounds like a 22V centre tapped device, and the presence of only 2 diodes on the board suggest the output voltage (unregulated) is probably around 16V, falling to maybe 13 or so under load. (

    This is very similar result to bridge rectifying 12VAC, which seems to be what is happening with the other unit. The one connected to the 12VAC plugpack seems like it has 2 output rails due to the pair of (what may be) regulators. The fact that they are not on heatsinks suggests that the output is quite low power.

    Your major issue in combining power supplies will be ensuring that you can supply sufficient power for all the things you're connecting to it.
    Last edited: Jun 27, 2012
  3. davenn

    davenn Moderator

    Sep 5, 2009
    Hi Steve

    I have just finished resizing and reuploading the images

    I was also going to comment to our poster to keep things a bit smaller both in physical kb size and also in pixel size :)

  4. oddjobmj


    Jun 27, 2012
    Sorry for the image sizes guys. My experience in the past with uploading images lead me to believe they would automatically be re-sized. I will make sure to adjust accordingly next time. Thank you for fixing them!

    Also, thank you both for your advice!

    This is basically the reason I'm posting. There is a UPS board that I installed inside the turntable that has separate jumpers for multiple items. Are you talking about something different, more robust, or this? Here is the one I put in. It is visible on the right hand side of the third picture I posted.
    (regulated 12v version)

    What do you mean by two jumpers? I'm very new to the electronics crowd as I'm sure is apparent. Please forgive my ignorance. I do see 6 places for output jumpers to be installed (along the top of the PCB) if that's what you mean.

    I believe your post was cut short so maybe you were about to address my next question but I'll throw it out there anyways. Is it reasonable to suspect that the power-supply board we're talking about would be able to deal with the 22V transformer and ultimately not fry the pre-amp in the process if they were all hooked to that board? How can I test this if it's not obvious?

    If I can use that 22v transformer to power the universal power supply board and split the power back to the turntable and to the pre-amp that would make things a lot easier. I just don't want to overload the board and/or the pre-amp.
  5. CocaCola


    Apr 7, 2012
    Another thing to consider, when you are dealing with 'hobbiest' grade kits like this, you can open up a big can of worms with ground/feeback loops when you try to combine power supplies... So take it as a fair warning that the feedback gremlins might rear their ugly heads once you plug it all together...
  6. oddjobmj


    Jun 27, 2012
    I would love to be a part of you hitting 1,000 posts so please elaborate! :)

    Seriously though, is there a way I can look at my setup to see if this might be the case? Maybe tips in avoiding it? What might cause this affect in my setup?

    I understand feedback loops in the context of turning my guitar towards an amp so the strings pick up the output thus looping and yielding that signature sound but I have no grasp of their form in a circuit.

    Either way, if that is the case I suppose having an extra cord to plug in might not be too harsh :p I mainly want to learn and do it 'the right way' which is why I'm going through the trouble.

    I just now realized what you meant here and really like this idea. Bring in one mains connection to the enclosure and split it off to the two original transformers. I'm sure I have space to do this. Anyone have any pointers for getting started on this in a safe manner?
    Last edited: Jun 27, 2012
  7. CocaCola


    Apr 7, 2012
    Man, you are forcing me out of my comfort zone for my 1000th post :)

    To be honest, I'm far from a professional in this regard, there are highly educated people that work day and night fighting and designing audio equipment and countering feedback issues... I'm not one of those :) I have only scratched the surface because I have had the unfortunate luck of the feedback gremlins popping up in some audio circuits I have worked on and thus was forced to bang my head against the wall and do brain hurting trial and error debugging trying to eliminate them... My first experience was trying to interface a small hobbiest kit amplifier to an existing system that had an audio circuit, quite similar to what you are doing... When powered on their own they worked fine, but as soon as I combined them into a single supply, it was a nightmare... Once combined the amplifier was picking up all sorts of noise from the now closely shared ground connections, trust me you will notice right away... Since I didn't want to redesign the entire thing and track down the specific source, I cheated and used an isolation transformer to break the loop :)

    My advise is to not worry about it all that much and just take my warning as a simple warning, don't dread on it now... When and if issues pop up then you can start to worry :)

    If you are really curious about the subject, this page will give you a good primer... Yes, it's a long read... seminar.pdf

    And there you have it post #1000
  8. oddjobmj


    Jun 27, 2012
    Congratulations :D

    I will heed your advice, thank you. I think the mains split (if I can figure it out) will bypass this issue altogether but I'm taking to the reading now anyways because that's what I started this project for anyways; to learn.

    Thanks again!

    EDIT: Oh no, the first paragraph in that link is already making me want to go back and re-solder all my crappy work:

    Last edited: Jun 27, 2012
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