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Varia-Geddon

Discussion in 'd.i.y.' started by sybil, Dec 13, 2012.

  1. sybil

    sybil pfm Member

    Hello All,

    I was having a look through the various Geddon threads and it led me to Norton AirPower psu (I believe Leigh posts here?):

    http://www.cherrynorton.pwp.blueyonder.co.uk/nt/APKit.htm

    This shown a PSU made with two variacs instead of fixed TXs. I had a look at the various options for variacs on ebay and I came across this vintage 2kVA Variac:

    http://rover.ebay.com/rover/1/710-5...0001&campid=5338728743&icep_item=150959954554

    Could this be used instead of the standard 2x55V 500VA TX that most tend to use in a DIY Geddon, but with the additional possibility of varying/optimising voltage to minimise motor noise?

    Additionally, sq291365547 (sp?) used two selected 0.1uF WIMA caps for the phase-shift element that were optimised for his Premotec motor. How do you assess this? Can you take a measurement on the motor or do you need to trial and error whilst holding the motor out of the plinth to assess?

    Motor is a a 110v AC motor in a Pink Triange PT TOO (not sure if it is Airpax or Premotec or what). The PT Too PSU works fine but they are pretty badly built so I was considering building a Geddon or AirPower for longer term use.

    James.
     
  2. martin clark

    martin clark pinko bodger

    It could but you'd still want a main isolation transformer ahead of it all - remember variacs are not usually isolated from the mains, but variable autoformers. This means the motor circuit would be live. Be careful!
     
  3. sybil

    sybil pfm Member

    I see - so this is in effect a step down device rather than an actual transformer where the primary and secondaries are not directly connected. Not so useful in that case - perhaps I just wanted the enormous control wheel!

    In the variable Airpower device how does this work? Would you need 2 transformers + 2 variacs or can you get small variacs which provide isolation too? I would like to fit everything inside the PT PSU wooden sleeve if possible which limits the size of TX in a Geddon - ie. I think a 500VA TX would be too wide at c. 150mm dia. The AirPower seems to be effectively a Geddon but with a separate small TX for each phase, and utilising two different voltages to reduce motor noise. Correct?

    James.
     
  4. sq225917

    sq225917 Bit of this, bit of that

    It utilises two different voltages, but whether that reduces noise of just adds more speed instability is a point for discussion. The DIY 'turntable speed' thread should answer most questions for you.

    I selected capacitors by hand, the differences were obvious, as was the effect of increased viscous drag.
     
  5. Barrymagrec

    Barrymagrec pfm Member

    This looks exactly like the one I`m looking at beside my bench at the moment - I really ought to get a case for it...
    Don`t really know about vintage - I found it in a closed down factory only about 30 years ago.

    It`s quite chunky, if you were hoping to reduce noise and vibration you`d want to mount it some way away.
     
  6. 337alant

    337alant Negatively Biased

    SQs is the best method simple and neat IMO and the 0.2UF is the correct value for 90 deg phase shift, people only use 0.22 as it is the nearest prefered value.
    Dont use a variac, tried it crap and one of the 240v phases goes straight to the motor so heed Martins warning :eek:

    Alan
     
  7. sybil

    sybil pfm Member

    Ok, thanks for the advice. I agree SQ's psu looks great.

    I have measured the wooden PT TOO psu sleeve - inside clear width is 130mm, although this reduces to 120mm at the top due to the lip which locates it on the metal box inside. The psu looks like this: http://www.vinylengine.com/pink-triangle-pt-too.shtml . I would like to make a psu the same size as the original (which is in a metal box that slides in to the wood sleeve) so it keeps the supply looking OEM. I will probably leave the 'clever' switch in the PT and the psu lead/original psu intact and just fit a second power cable to the motor with a 3-pin XLR and a switch on the secondaries for startup.

    If a Pink Triangeddon is the best bet then I guess a 300VA TX is the biggest that will fit the enclosure - at least some of these appear to be just under 120mm in diameter. Will this adversely affect performance compared to the recommended 500VA/Naim 430VA? Would two equal 250VA transformers work the same (one in phase, one at 90 degrees - a bit like the Airpower but with equal values)?

    Kind regards,

    James.
     
  8. martin clark

    martin clark pinko bodger

    The motor draws under 2w. The marginally smaller transformer will have no effect whatsoever.
     
  9. sybil

    sybil pfm Member

    Martin,

    So I can just use a transformer that fits under the hood, so to speak? On the other Geddon threads there has been some mention of the fact the large TX 'filters' out mains nasties. Is this BS or is there any other reason 500VA seems to be the most used value other that Naim's original choice (430VA ala HiCap) or, say, a 100VA TX?

    I would quite like an encapsulated transformer from a build POV so would 200VA be ok?

    James.
     
  10. sq225917

    sq225917 Bit of this, bit of that

    A bigger traffo will smooth more than a smaller one, marginally. 500va is just an easily available size and close to the Naim value so people use it. I use it because I picked up a couple that size very cheap. stick to 300va as a minimum and you'll be fine.
     
  11. sybil

    sybil pfm Member

    Simon,

    The 'Turntable Speed' thread I found was about 18 pages long and started with listening to needle-drops or something more elaborate? Is this the correct one? There was a thread part 2 also.

    James.
     
  12. martin clark

    martin clark pinko bodger

    Sybil - I'm sure it would be. I've had a good think about this over the years and cannot see why a large transformer is an advantage unless you sell PSUs by the lb or have a large pile of NAP250 transformers to hand, or both ;)

    Smaller-VA transformers have larger leakage inductance, which is a good thing if you really want filtering (and for which you'd also probably go for a dual-bobbin E-I type rather than a toroid... sometimes the 'poorer' transformer design is a good choice.)
     
  13. batjodel

    batjodel pfm Member

    Stupid question really

    I suppose you are talking about the Norton Airpower that was produced commercially and that there is a published schematic on the web?

    I have a Norton power supply (originally bought through pinkfish). And brilliant it is. Much better than the 500 VA DIY geddon I had before. Or the valhalla on my LP12 before it. I can thoroughly recommend the Norton

    When I first got it I opened it up, and from memory it consisits of a custom wound transformer with two secondaries (500VA?) and a small circuit board. I guess I could open it up and post a few pictures / details if you want, and its not commercially sensitive (probably not because its not in production anymore). It doesnt have a veriac or anythink like it.

    Merry Xmas to everyone

    Dave
     
  14. stackowax

    stackowax pfm Member

    Yes, pictures Dave! (When you get a chance.)
     
  15. flashgo

    flashgo DIY Practitioner

    There are pix in the OP's CherryNorton link. It's for a kit with smallish traffos, but he says that there's minimal compromise in SQ.
     
  16. nortonl

    nortonl pfm Member

    Hi Alan

    0.22uF is the value recommended by the motor manufacturer http://www.farnell.com/datasheets/526545.pdf.

    I've never seen an isolating variac, but using a non-isolating one should not be a problem. Voltages at the motor are no different to normal (or you'd fry it). The only difference being the grey motor wires are connected to mains Neutral (0v). Obviously, you need to take care not to turn the voltage up too high though (I wouldn't go over 120v).

    If you have it wired correctly, sound-wise, it is no different from a production AirPower. In fact this is how the AirPower was originally developed and the optimum voltages determined. I know of at least two people who have built units using variacs and use them daily. Since you can fine tune the values to your particular motor, it will actually sound slightly better.


    Leigh
     
  17. nortonl

    nortonl pfm Member

    It reduces motor frame vibration (quite a significant amount). If you hold the motor in your hand and adjust the voltages, the difference is very obvious.

    During development, I had a friend adjust the voltages while playing music and you can easily hear when the voltages are best. We were able to repeatedly get the same voltages blind and the results exactly coincided with the above hand test.
     
  18. sq225917

    sq225917 Bit of this, bit of that

    Leigh I found that by selecting an optimal rather than nominal cap value that there was nothing to be gained from adjusting voltages. Might be worth looking into.
     

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