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Very Simple Phono Stage questions

Discussion in 'DIY' started by ocmsRzr, Jun 21, 2011.

  1. ocmsRzr

    ocmsRzr Member

    Messages:
    90
    Hi all,

    I'm planning on building the very simple phono stage as a first audio diy project. The schematic is here:
    http://phonoclone.com/diy-pho5.html

    It says that R1 should be set relative to the cartridge manufacturer's value. Is it possible to substitute a potentiometer here so this resistance can be tuned as different cartridges are used?

    What does it mean that there is an optional capacitor?

    Finally, I'm a little worried about noise from my power supply (a wall wart from the thrift store). Is there a way to add a filter to the power supply to clean it?
     
  2. quadtech

    quadtech Active Member

    Messages:
    469
  3. avionic

    avionic " Black Knights" Subscriber

    Messages:
    34,148
    Location:
    Fort Dodge, Iowa
    Cartridge loading..A pot probably not a great idea. A switch or two would be better.Depending on the carts specs , the usual loads are either.
    100Ω ,1KΩ ,& 47KΩ resistance as well as 220 pf and 330 pf capacitance at least thats whats available with my Yamaha C-85 pre-amp.Using a switch to select load combinations is what is the norm.
    Would need to be closely matched pairs for the left and right channels.

    http://www.sound.westhost.com/project25.htm
     
    Last edited: Jun 22, 2011
  4. ocmsRzr

    ocmsRzr Member

    Messages:
    90
    Hi avionic,

    I'm very new, but I've seen matching mentioned many places, but what exactly does it mean?
     
  5. Detailman

    Detailman Addicted to tweaking

    Messages:
    2,138
    Location:
    Daytona Bch. Fl area
  6. nashvillebill

    nashvillebill Super Member

    Messages:
    2,853
    I've built this Very Simple preamp many years ago and it worked well for me. The original article included the split-rail power supply, easy enough--just your basic center-tapped transformer, bridge rectifier, and filter caps, followed by voltage regulators and another set of filter caps. Make sure the transformer is a little bit away from the preamp, maybe six inches or so. My first enclosure had the transformer too close and the preamp picked up some hum from the transformer, moving it away took care of that.


    I would use a resistor and not a pot for the cartridge loading. I went with the standard 47k. The optional capacitor note means that if you want to add more capacitance to the system, you could. I didn't, between the cartridge and interconnects the capacitance was within the range that the cartridge was intended.
     
  7. Fasterdamnit

    Fasterdamnit Super Member

    Messages:
    2,889
    Location:
    Charlotte NC
  8. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    Messages:
    5,961
    Location:
    Melbourne, Australia
    Fasterdamnit's super cheap CNC MM phone stage is fantastic, easy to build, great sound and low cost.

    We are getting PCB's made at the moment, stay tuned.
     
  9. ocmsRzr

    ocmsRzr Member

    Messages:
    90
    Hi,

    I built the Very Simple Phono Stage, and I'm trying to come up with a power supply.

    I have a wall wart that goes 120V ac ->24V ac. As far as I can see I have two options I'm weighing:
    1) Put two 12v dc wall warts in series. How identical do these have to be? Same amps/voltage or exact same brand? (I'll probably scavenge them from a second hand store or spare box)

    2) Use the 24V ac transformer with rectifiers as input into the voltage regulator circuitry in the vsps.

    The VSPS voltage regulators are here:
    http://phonoclone.com/diy-guide.html
    Under "Voltage Regulation and Filter Capacitors"

    And the rectifying circuit I'm planning on using is from:
    http://www.audioxpress.com/magsdirx/ax/addenda/media/stamler2874.pdf
    Under "Balanced Rails" (specifically Fig 1)

    My question here is: can I just use the diodes to create the three rails and input them into the voltage regulation circuit or do I also need to include the capacitors shown in figure 1?
     
  10. nashvillebill

    nashvillebill Super Member

    Messages:
    2,853
    The Elliott power supply here http://sound.westhost.com/project05b.htm is an improved version of the voltage doubler you listed, you could use the 24 VAC wall wart...(edited, oops, BinaryMike confirmed it'd be too much for those voltage regulators, nevermind!!) . This is a complete supply as he has drawn up, so the +/- 15VDC goes to the VSPS without any further regulation circuitry.

    I'd use that version rather than putting the two wall warts in series, to me that's an ugly way to do it plus you need to phase them.

    My true preference would be to get a new CT transformer for less than $20 and go with a full wave bridge rectifier (center tapped), much more efficient and smoother.

    You must have the capacitors after the diodes to smooth the ripple on the power supply, otherwise you will have a nasty hum.
     
    Last edited: Jul 15, 2011
  11. BinaryMike

    BinaryMike Pelagic EE Subscriber

    You'll end up with about +/-34VDC on the rails with that doubler circuit, and that's too much for many 3-pin regulator ICs. I've attached a rectifier and rail-splitter circuit that should do the job with a floating 24VAC secondary winding. Be advised that it's derived from obscure sources and hasn't been tested in my lab. Preamp power supplies often require extra design effort because of the need for excellent AC line noise suppression. Wall-warts typically have poor performance in this area.
     

    Attached Files:

  12. catman

    catman Addicted Member

    Messages:
    7,620
    G'day all, yes I've built quite a few ESP P05B's and it is an excellent split rail power supply powered by a single 16VAC AC plugpack (or 'wallwart').

    On the VSPS, well the simple one op amp DIY phono stage I've built is very similar circuitwise, with the VSPS being a variation of the commonly used full feedback op amp circuit. It works well enough.

    I have used pots on the input of DIY phono stages myself, and they are quite feasible except that hum pick up is a possibility unless everything is properly earthed and poor exact 'matching' between the two sections of a 'ganged' potentiometer tends to happen, even with supposedly high quality pots, unfortunately. On capacitor shunt loading, ideally it show be as low as possible as interconnects and tonearm wiring will probably contribute several hundred picofarads and much more in the phono stage will likely be too much for many modern phono cartridges. These things have been discussed entensively in the 'turntables' forum. Regards, Felix aka catman.
     
    Last edited: Jul 16, 2011
  13. ocmsRzr

    ocmsRzr Member

    Messages:
    90
    The more I get into this, the more KISS is making more sense. I think I'm going to use four 9v batteries so that I have +18V-0V--18V as inputs, with +/-12V out regulated? Is this OK?

    RJM, the guy that designed the circuit said that a 200mAh 9v will run for ~20hrs before needing to be changed. If its only on when I have the phonograph on, I think this is a reasonable amount of use. Then again, maybe I should just daisy chain two wall warts together to get a dual rail set up... Or maybe I can find a dual rail printer power supply...

    How do I figure out how high the input V++ and V-- can be and still have reliable +/-12V out?
     
  14. Detailman

    Detailman Addicted to tweaking

    Messages:
    2,138
    Location:
    Daytona Bch. Fl area

    From my limited experience the best options are just running two 9v's.

    Goes with the Kiss theory.
    If you think you need more voltage just use =+ and- 18. Most op amps operate safely in that area.

    Just the batteries as Hypno stated seem to give the best response to my ears.
    Eliminates a lot of noisy components.
     
  15. quadtech

    quadtech Active Member

    Messages:
    469
    For best results, use 4 of the Eveready 1209 or similar lantern
    batteries, connected in series, to form a 12-0-12 supply.

    I use a simple symmetrical supply based on the 7812/7912 3 terminal
    regulators, and the VSPS works well with that.
     
  16. nashvillebill

    nashvillebill Super Member

    Messages:
    2,853
    Again, running two wall warts in series would be the LAST option I'd consider.

    Running off of batteries would be far far better than the two wall warts.

    Want a cheap center tapped transformer to run a good symmetrical power supply? Go to GW or your favorite thrift store and find a cassette deck for a couple of bucks, you just need one that powers up. Rip the transformer and AC power cord out of it--I used the tranny out of an old Sharp tape deck for my VSPS power supply and I then used a conventional 781x/791x voltage regulator setup.
     
  17. tube mania

    tube mania Active Member

    Messages:
    140
    Location:
    Denmark
    This will work to...phono stage is sensitive to ripple so voltage regulators are needed...

    [​IMG]
     
    Last edited: Jul 16, 2011
  18. ocmsRzr

    ocmsRzr Member

    Messages:
    90
    picked up 4 9V and a set of connectors from Radioshack. I'm gonna wire everything in and see how it looks. Is there anything I should know before I pile in? Should I hook up two and then go with 4 if I need it or should I stick with 4. Which will give me the best battery life?

    How do you find these things out so I don't have to pester you guys in the future?
     
  19. tube mania

    tube mania Active Member

    Messages:
    140
    Location:
    Denmark
  20. HypnoToad

    HypnoToad Ms Puss Puss Subscriber

    Messages:
    5,961
    Location:
    Melbourne, Australia
    Walmart sells twin packs of Rayovac 6v lantern batteries, buy two and have 12v per side. Will power most opamp based phono stages for over a year.
     
    Last edited: Jul 16, 2011

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