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AD797 Phono Stage Build and Help Desk Thread

Discussion in 'Turntables' started by HypnoToad, Feb 11, 2013.

  1. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Hi Wyn
    Thanks for the info, just a few questions as I not too sure with electronics and this is my first build so bit unsure with terminology, I have found diagrams of f the Opamp pin inputs and outputs so no where to probe with my dc meter but do I keep the black probe on the ground dc input on the board and then use the positive red probe on the opamp pins to take the readings. Also what is meant by railed? I have taken readings from the plus and minus inputs supply pins and my positive is 9.9 and my negative is 10.5, this seems a bit lower than you stated, I rechecked my power supply and when disconnected and that's still bang on 12volts on the negative and positive?
    Many thanksgiving
    Phill
     

     

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  2. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    Yes, black probe on ground. The supplies have 10-20 ohms of series resistance to the opamps. For there to be 2v of drop you need to have 100mA of current or more. The opamps have c. 12mA of quiescent current combined. This also assumes that the electrolytic caps are undamaged.
    This is what I would do, start by removing the opamps. Measure the voltages at the sockets where the supplies to the opamps are supposed to be. The voltage should be pretty well exactly what the supply input voltages are. If not, you have a problem. If OK put the opamps in one at a time, starting at the input, and check the supplies again and if they're OK check the input/output terminals.
     
  3. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Hi Wyn, will give these a try, just been looking at RS to get my ad797anz if I need them and some replacement input opamps, they have in stock some lme49720na/nopb, have checked and the only difference between these and the lme49710na/nopb is the noise density is 2.7nv for the 20 and 6nv for the 10, can I use these if needed?
    Many thanks Phill
     
  4. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
    Bharat,where Tigers rule
    LME49720 are dual opamps. They won't work in this phonostage. You will need to use mono opamps in this circuit. I would suggest first you rectify the problem and later upgrade lme49710 to OPA627 etc.
    There could be some electrolytes shot due to incorrect voltage. First check the voltage without opamps as suggested by Wyn and later with opamps. You may find out the actual problem. i would also suggest to check the RCA connections if voltages are correct.

    Regards
    Sachin
     
  5. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
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    Location:
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    Most probably all your electrolyts are shot. I would suggest you to change all your 10uf electrolyte caps. That should solve your problem.

    Regards
    Sachin
     
  6. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Thanks Wyn and Sachin, looks like I fried fried all the opamps, voltages to each opamp supplies are ok when put in one at a time but out puts are all 11.6v or around that. All RAC connections are good. I will go through all the electrolytes to see if there ok. That's what I get for being impatient and not checking everything first. Many thanks guys for all your help and even though I'm out of pocket and got to buy some more parts I've really enjoyed this bit of fault finding and have learnt a lot. I will update when parts are here and hopefully will get to do a bit of listening.
    Thanks again
    Phill
     

     

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  7. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    Oh and by the way, be very careful with your AD797s. The ones from China seem, demonstrably, to be fake. Best to buy from a reputable source. This is a particular beef of mine. It's actually pretty easy to be fairly certain that the opamps are real with a DC meter, but by then it's a bit late. It also doesn't matter what grade/package of the AD797 you are using, just as long as you can socket them.
     
  8. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
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    I would still suggest to change all electrolyte caps for safer side.

    Regards
    Sachin
     
  9. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Will be getting my ad797 from RS as not bad price and come next day and also change my caps as well to be on the safe side. Many thanks again
     
  10. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Any suggestions please on the best wire to use between the RCA,s and phono outputs and inputs? I'm thinking of changing mine while I wait for my replacement parts to some spare sheilded van dame cable I build my interconnects from and solder them to some spikes fitted into the board.
     
  11. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    I just use ordinary insulated stranded wire and twist the ground and signal lines together. It works quite well. If you are concerned about rfi pickup the best solution, in my opinion, is to make the change to the schematic, in particular the additional hf roll off, that I recommended earlier.
     

     

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  12. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Well parts came, I got some opa727 ap Burr browns from eBay that had been pulled from a navel vessels intercom, 2 for £11, fitted new caps and the the op amps, rechecked everything, plugged in everything and turned on, fingers crossed and waited for my tube amp to warm up. But the needle on the record and a wonderful noise stared coming out the speakers, ive been playing records all day and it's getting better all the time, thanks Wyn and Sachin for all your help. Had a little hum at first when volume turned up, thought it might be as I had the PSU too close and inside the same enclosure so moved it outside and away from the phill no stage but still there, though that I try and disconnect the ground from the tt and total silence, no hum what so ever even after I replace the PSU in the same enclosure. What a fantastic sound phono stage and all the fun making it. Next stage is to finish my CNC mm phono stage and finish the enclosure. Will post pic when all complete.
    Cheers
    Phill
     
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  13. tubesguy

    tubesguy New Member

    Messages:
    13
    I just finished the assembly on my board, and when I tried to adjust impedance and gain I ran into some odd results.

    The right channel adjusted just fine to my targets, but the left channel gain trimmer resistor would not adjust to any value higher than approximately 580 ohms. In addition the two designated 390R resistors in the left channel read approximately 330R, measured in circuit. Right channel were spot on at 390R. (These resistors were part of a Mouser purchase of four pieces, and the color codes match on all four.) Without a schematic, I'm kind of lost regarding troubleshooting. I'll try replacing the left channel 2k trimmer resistor for gain, but I'm scratching my head regarding the "390R" resistors. Any suggestions before I do my least favorite thing and start desoldering components? Thanks in advance - Pat
     
    Last edited: Nov 25, 2018
  14. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    You're measuring them in circuit? Are the opamps present? Is the circuit active? In either case it's really not recommended.
    At the very least remove the active devices and try again. Remember that if you measure a resistance between two nodes it's not just the resistor directly between the two nodes but the resistance of all circuit paths between the two nodes that you measure. If you're going to set the gain up it's much better to apply a 1kHz ac signal to the input and trim the gain with the circuit active as that's the frequency that the cartridges are all specified at and the RIAA compliance is referenced to. The circuit is pretty simple and you can figure it out from earlier postings starting at about page 58 I believe.
    It's extremely unlikely that resistors with the same color codes are non-compliant to the values/matching defined by the codes, in fact it's entirely outside of my experience.
     
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  15. tubesguy

    tubesguy New Member

    Messages:
    13
    Thanks Wyn, I was measuring with the opamps in place. Removed them, and I was able to measure and adjust both the impedance and gain trimmers in both channels as set forth in one of the introductory posts. Once I have power to the circuit I'll try to trim the gain as you have suggested. Appreciate the help.
     
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  16. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
    Bharat,where Tigers rule
    Good to know Phill, that phonostage is working now.

    Regards
    Sachin
     
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  17. Gricko 1964

    Gricko 1964 New Member

    Messages:
    2
    Hi all !After long search i dont have LME 49990 so what will be good replacement,OPA 1611 ?Thank you for your help !
     
  18. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    The OPA1611 is TI's recommendation .The LME49710 can still be had.
     
  19. Gricko 1964

    Gricko 1964 New Member

    Messages:
    2
    Thank you!
     
  20. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    I used opa727, there plenty on eBay and you don't need to use the adaptor.
     

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