1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

AD797 Phono Stage Build and Help Desk Thread

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

  1. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    You could put a relatively large cap in series with the resistor to ground at the inverting input of the output amp.
    This will reduce the output offset to more or less just that of the input- which if the impedances are correct will be less than 1mv . The cap has to be non-polar, but it has a signal across it which is both very low frequency and very small, so distortion is extremely small and inconsequential and ONLY at frequencies close to the -3dB point and their harmonics/intermod products. If you really wanted to you could shunt it with smaller value "better quality" caps.
    If the resistor is 390 ohms, then a 47u cap will give a -3dB point at 6.5Hz, a 22u at about 15Hz, etc.
    The feedback amp I previously added only has the advantage of requiring a smaller cap and providing a "true" DC coupled design. Personally, I don't think it matters, but some do.
     

     

    Please register to disable this ad.

  2. elofan15

    elofan15 New Member

    Messages:
    1
    ....Do not intend to steal the thread but since my question relates to this project that im trying to complete...What would be the best way to implement Mono-Stereo Switch ? :bowdown:
     
  3. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,020
    Location:
    Eastern Bamastan
    I did it on another similar type with a film capacitor bridging the two inputs.
     
  4. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    Short the resistors that go to the non-inverting input of the second opamp together at the input terminal of the opamp is how I would convert to mono.
     
    Pio1980 likes this.
  5. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    There are, albeit theoretical, reasons why you would not wish to do it this way. The series coil resistance for a LOMC cartridge is in the c.3-20 ohms region. To get true mono the impedance at all audio frequencies has to be significantly lower than this. If the cap is sufficiently low impedance to allow for true mono down to LF then a very large cap will need to be used.
    Even if you assume that frequencies below, say 100Hz don't need to be truly made mono, the cap is still huge.
    In addition, the low impedance will allow a fairly large signal current to flow through the windings which will create a significant back EMF due to Lenz's law and roll of the HF response and alter the transient response of the cartridge.
    Shorting the resistors that are after the input amp outputs together does not have these effects. Just a switch will do. The large-ish resistors protect the opamp outputs. The disadvantage- each output sees a 6dB loss in amplitude as you now have a voltage divider (the output is (A+B)/2) - but this is exactly the same as ideally shorting the outputs from the cartridge together ignoring some of the other issues.
     
    sachu888 and Pio1980 like this.
  6. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
    Bharat,where Tigers rule

     

    Please register to disable this ad.

  7. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    Hi Sachin- what value are the resistors? I assume that the switch is applied directly at the output of the amps, and some care needs to be taken to avoid either overloading the output opamps or increasing the output impedance too much that the HF is rolled off with the cap load that the next stage/cables represent.
     
    sachu888 likes this.
  8. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    27,020
    Location:
    Eastern Bamastan
    Makes sense, as does your suggestion that leaves the front ends as gain buffers to prevent the unintended consequences.
    A better 'global' approach.
     
  9. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
    Bharat,where Tigers rule
    I really don't know the resisters values, but he is getting good results.

    Regards
    Sachin
     
  10. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    The reason I'm asking is that the picture seems to have brown black black red- then brown as the color codes. I thought that was 10k ohms 1%, but it's been a while since I've been completely facile with the color codes. If so then the mono output resistance is 5k to each of the two outputs in parallel, which is pretty high.
     
    sachu888 likes this.
  11. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Hi, after a bit of help, I've just finished putting together one of these thanks to Sachin who put me on to it and supplied the excellent pcb and One for the power supply, i used all the exact parts from the bom list and am used to soldering. Checked I'm getting exactly a steady 12 volts dc supply from the power board but all I get is hum and no sound, have ch checked all leads and the cartridge I know works ok, any one help me please?
    Many thanks
    Phill
     

     

    Please register to disable this ad.

  12. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
    Bharat,where Tigers rule
    can you post some pics? It runs on +/- power supply(-12/0/12)

    Regards
    Sachin
     
  13. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Hi Sachin, here are some photos, very strange but I double checked my voltage and found it at 9volts and dropping slowly, checked and one of the 4700uf power caps was very hot and staring to expand, I checked and polarity was correct so swapped out with a spare and rechecked voltage at a very steady bang on 12 volts, reconnected everything up and now no hum but no sound until volume at full on my amp and it can only be heard if you put ear to speakers. Double checked gain and impedance at still bank on at 1k for gain and 100ohm for impedance, I have a Denon dl 103. Any ideas? My ad797 are an, are these ok or do they have to be anz?
     

    Attached Files:

  14. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
    Bharat,where Tigers rule
    I think I have found the problem, as I said this phonostage needs sual power supply + / 0 and minus. I see you only use 2 wires from power supply output to phonostage power input.You have to use 3 wires and set the voltage to +12/ center is 0 and -12V.
    There are two trimpots on power supply, 1 for settng positive and 1 for negative output supply. Which transformer/ wall wart did you use?
    PS: please don't reverse +/- on phonostage board.

    Regards
    Sachin
     
  15. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Hi Sachin, I see now, I was trying to find pics of other peoples builds to check that the power wiring was correct, so I should have a wire coming out of the power board -v marked as number 1which is the negative and this will go to my negative on the phono stage, a wire from the ground on the power board marked as number 2 going to the ground on the phono board and a wire going from +v marked number 3 going to the plus on the power board? Then I should set up the voltages as per the original instructions to get plus 12 volts on the plus side measuring between +v and ground and minus 12 volts measuring between -v and ground? Is this correct?
    I'm using a wallwart transformer, 12volt AC 1amp output .
    Many thanks again
    Phill
     
    sachu888 likes this.
  16. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
    Bharat,where Tigers rule
    This is correct
     

     

    Please register to disable this ad.

  17. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
  18. Phillamon

    Phillamon New Member

    Messages:
    46
    Location:
    South east Essex
    Didn't get a chance to try it out with correct wiring of power supply till this morning but it's still the same, do you the think i might of fried something when I connected it up incorrectly before? When it was wrong I had the ground from the power supply to the minus connection on the.phono stage and the +v in the power supply going to the plus on the phono stage?
    Many thanks Phill
     
  19. wyn palmer

    wyn palmer Active Member

    Messages:
    345
    Alas, it's possible that the opamps are dead. In particular the 797s have input protection diodes that could be toast. If you have a DC meter it's easy to check if the opamps are OK. Probe the + and - supply pins- they should be no more than about a couple of hundred mv lower than the board edge supplies, then probe the + and - input terminals and the output terminals of the opamps. The input terminals should be equal and the output terminals should not be railed. The actual output voltage value for the input opamp should be pretty small- tens of mv at most, but the output opamp value is quite variable- up to 2.5v or so depending on the opamp trimmed offset values and whether or not the circuit resistors were modified to give the best output offset as I described about 10 pages ago.
     
    Pio1980 likes this.
  20. sachu888

    sachu888 Spare the tigers, kill me Subscriber

    Messages:
    1,295
    Location:
    Bharat,where Tigers rule
    Hi Phil
    Opamps can be damaged by wrong supply voltage. You can test opamps as suggested by Wyn or can try some cheap mono Opamps in place of AD797 and see. Have you checked all RCA connections Input/output?

    Regards
    Sachin
     

Share This Page