Converting the AD797 MC Phono Pre to a MM Phono Pre - A Mixed Design.

Discussion in 'Turntables' started by USMC Spike, Jan 1, 2019.

  1. USMC Spike

    USMC Spike AK Subscriber Subscriber

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    This thread will discuss the changes necessary to convert the AD797 Moving Coil (MC) phono preamp to
    use a Moving Magnet (MM) cartridge.

    Hypno Toad worked to create the design of both the AD797MCPRE Printed Circuit Board (PCB) and a MMPRE PCB. These are through hole designed boards.
    AD797 thread located here:
    http://audiokarma.org/forums/index.php?threads/ad797-phono-stage-build-and-help-desk-thread.501186/

    MM thread located here:
    http://www.audiokarma.org/forums/in...igh-end-phono-stage-no-expense-spared.371889/

    sacgu888 worked to have these boards produced along with a power supply board
    for AK members, here is the LINK

    My inquiry started with this post:
    http://audiokarma.org/forums/index....help-desk-thread.501186/page-71#post-12273730
    followed by some additional questions, answers, suggestions, etc.

    Wyn Palmer, a retired Analog Devices, Inc. engineer, graciously suggested we create a new thread.

    Here is a summary of previous information:
    Is there a way to adjust the gain of this Phono amp so that It can be used with
    a MM Ortefon red cartridge? Specs as follows:
    Output: 5.5 mV
    Frequ at -3dB: 20-22000 Hz.
    The following are recommended:
    Load R: 47kOhms
    Load C:150-300 pf

    Devices in my inventory:
    AD797
    LME49990
    LME49870
    LME49710HA
    LME49720HA
    LT1115
    LT1028
    LT1128
    LT1468
    OPA 134
    OPA2134
    OPA1641

    I've read the RIAA Phono Preamplifier section from Jung, W. (2002). Op Amp Applications,
    Analog Devices, Inc. (ch 6.11 - 6.27). While it's good background material, it's not applicable to this discussion.

    My turntable is nothing fancy, a working Thorens TD-147, not currently in my system
    as it's slated for electrolytic cap replacement and cartridge fitting.

    The cartridge I've chosen is suitable for a reentry into vinyl of which I have one milk
    crate left of various LPs. The cartridge specs follow:
    Ortefon Red MM Cartridge
    Output: 5.5 mV
    Freq at -3dB: 20-22000 Hz.
    The following are recommended:
    Load R: 47kOhms
    Load C:150-300 pf
     
    Last edited: Jan 1, 2019

     

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  2. USMC Spike

    USMC Spike AK Subscriber Subscriber

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    Here is a brief summary of our discussion. I've tried to edit out some of the non essential information.
    Can the AD797MCPRE be changed so that I can use a MM cartridge with it?
    Ok, I'll have to figure out how to measure it.
    Yes
    Ok for the moment.
     
    Last edited: Jan 1, 2019
  3. wyn palmer

    wyn palmer Well-Known Member

    Messages:
    583
    So, how do you want to proceed with this? Do you want to ask questions and I will answer- sort of how the Leach redesign thread went?
    I'm willing to add fairly detailed explanations with schematics and simulations- then instructions on how to actually implement the changes on the board.
    It's a bit of a trade off between too high levels of technical detail that many people will not follow and providing sufficient information so that the changes are not opaque.
    It would be really good if you could "follow along" by running LTspice simulations in parallel, just as happened in the Leach thread. It does require a bit of learned expertise to do so, but you might enjoy the experience.
     
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  4. USMC Spike

    USMC Spike AK Subscriber Subscriber

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    Hi Wyn, Yes, and no. I'm more proficient at using NI Multi sim as that is what we use at our community college.
    So I can simulate it with that. Also, I have a various general types test equipment.
    I think there is a way for me to import spice models into Multisim. :)

    The problem is I don't have a schematic to start with.
    I just ordered a couple of boards, so it will be a little
    bit before they arrive. In the mean time Lets go.

    Thanks,

    I sent you a PM (conversation)
     
    Last edited: Jan 1, 2019
  5. wyn palmer

    wyn palmer Well-Known Member

    Messages:
    583
    I don't have access to NI's Multisim. LTspice is free, straightforward/intuitive to use, and has embedded models for several of the opamps you have and adding the LME49710 is simple and I will provide instructions. Heck, although not exactly the same (amongst other things the model has some odd HF behavior), the Lt1115 model that is included in the LTspice libraries is a near drop in replacement for the 49710.
    I also can send/post circuit files that you can use.
     
  6. USMC Spike

    USMC Spike AK Subscriber Subscriber

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    Looks like LT Spice it is.

    Hypno Toad send me the link to the schematic, which I'll include where
    as the starting point.

    Well the image that was below has been removed.
    [​IMG]
     
    Last edited: Jan 26, 2019
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  7. USMC Spike

    USMC Spike AK Subscriber Subscriber

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    Oh the joy, trying to add j2 into a new schematic. one hour and counting.
    LTSpice XVII doesn't like me.
    Help doesn't recognize "input" RCA Jack, Jack etc.
    Can't even place and AC Signal input on the screen.
    I have a blue screen with nothing on it in my sim.
    Now it wants a search string and won't do anything else.
    ESC key does nothing.
    Enter closes that dialog box.
    I click on the thing that looks like an electrical plug.
    select voltage, click ok, then my mouse pointer disappears.
    I thought I'd left click and it would place it, NOT.
    I right click and zoom out the screen turns gray.
    Okay I try again, to place a voltage, no dice.

    Now I'll try to place something else.
    I picked the resistor ICON and then clicked
    into the work gray work space twice.
    There were two dots placed on the gray matter.
    So small I can't see them, never fear, the + magnifier is there.
    I click it, and make is square over the two dots.
    the dots disapear. -- More Joy.

    I clock the capacitor icon then I
    click into the gray space. 10 somethings
    that look like fleas on a dog. I can't wait
    to see how these will disappear also.
    They don't this time, as I went View:ZoomArea.
    The small flees moved to the middle of the screen.
    Now when I left click, I get more fleas. -- Even more joy.

    Back to view: Zoom to fit
    whamooo, they move to just below my screen, with
    just a tiny piece of the flea on the boarder. I check and
    see that both the application and the sim screen are maximized.
    somehow I fake it out with the zoom icon and magnify it.
    It moves to center screen, by this time, I click the zoom icon
    again and drag over the fleas. About 10 times.
    I have something, but now it's too big.
    I press Ctrl + Z keys to undo what ever I did.
    That doesn't work. Wow, user friendly.
    There is no more joy to me. I'll post screen shot for
    Everyman to view.
    Sadly I'll now quit--and I'm not even ahead.

    Fun with LTSpice XVII.jpg

    Oh the More, More, More, More, did I say more joy?

    What a way to start the new year.
     
  8. wyn palmer

    wyn palmer Well-Known Member

    Messages:
    583
    I'll send schematics tomorrow. It's pretty simple so I've no idea what the issue is.
    Hopefully that will help. Are you using a PC?
     
  9. sregor

    sregor AK Subscriber Subscriber

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    There appears to be an error in the feedback loop of the second IC I believe it should be:
    upload_2019-1-1_20-27-53.png
     
  10. wyn palmer

    wyn palmer Well-Known Member

    Messages:
    583
    Yes, that's correct. There's also the supply decoupling network. I have updated the schematic to include it. I'll send it and my modified MM schematic with the updated RIAA values tomorrow am. Then I can send the split design and simulations.
     
  11. USMC Spike

    USMC Spike AK Subscriber Subscriber

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    Yes, I'm on a PC. I did finally manage to place a ground arrow. :)

    Then I got about 1/2 devices in the sim, when I tried to rotate
    the resistor between ICs I fat fingered the mouse on something
    everything disappeared, nothing remains.

    F9 and shitF9 says there is nothing to redo or undo.

    I can tell this feels like free ware.
    So where can I find pots, and RCA, BNC, Jacks and connectors.
    LT doesn't seem to have any. Couldn't find the rotate button.
    LT can't find "rotate" device, object, jack, etc.

    But I did manage to come up with this
    in about an hour:

    2nd Schematic.jpg

    Then I tried to add power (more joy):

    2_1 Schematic.jpg

    Now I can't tell you how much longer that took, because
    the first time the AK system said it was 58 minutes.
    When I looked at the AK system it now says 8:50 PM.
    Where is that happy banana, I want my little girl to start
    screaming with joy!

    :banana::banana::banana::banana::banana:
     
    Last edited: Jan 1, 2019

     

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  12. wyn palmer

    wyn palmer Well-Known Member

    Messages:
    583
    Below is the left channel full schematic- original design. The device numbers do not necessarily conform to the identifiers on the board.
    leftold.PNG
    Below is the modified (new) design, with a tighter RIAA characteristic and RF rejection added.
    The sensitivity for 1v rms out at 1kHz is 432uv.
    leftmod.PNG
    Below is the MM mod of the above (new) design with the AD797 at the input replaced with an LME49710 and the sensitivity reduced to 5.5mv. RIAA compliance is about +/-30mdB 20Hz-20kHz.
    MMnew.PNG

    Below is the MM mod- but with a distributed pole-zero implementation of the RIAA (mixed).
    mixed1.PNG
    Either an LT1115 or a LME49710 can be used interchangeably in this last one with essentially identical results.
    The 10pF (C27,C28) is necessary for the LT but not the LME. Without it the LT is marginally stable at HF.
    The RIAA compliance is c. +/- 25mdB nominally.
    If you would like .asc files of any of the above please ask.
    If you have questions about the difference, performance levels etc. please ask and I will add simulation results with commentary.
    All of the mods can be relatively easily made on the AD797MCPRE board as they consist mostly of the replacement of components or the placing of two Rs in series or the addition of a shunt cap- either across a resistor or to ground from a resistor. I've tried it. It's not terrifically pretty- but it works.
     
    Last edited: Jan 2, 2019
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  13. sregor

    sregor AK Subscriber Subscriber

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    Good work - almost correct. You need to remove the connection between the output of U1 to R4, and to move the connection between R5 and C4 to the other end of C4. I don't see any other errors, but....
     
  14. wyn palmer

    wyn palmer Well-Known Member

    Messages:
    583
    There are a few errors.
    I've attached a schematic that might be of some help.
    It includes a couple of the prior mentioned schematics with the LME49710 replaced with the LT1115.
    It does include an AD797A which you will not be able to simulate without adding the device to your libraries, but it should get you off to a good start.
    You need to change the .txt extension to .asc then open it in LTspice. I think that works even if you don't have the model for the AD797A.
    By the way, I should make it clear- I prefer the LME to the LT. The LT model has some pathological issues in the 2-10MHz region that I don't like. The LME has none of these issues and in my opinion is a more solid design- assuming that the macro models are accurate.
     

    Attached Files:

    Last edited: Jan 2, 2019
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  15. USMC Spike

    USMC Spike AK Subscriber Subscriber

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    Ok, I see them. I am unsure of what they mean, so please forgive my ignorance.
    One thing that would help, is how to rotate components in LT spice.

    AND

    Generally, how do the sims correlate to real world listening of the devices that
    we are discussing?
    AD797
    LME49990
    LME49710
    LT1115

    Can you also send the LTSpice info for the devices to include into my library?

    I'm not sure where to go from there at the moment. So, if there is something I need
    to know or that would be helpful background wise for those following along please
    continue.

    That being said I thought of some generalities perhaps you could explain.
    I read alot and see in the data sheets specifications for the following:
    Power Supply Rejection Ratio (PSRR)
    Common Mode Rejection Ration (CMRR)

    For example, the LME4990 has the following:
    PSRR: 144dB
    CMRR: 137dB

    If I understand it that would be the ICs ability to separate the music signal from the noise
    that is in the background resulting from the powering the IC and the electro-mechanical noise
    of the devices on the board etc.

    Somewhere among the PCB layout there is susceptibility of RFI that can enter the signal
    chain and corrupt the music signal and cause problems with the various SS devices, yes?
    I'm not sure I have a good understanding of the causes. For that matter, I'm also not sure
    that I understand the solutions...other then button things up and keep parts isolated from
    other parts in the circuit.

    I've read about this on the boards and forums and also we discussed it briefly in my some
    of my EE tech classes. When I asked for specifics they were short on answers.
    When I inquired as to why Florescent light figures were noise or mentioned that
    cheap light dimmers can be noisy I was flat out told they were not. For the latter,
    I am not talking about the large rheostat type dimmer on isolated plates that are
    used for studio control room dimming.

    In the mean time I'll try and get the attached sim into my LTSpice and get it running.
    If you don't mind you can send the the other 2 or three sims that you provided images
    to, so that I have them in my sim library.

    Thanks,

    Yes, there is a void where the IC would be....right dead smack in the middle.
    ALackThereof.jpg
     
    Last edited: Jan 2, 2019
  16. wyn palmer

    wyn palmer Well-Known Member

    Messages:
    583
    The rotation commands are the clockwise and counterclockwise circular arrows about four icons from the right on the control bar at the top.

    http://audiokarma.org/forums/index....age-build-and-help-desk-thread.501186/page-61
    Post 1210 explains how to add the AD797A model. The LME49710 is a bit different and I'll have to dig through my memory to remember how I did it.
    I'll post the info later.

    How do the sims relate to real world listening?
    Who the heck knows. People claim all sorts of preferences based on, well, not very much that makes any kind of sense from an engineering perspective, and there's an awful lot of confirmation bias out there. So, ultimately caveat emptor...
    There really is not much that I can say in a few paragraphs that will not create controversy- which is not my goal- so I'll try to be as objective and as terse as possible while not pretending that the lists of the relative merits are all inclusive.
    In my opinion, the one that generally makes the least sense from an engineering perspective is the passive design (the CNC preamp)- it tends to have the worst noise performance and the greatest susceptibility to overload- but it does have the magic of no capacitors in any feedback loop and the interstage filter that defines the equalization has excellent RF rejection. And yes, I know that capacitors have non-linearities, but people also prefer caps in some cases that are measurably worse than others they dislike- so what can I say?
    It's also possible to design a passive equalization stage that doesn't have the built in voltage attenuation, but that's beyond the scope of this exercise.
    The "all the equalization at the input stage" approach has the best noise performance and the best overload characteristic, but has poor intrinsic RF noise rejection as the gain remains unity up until very high frequencies, and having all the gain in one stage leads to less feedback at HF which can increase the deviations from Ideal for the RIAA stage and also the distortion.
    The mixed design has excellent overload characteristics as the HF pole is placed directly at the input. The input opamp becomes unity gain at frequencies of a few kHz and above which counteracts the RIAA preemphasis boost prior to the low pass needed for the LP cutter at or above 40kHz . It is then followed by a second HF pole which provides RF rejection and even better overload protection for the output stage. The final opamp provides the low frequency boost needed for the equalization- at frequencies where there is lots of gain for distortion reduction- then it becomes constant gain at frequencies above 500Hz or so. It also has good noise performance so it's a decent solution on all fronts.
    Yes, opamps can have very large power supply rejection and common mode rejection. Both of these decline with increasing frequency. for a variety of reasons.
    The mechanisms for noise pickup are complex and beyond the scope of this write up. However, it's beneficial to have good HF power supply rejection and good RF rejection in the amp signal path. RF components can cause intermodulation products with desired signals that can fold down into the audio band and cause audible issues.
     
    Last edited: Jan 2, 2019

     

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  17. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    This thread is fun. Subbed.
     
  18. USMC Spike

    USMC Spike AK Subscriber Subscriber

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    Getting closer, it still doesn't work.
    I was able to clean up the schematic a bit.

    2_2 Schematic.jpg

    Maybe someone can assist who's following along.
    Wyn, do we have the LME49710HA Data or do we need
    to find it?

    I've spent over and hour on the ADI website reviewing a video
    it was overly complex so I stopped. I placed something in the
    sim relating to the AD797 but got stuck over in the
    I for get the name, I used opamp2, then right clicked
    it and remained it. I'll be damned if I could figure out how
    to place the device in LTSpice.

    So then LTspice is ok for getting the electronics part working
    to see if we are in the ball park and hopefully won't blow everything
    up; (i.e., letting the sacred smoke out of the devices.)

    From there we have to assemble the PCB with components, test it
    then start listening and measuring.

    Engineering versus Listening, I recall something from that guy from
    the U.K., who spent time in India's sunny climb, a serving of Her Magistracy
    the Queen....

    Now east is east and west is west and ne'er the twain shall meet. -- Rudyard Kipling.

    At least we know that going in.

    NOTE: Did the additions according to the post#1210.

    How do I create a, "A .lib spice directive needs to be added for AD797.sub"?
    I know these are kind of basic questions... bare with me.

    Post Script - Yes, I found the rotation icons up there finally.
    Before, I was using the undo circular back arrow which was
    causing me fits.

    Post Post Script, I'll try adding the AD797 to it and see if it works.
    Realization, that now I have included the device to /subs and /lbs
    Your schematic might work on my 'puter.
     
    Last edited: Jan 2, 2019
  19. EngineerNate

    EngineerNate AK Subscriber Subscriber

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    I see your problem, at least one of them. You've set V3 and V4 to -15V and then hooked the opamps up to the '-' side of the voltage source. If you input a negative voltage they'll output a negative voltage on the "+" side of the voltage source. As a rule I hook the minus side of DC voltage sources in LTSpice to ground.

    My preference, if the schematic can accomodate it, is to setup Vplus and Vminus "rails" in LTSpice with a single voltage source for each voltage. I'll only add a second if it's really really cumbersome to route that voltage where it needs to go with wires. In your schematic, I'd have a +15 and a -15 off either to the left or right above and below the existing schematic with horizontal "rails" drawn in and run wires down to each opamp for power.
     
  20. sregor

    sregor AK Subscriber Subscriber

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    Looks like you have a ground on the input signal line.
     

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