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. wyn palmer

    wyn palmer Active Member

    I hope that it's not really your intention to apply 1vrms at 1kHz into the preamp phono input (that would be bad...). The input should be 5.5mv rms at 1kHz and conform to the RIAA preemphasis curve if you deviate from 1kHz. The output from the new phono stage is intended to be c.1vrms.
    Theoretically, the distortion should be more or less unmeasurable using the new design. However, it might be interesting to explore the overload characteristics.
    Last edited: Jan 12, 2019


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  2. wyn palmer

    wyn palmer Active Member

    That does not look like what I would expect.
    noise1.PNG noise2.PNG
    The first is the unweighted noise for the 5.5mv rms sensitivity preamp.
    The second is the A-weighted noise.
  3. USMC Spike

    USMC Spike AK Subscriber Subscriber

    Tejas, North Central
    1. Pre amp on hold. No, I haven't done anything with it yet.
    Completly untested.

    2. Let me find the last unmodified version of the schematic and I'll start from there.

    3. what does the c. mean when you say c. 1vrms?

    4. This v(onoise) plot is different than yours.
    Last edited: Jan 11, 2019
  4. wyn palmer

    wyn palmer Active Member

    c. is a contraction of circa (Latin) which means about, approximately, close to, in English.
  5. USMC Spike

    USMC Spike AK Subscriber Subscriber

    Tejas, North Central
    Starting over again.
    Here is my first v(onoise) plot:
    01.Post245 RestartDataFixed.jpg 02.RIAA restart Output.jpg
    Here is the noise data for this plot:
    01.POST245 RestartData.jpg

    Confirmed, they are the same.
    Last edited: Jan 11, 2019
  6. nj pheonix

    nj pheonix AK Subscriber Subscriber

    New Jersey
    Not sure about anybody else but I'm still lurking in the back ground.
    When I started my intention was (Still is) to build a better phono stage.
    I have wanted to get somewhat acquainted with LT spice time permitting.
    I'm not seeing the free time being available anytime soon but hope springs eternal.
    If so I'm sure thread will prove a valuable reference and I thank both of you for continuing it.


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  7. USMC Spike

    USMC Spike AK Subscriber Subscriber

    Tejas, North Central
    Thanks NJ. It is always good to know that someone is following along.
    I'd like to do a series of measurements to confirm what I already have
    in my pre amp phono stages.
    Audio Research SP3-A1
    Audio Research SP-10MKII
    Marantz 3250

    Then when I do the boards:
    MM LME49710 (from AD797)
    MC AD797 Board

    I'll have a starting point with baseline data for each individual
    stage. At some point whole system data too.
    When I put something in the system, what ever I pull out
    can go to the bench for a Measurement.

    It's good to know and it should be of benefit those following
    along or those who are in need of baseline info for their units.
  8. USMC Spike

    USMC Spike AK Subscriber Subscriber

    Tejas, North Central
    Yes, it is a phono bad.
    5.5mv rms Input.
    We'll see what the output looks like at 1vrms output :)
    What would be a safe reasonable overload voltage NOT 1vrms?
    There must be some general rules for testing of phone stages, yes?
    Anyone know?

    Google is our friend. These look promising from Hi-Fi World:
    Phono Stage Tests: LINK
    All Pages

    Time to read what the site has to offer.

    BTW anyone offering free tickets and passage
    from Tejas to the Birstol Hi-Fi Show
    February 22 thru Feburary 24 2019?
    Last edited: Jan 12, 2019
  9. wyn palmer

    wyn palmer Active Member

    At 1kHz the output stage clips at about +17dBv. This is about 15mv rms at the input or about 22mv peak. Why go beyond this at 1k.
  10. USMC Spike

    USMC Spike AK Subscriber Subscriber

    Tejas, North Central
    That is the maximum range I should monitor.
    I should start 0mv /shorted input for Noise floor.
    Then stepped 1mv, 3mv, 5mv, 5.5mv for standard MM cart.
    then for overload:
    6mv, 7mv, 9mv, to onset of distortion...
    estimate to happen around15mv.

    I'll keep my eyes on the scope and when the signal begins to flatten....
    document that point. I posted a link to Hi-Fi world that covers this topic.
  11. USMC Spike

    USMC Spike AK Subscriber Subscriber

    Tejas, North Central
    Great, I see now HIFIWorld (HFW) starts with white noise.
    Through an accurate inverse RIAA network.

    That is for frequency response,
    measured using a manual swept sine wave generator?
    If manual, then each frequency's output has to be adjusted.
    And we do have the Reproducing Characteristics B.S. 1928:1965
    table of values. :)


    MM x100 gain = 40dB
    MC x1000 gain= 60dB

    MM x200gain 46dB
    MC x1k-3kgain 60dB - 70dB

    Well, HFW measures theirs phono sections at the following input levels:
    MM 10mV
    MC 1mV

    Ideal (my observation)
    10mV is roughly twice the standard MM cartridge output rating of 5.5mV.
    So they are in effect measuring and amps overload characteristic above the cartridges
    standard output value...if I understand what the article is saying. I don't know this is
    my first time. So if I'm open for suggestions to cut measurement time.

    My question about "How they measure it"
    Gain at 1kHz nput voltage is measure direct (i.e. p.d and not generator emf).?

    I have no clue what "p.d" vs. "generator emf"?

    Here is a link from the national archives, it is the
    Tektronix Cookbook of Standard Audio Tests: LINK

    Having this reference will at least give us a common vocabulary
    and describe general test procedures, test setups, load matching
    etc. Is it dated, yes. These tests should be able to be made by
    anyone who is a semi-serious hobbiest.
    Last edited: Jan 12, 2019


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  12. wyn palmer

    wyn palmer Active Member

    The "normal" MC and MM output levels are 0.25mV and 2.5mV, respectively, measured in the usual way- 1kHz, 5cm/sec. Generally, the max level that is recorded on an LP, and the point at which the cartridge must maintain tracking, is 20cm/sec- or 4x. This leads to a max input level of 1mv and 10mv- hence the test conditions.
    Your signal generator most likely has a 50 ohm output impedance and in my case I "pad down (p.d.) " the output by putting it through a resistive attenuator to reduce the level and also the source impedance- for example loading a 50 ohm generator into 5.6 ohms drops the level to c.1/10 of the unloaded (generator) output level and drops the output impedance to 5.0 ohms. This also improves the effective resolution of the output by a factor of 10 which might also be helpful. You can increase the attenuation, reduce the output impedance, and improve the resolution by padding down even more.
    In order for the distortion measurements to be meaningful you need to determine and remove the source distortion levels from the measurement. There are various ways to do this.
    Last edited: Jan 13, 2019 at 8:20 PM
  13. USMC Spike

    USMC Spike AK Subscriber Subscriber

    Tejas, North Central
    That makes sense.
    I have a couple of choices here, The HP339A is very accurate and has clean sine waves
    with minimal distortion, <= -120dB. It is 600ohm.

    I redid my Leader LFG1300S which is only .05% distortion at 50 ohm output.
    To pad down (p.d.) the output, I can feed it into my HP350D attenuator.
    As you mentioned, I can drop that 50ohm output impedance even lower.
    To do that, I'd have to include something in parallel with the output to bring
    the impedance down to 5.0 ohms.

    I'd have to figure this out as I've not done this before. It will take me a bit
    to find and calculate what I need. which will take a bit.

    Once I have the process down and documented the next unit should just
    be a simple job to replace the DUT (Device Under Test) interfacing to
    what is already in place, right? We know there won't be any surprises
    and please, no one invoke the "M" work or correlarrys (sp).

    The file is too large to upload. :(

    ARC SP3-A1 Specs FrontSM.jpg ARC SP3-A1 Specs RearSM.jpg

    From what I think I understand now, at least with some vintage
    gear, they generally weren't designed to accommodate a MC cartridge.
    Max Input = 300 mv rms @ 1kHz (Magnetic phono input)
    high level inputs overload proof. (unless I forget and swamp
    it with 1V rms) NOT.

    The Frequency Response has there are two specs preamp and phono section.
    10Hz to 30 kHz, IHF load
    30Hz to 30kHz, RIAA.

    Here we've been discussing the RIAA curve and designing the modded
    AD797 MC PRE for the LME49710HA MM PRE in a mixed configuration.

    The two gain specs;
    57dB magnetic phono input, main outputs IHF load.
    34dB magnetic phono input, tape outputs IHF load.

    Input Impedance:
    Magnetic phono 50k ohms typical (150pf shunt)
    High Level; 100K ohms typical.

    Looking at the schematic, I should see the 150pf shunt capacitor, also.

    Here is phono stage and the preamp.
    01.01 Upper Left SP3-A1.jpg 01.02 Upper Right SP3-A1.jpg

    Here is the power supply:

    01.03 Lower Left SP3-A1.jpg 01.04 Lower Right SP3-A1.jpg

    For the power supply board, I'm measuring it
    to fit with individual shunts for all three output
    sections of the secondary, along with upgrading
    the bridge with modern diodes. The 1n400x diodes
    have just about the worst specs of diodes.

    For the diode bridges in the preamp I'll go with the
    VS-ETH0806 (8A, 600V) FRED pt $0.86.
    Ranked from best in class, citation below.

    Johnson, M. (2015). Soft Recovery Diodes Lower Transformer Ringing by 10 - 20X.
    Linear Audio, Volume 10. Linear Audio of Turnhout, Belgium.

    Individual article available for purchase here: LINK
    Last edited: Jan 13, 2019 at 4:23 PM
  14. USMC Spike

    USMC Spike AK Subscriber Subscriber

    Tejas, North Central
    Have now a nice selection of parts for the boards.
    What I don't know,m Wyn, which are the noiseless resistors
    that you favor etc? Are we talking the bulk foil types here?
    Or, some of the tantalum flavored resistors?
    Or, a surprise?

    In the mean time, got the internet up and running again.
    No boards yet.
    Ran into some issues with the preamp on the bench
    along with life and then school starting up again.

    Lot's of good stuff going on...stay tuned.

    Another phono question, going through some references,
    Any one know what happened to the ceramic cartridge?

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