C37 phono mod?

Discussion in 'McIntosh Audio' started by nowinowski, Sep 7, 2017.

  1. nowinowski

    nowinowski New Member

    Messages:
    23
    After comparing it sounds much better with the step-up in line. On the volume 01 is pretty quiet - 10 is foundation shaking. i will note that 10 is about 11' o-clock on the volume knob. I think I'm just going to use it like this and I'm also going to follow up with the shop to see about checking the pice and seeing if it needs to be restored to stock.
     
  2. c_dk

    c_dk Addicted Member

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    5,498
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    I do not believe there is a way with 1 part replacement to turn the mac phono stage into a MC stage.

    If you research the OP amp based MC stages out there on the web they are quite a bit more complicated than just a single part change to implement.

    The second generation C31,35, and C37 all use the same volume control, the C37 adds a rotory knob to the moter driven function.

    I have both a restored Ortofon head amp and a AT transformer and have no overload issues with a number of low output MC and my C35 or C37.

    I would suspect the Sony transformers are improperly loading the Denon MC.

    In the past Mac's MC head amp paralleled 4 low noise transistors to get the best S/N if I remember correctly.
     
  3. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    In the past Mac's MC head amp paralleled 4 low noise transistors to get the best S/N if I remember correctly.

    From MCP-1 owner's manual :

    Eight transistors are parallelled in the MCP 1 which gives a reduction of 9 dB in noise level over using just one transistor.
    This design approaches closely the theoretical optimum.
    To guarantee performance each input transistor is individually tested for noise voltage and noise current at 100 Hz, 1 kHz, and 10 kHz before it is mounted in the MCP 1.
    This testing is time consuming but assures noise free operation.
     
  4. TSmith8605

    TSmith8605 Senile Member Subscriber

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    Location:
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    But I believe the OP said the 103 / Sony transformers did not overdrive the C31V but are drastically overdriving the C37 indicating the C37 is working differently than the C31V. Also, OP said the shop owner believed the C37 phono section had been modified. If it has been modified, I would expect this hack would not result in McIntosh-level (MCP-1) noise and distortion performance.

    If it were me, I would return this C37 to the shop for a refund and find another.
     
  5. clinic-audio

    clinic-audio all on YAMAHA untill 1990

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    1,754
    Location:
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    As I said at the beginning of this discussion :

    Sensibility of your Phono input is 2,5mV with C37 and 2mV with C31V

    So you can't get : Sony transformers did not overdrive the C31V but are drastically overdriving the C37 this is not possible because C37 need more voltage to saturate 2,5mV .

    You talk about mod in this C37 , after carefully watch the C37 pictures none mod has been made on these boards and I honestly think that this is not true !
    Try to find another transformer and you will see that your C37 is perfect !
    Again , as Chris said , this is not possible to change a MM phono input in MC input with one component (resistor) replacement !
     
  6. nowinowski

    nowinowski New Member

    Messages:
    23
    Yep that makes total sense. I got some clarification from the shop that a resistor was changed for the purpose of MC loading reducing the 47k input to something more like 500 ohms. That might have some effect on gain I suppose but it shouldn't be huge?
     
  7. c_dk

    c_dk Addicted Member

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    5,498
    Location:
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    The only resistor I can clearly see in the first pic, just this side of the RMC 470pf ceramic capacitor is the load resistor yellow, violet, green, red, brown is 47.5k ohms at 1%.....it also appears the phono stage is pretty much identical from the C31 thru the C38 except for the DC input blocking cap......both 10 mfd, polarized and non polarized are used.

    My C35 and C37 schematics are not available right now but I laid open a C31 schematic and compared it to your photographs.......not much difference....

    It appears the resistor soldered to the RCA center pin is brown,black,red, gold; 1k at 5% stock value for C31 thru C38.....

    I think they are telling you a tale........sorry.

    I would concur though that you are approaching over driving your Adcom amp with the Mac preamp.
     
    Last edited: Sep 13, 2017
  8. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    1,754
    Location:
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    Here is a view of the board and schematic of C37 Phono input

    C37-R.jpg

    C37-pcb-R.jpg


    is it enough as evidence ?
     
  9. TSmith8605

    TSmith8605 Senile Member Subscriber

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    755
    Location:
    Sunnyvale, CA
    So your post #1 said the usable volume for the phono was between 1 and 3 on a scale of 100 and now your saying 1 to 10 on a scale of 100. I assume this number is the volume level displayed on the front panel.

    OTOH, "foundation shaking" with the volume control at 11-O'clock sounds more like what you might expect with 6 mV into the phono preamp / 6 Volts into the line section. So another possibility is that the C37 is not displaying the set volume level correctly on the front panel. Do you have other sources hooked up (like a CD player) and if so, how does the volume control work and what is displayed as the volume level for the other sources?
     
  10. nowinowski

    nowinowski New Member

    Messages:
    23
    Very helpful. Thank you all for taking the time to look at this. I hooked up my phone to an aux jack and 25-30 was very loud - where with the phono 5-10 is very loud. The readout and the knob do not seem correlated in a linear way as 1/4 turn or 9 o clock is about 5 and 1/2 turn or 12 o clock is at least 40. I like the idea of getting a mcintosh amp as funds allow I regret imensely the 7200 I sold.

    After experimenting with it the usable range is a bit higher than I thought at first - I think I was shocked by how loud it was and also using an old Nancy Sinatra album that was mastered quite loud.
     
  11. 62caddy

    62caddy Trust but verify Subscriber

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    Location:
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    Speaker sensitivity will dramatically affect the SPL levels for given volume settings.

    The one thing that stands out here is the fact that higher volume is obtained with phono than for a high level source at the same volume setting. The exact reverse has always been the case for me throughout many years for countless systems.
     

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