Hiss in McIntosh MC2105

Discussion in 'McIntosh Audio' started by straycat23, Feb 7, 2019.

  1. straycat23

    straycat23 New Member

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    I just received my new to me MC 2105. Hooked it up and it has a hiss at full Gain in both channels. I called the vendor who sold me the amp and was told to turn the gain down to 12 o'clock. It still has hiss at the 12 o'clock position although lower. I have no hiss with my MC240 even at full gain. Is this normal with McIntosh solid state amps? Does the MC2505 have hiss as well? I don't remember a hiss with my old SAE amp.
     

     

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  2. c_dk

    c_dk Addicted Member

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    With the amp off plug in some shorting RCA plugs, like might be in a unused phono input, and then turn the amp on and see how much noise there is.

    Any noise would be from the amp ill regardless of the preamp/amp interaction, Rf interference etc.

    I would guess that the MC2105 hiss just could be a frequency you or your speakers are more sensitive to than that out of the MC240.

    As to compared to a different amp who knows until you remove all the possible interactions.
     
  3. jbailey930

    jbailey930 AK Member Subscriber

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    did you reverse the plug polarity in the outlet?
     
  4. twiiii

    twiiii Addicted Member

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    A few owners with Klipschorns used new just introduced 2105's and complained of Hiss, it turned out to be the C-26 and 24 pre-amps. Had to go back to the original C-22 that was still I'm production at the time. We set the volume controls on the amps at 12:00. I used 2100's for a while and set the controls at the 11:00 position with my C-28.
     
  5. straycat23

    straycat23 New Member

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    This sounds like a good idea. I don't have any shorting RCA plugs, but I can get some. Should I use shorting RCA plugs in all my unused inputs to cancel RF interference? Although, I don't think the hiss is from RF interference. I got the impression from the tech at Audio Classics that the hiss was inherent with solid state amps. Not with tube amps. I plugged in my old SAE again, and now I noticed it has hiss too. I didn't get so critical and picky until I started getting into McIntosh. Apparently the preamps, at least with McIntosh and Marantz, don't have hiss through the headphone jacks. At least, mine don't. I have a Mcintosh C28 and a Marantz 7T, which is the first Marantz solid state preamp I've had for 50 years. I think they both have separate headphone amps.
     
  6. c_dk

    c_dk Addicted Member

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    The shorting plugs only go in the high gain inputs, tape head,mic,phono.

    Neither of those particular preamps are especially quiet compared to units 10 years newer with dual rail power supplies.

    What speakers are you using as you reference.
     

     

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  7. damacman

    damacman Blown and Injected Subscriber

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    You can make a shorting plug with any old RCA cable. Cut it in half, strip the insulation off of both the shield and the center conductor, and then twist those together. Wallah. Of course, you don't want to do that with a nice RCA cable . . . any rubbish will work just fine.
     
  8. straycat23

    straycat23 New Member

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    JBL. Don't ask me which one's. I built them with my Dad 50 years ago too.
     
  9. SaturationPt

    SaturationPt Fickle Collector Subscriber

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    My 2105 has been completely restored including outputs and I too get a low-level hiss with no input, so I would expect that it's just the nature of the beast. My 240 OTOH, no hiss.
     
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  10. 62caddy

    62caddy Trust but verify Subscriber

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    I'm guessing these are high sensitivity speakers so I would try adjusting the gains down until the hiss is more tolerable.

    Try the following procedure:

    1 - With gains fully turned fully down set the volume control at the preamplifier at the 2:00 position, then play music.

    2 - Increase the gains until the sound is slightly louder than the maximum tolerable listening level.

    The gains are now set for the lowest noise floor and the maximum useful range of the volume control. .
     
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  11. straycat23

    straycat23 New Member

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    Thanks for the responses everyone. As always, very helpful. I believe now, it is "the nature of the beast". I guess this is one reason many prefer the tube amps.
     

     

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  12. c_dk

    c_dk Addicted Member

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    In my experience the preamps you mention are quite a bit noisier than either Mac amp, however it is a given that later Mac amps are considerably quieter than the first generation transistor amps.

    Speculation over why and possible solutions though I have never explored like I have for the C26 and 28 preamps.

    I did stumble on a interesting article out of the University of Auburn (any paper that in the third paragraph references Einstein and one of his theories is worth study) and mentions Johnson's proof of said theory.

    I do have a mostly restored MC2505 around, I just might have to do a noise study of it.

    I do believe your quite efficient speakers and the specific noise spectra of the amp and preamp are synergistically creating a bad experience for you. Most of us found relief with just turning down the gain since that also put the preamp volume control in the correct range.

    Of coarse most also succumbed to the siren call and upgraded to a MC2125 or 2155, or etc.
     
  13. 62caddy

    62caddy Trust but verify Subscriber

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    The MC240 & MC2105 have identical S/N specs of 90 dB so there's little reason either should hold the advantage in that department (assuming all's up to snuff).

    With the gains of the [same generation] MC2505 at the 12:00 position, I've found it provides identical gain as the MC240 in STEREO mode which is recommended when using a McIntosh preamplifier.
     
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  14. c_dk

    c_dk Addicted Member

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    I think it is genetically impossible for a guy to not want to run his amp wide open.....or better yet to 11.
     
  15. damacman

    damacman Blown and Injected Subscriber

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    Ironically, gain controls and accelerator pedals have the very same function - limit output ... whereas the perception is that they do the opposite.
     
  16. TSmith8605

    TSmith8605 Senile Member Subscriber

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    So to sorta wrap all this up, the gain control is the first thing seen by the signal at the MC2105 amplifier input so turning the gain all the way down is (almost) the same as shorting the input. So, any hiss you hear when the gain is all the way down is the hiss coming from or generated within the MC2105 amplifier. The hiss that gets louder as you turn the 2105 gain control up is not coming from the 2105, it is in the signal going into the amplifier and is being amplified by the 2105 amplifier as it should. I.e., that hiss is coming from the preamp and/or the original source.

    With the gain all the way up, the MC2105 has more gain (more amplification) than the MC240 so that the amplified noise going into the amp will be amplified more by the MC2105 (as it should) and so the hiss sounds louder at the speaker. The music will also be louder. If you use the gain control to lower the MC2105 gain to match the MC240 (per post 13 above), the hiss level from the MC2105 should be about the same as the MC240 - same 90 dB ratio between the music (signal) and the noise.

    So if you hear a lot of hiss when the input gains are all the way down, then the hiss is coming from inside your 2105 and you may have a problem. (But I've never heard a broken power amp hiss, they usually distort or hum when broken.) If it is quiet when all the way down, the amp is working correctly and it does not "have" anymore hiss than the MC240.
     
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  17. c_dk

    c_dk Addicted Member

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    Tony, we all know you are a 11.
     
  18. damacman

    damacman Blown and Injected Subscriber

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    Cat out of the bag is it?
     
  19. c_dk

    c_dk Addicted Member

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    The jimmy blower sticking out of the Olds hood might have been a tip off.
     
  20. damacman

    damacman Blown and Injected Subscriber

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    LOL - I was cruising it down the 202 here last night at about 70mph and applying just enough throttle for the vacuum / boost gauge to read “0” ... you’re doing 100 instantly!
     
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