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    got to get the hum out

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    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    got to get the hum out

    Post by j beede on Mon Mar 07, 2011 1:18 am

    I am making good progress on my Mark III restoration project... unfortunately I am mostly unable to enjoy the fruits of my labor due to hum. The amps are reasonably quiet (tolerable noise floor) with inputs floating or with a shorted male rca inserted--so I think my power supply filtering is good. As soon as I connect any sort of input cable--with or without a source I get a low level hum that stops the show. CD player straight in? Hum. Preamp output connected? Hum. Running a ground wire from the CD player or preamp to the Mark III makes no difference. I need some ideas for some additional experiments to run.
    ...j

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: got to get the hum out

    Post by GP49 on Mon Mar 07, 2011 12:48 pm

    You very likely have a ground loop. Did you replace the two-wire power cord with a three-wire? If there is another three-wire cord in your system, that can be a good way to create a ground loop.

    Sal

    Posts : 221
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: got to get the hum out

    Post by Sal on Mon Mar 07, 2011 2:20 pm

    I second what GP49 said.

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: got to get the hum out

    Post by j beede on Mon Mar 07, 2011 4:23 pm

    Yes, of course I replaced the "petrified" original white(?) two-pronged, ~18 gauge cord with a nice, new 14 gauge IEC three-pronged cord. It will be an easy test to open the ground wire and see if that improves the problem. Maybe I'll just put the orignal cord back in so I can flip the hot and neutral too.

    If the problem was a ground loop wouldn't you expect that to go away when other "appliances" are disconnected from the Mark III? The hum appears even if I just connect a phono cable to the input (antenna) with or without anything driving or even connected to the other end. Maybe I have created an internal grounding issue by trying to adopt a central grounding scheme inside my Mark IIIs... I am trying make all the ground connections to the terminal strip where the red/yellow from the PT attaches.

    quad44

    Posts : 18
    Join date : 2009-06-08

    Re: got to get the hum out

    Post by quad44 on Mon Mar 07, 2011 7:31 pm

    A forum member danf has contributed his solutions to MK 3 hum problems and if memory serves me, he may have a solution for your hum problem.
    Do a search for danf and all his posts and see if there is a something there which applies to you.
    I can also vouch for what he states about the star grounding in a MK3 because I had a similar problem on a new MK3 build with the VTA boards.
    Hope you find your solution.

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: got to get the hum out

    Post by j beede on Mon Mar 07, 2011 11:53 pm

    Finally! There is finally music again in my listening room. I used 3-to-2 prong adapters to isolate the grounds on my Mark III amps with three-pronged power cords. There is still a trace of hum but you have to put your ear right up to the woofers on my little Quads to hear it. On the Martin-Logan CLS there is nothing audible. I will either disconnect the ground wire inside the chassis or I may see if I can find some nice, heavy, molded two pronged cords to cut and splice in. This is actually the first time I have heard Mark III amps. They have some horsepower (I am running 6550 biased to 1.56V) and the imaging is quite entertaining. With the suggested diode updates and freshly built series 450WVDC cap boards in place I may try KT-88s with silicon bridges in place of the 5AR4. The way I am set up now I am seeing 525V at Pin #8 on the rectifiers. B+ might go to 575V with bridges. I am so glad to have those 525V quad cap cans out of the circuit. Smile. Now I am having fun.
    ...j

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: got to get the hum out

    Post by j beede on Mon Mar 07, 2011 11:54 pm

    quad44 wrote:A forum member danf has contributed his solutions to MK 3 hum problems and if memory serves me, he may have a solution for your hum problem.
    Do a search for danf and all his posts and see if there is a something there which applies to you.
    I can also vouch for what he states about the star grounding in a MK3 because I had a similar problem on a new MK3 build with the VTA boards.
    Hope you find your solution.

    Thanks for the suggestion Quad44. I will do a search. My Mark IIIs are now easily quiet enough to listen to, but less hum is always welcome!
    ...j

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: got to get the hum out

    Post by j beede on Mon Mar 07, 2011 11:56 pm

    GP49 wrote:You very likely have a ground loop. Did you replace the two-wire power cord with a three-wire? If there is another three-wire cord in your system, that can be a good way to create a ground loop.

    Thanks for the recommendation GP49. I picked up a couple of 3-to-2 prong adapters and sweet music has been playing for about 2.5 hours now!
    ...j

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