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    ST70 VTA board slight hum. Preamp?



    Posts : 18
    Join date : 2012-03-26

    ST70 VTA board slight hum. Preamp?

    Post by rockinrob86 on Mon Mar 26, 2012 5:48 pm

    I just finished installing a VTA board into my st70, and it sounds great.

    However, I am getting more hum than I anticipated.

    I bought the low gain version

    I am using my thorens turntable, playstation, or ipod into a Marantz pm500 integrated and using the headphone out of the Marantz to get into the st70 which goes to Klipsch Chorus II’s

    If I have the unit on and connected to speakers, but with no source, it is dead silent. When I leave the cables into the st70, but short them at the other end, it is still silent.
    When I plug the cable into the headphone jack and turn the Marantz on, the slight hum starts.

    The hum fades into the background eventually, but at that point it is LOUD. I think I can thank the highly efficient chorus II’s for this

    For watching tv or using the playstation, I can only get the gain up to about 8 o'clock for normal listening levels. At 9 o'clock the house is shaking. With the turntable, 9:30 is normal volume. at 8:30 you can hear the turntable, but it is 50/50 hum. at 11 it is almost painful. At about 10 o'clock with the turntable the hum is no longer noticeable, maybe 98/2 or so.

    I have recapped most of the Marantz, but maybe I need to look into the headphone section. I have never noticed a hum when using headphones.

    I think the problem is that the headphone out is slightly noisy, and that when that is further amplified, the hum becomes noticeable. Sound right?

    If I bought an earlier Marantz integrated with a preamp out, could I use that and would that be quieter? I could also open up the pm500 and find the preamp/power amp function and run a line to the rca's on the back...

    I cant really afford a tube pre, but maybe in the future I will get a dynaco pas3 and modify it with the other vta boards for it. This will be awhile in the future though…

    I know I can adjust the feedback resistors on the vta board, but it seems to me like the problem isn’t the vta board.

    Any ideas for solutions to this?

    Posts : 1435
    Join date : 2008-11-30

    Re: ST70 VTA board slight hum. Preamp?

    Post by tubes4hifi on Mon Mar 26, 2012 6:16 pm

    you shouldn't be usiing a headphone output as the input to the amplifier, although this will usually work with something like an iPod, but not recommended for something like the Marantz. Your Marantz should have a pair of RCA output jacks labeled line output.
    The headphone output obviously has way too much output, also, it should have a volume control on it, and that should be turned down.
    If your Marantz doesn't have a line output it should have a "record output"


    Posts : 58
    Join date : 2009-01-19

    Ground loop?

    Post by danf on Mon Mar 26, 2012 6:50 pm

    Do any of your sources have a grounded chassis (3 wire plug?) If so, this might well be a ground loop if your ST-70 chassis is grounded (a good idea for safety).


    Posts : 18
    Join date : 2012-03-26

    Re: ST70 VTA board slight hum. Preamp?

    Post by rockinrob86 on Mon Mar 26, 2012 9:16 pm

    this marantz doesnt have preamp outs or a line out.

    The only outs on the back are the tape ones, but those are unaffected by the volume control, so I have no volume control.

    The headphone jack is the only option I have at the moment.

    Does anyone else use one of the 70's marantz integrated amps that have a preamp out?

    I can get several of those for less than $100, and $50 or so if it has problems (which are usually the output stage, which I wouldnt need if I was using it as a preamp)

    I think that would be my cheapest route until I can get a real preamp


    Posts : 18
    Join date : 2012-03-26

    Re: ST70 VTA board slight hum. Preamp?

    Post by rockinrob86 on Wed May 09, 2012 9:47 am

    Just to follow up, I built an sp8 and the humming is reduced substantially!

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