Conference hips::uk_audioo

    I Have had this before with other systems.
    
    Questions:  1. Does this happen with Headphones ?
                2. Is everything ok with all other inputs ?
    
    
    Try this:   Put LP on turntable leave the turntable plugged into the 
    		Amp but disconect from the mains. Place arm on the record
    		And then turn the volume up SLOWLY. Do you get feedback
    		like you would from a microphone ? 
    
    		If so the turntable needs to be isolated. Try moving it
    		away from the speakers. Get a proper Turntable Table. 
    		(IE Target). Put you Speakers on Proper Stands.
    
    		If that doesnt do the trick, Its probably a mains problem.
    
    		PS What Cartridge & Speakers are you using.
    
    		PPS If you get the propers stands etc. Your system will
   		sound better too. :-)
    
    		Have fun.
    
    		Rob.
    

    
    Is the Turntable grounded on its mains plug as well as through the
    connection on the amp ? If so, I think it could be a ground loop, but I
    was under the impression that ground loops caused hum problems that are
    either ON or OFF - I didn't think that they slowly got worse.
    
    Trevor

    Thanks for your replies,
    
    Re .1 :  Yes the problem does occur with headphones and yes everything
             seems to be ok on the other inputs.
    
             I tried your suggestion about selecting phono and not plugging
             in the record deck and the problem recurred. However I dont
             think it is acoustic feedback. The sound I get from the
             speakers is not the high pitched whine usually associated with
             microphones. It is more of a bass hum, and sounds rather like
             a truck passing by the house. 
             The cartridge I am using is an Ortofon TKS 49S and the speakers
             are AR 18EJs. The speakers are not mounted on proper stands
             and neither is the Dual.
    
    Re .2 :  I am not sure if the Dual is grounded through the mains plug.
             I will check this tonight. If it is not what should I do?
    
    Regards,Tom

    Ok we have proved what it isnt :-)
     
    If all other inputs are ok I would think that Its the deck or cartridge
    that is at fault. Or the suggestions in .2. Can you borrow another deck
    from anywhere (frendly dealer maybe). Also a new cartridge may help.
    
    FWIW Acoustic feedback in hifi systems is low frequency. Its is caused
    by the sound from the speakers going back into the Cartridge. Hence my
    question about headphones.
    
    
    Have fun.
    
    Rob.
    

    Rob,
       I will be able to try another deck tommorow night although it will
    not be another Dual. If it turns out that this solves the problem do
    you think that I should go ahead and buy a new cartridge and if so can
    you recommend a decent cartridge which wont break the bank. Keep it up
    Rob youre playing a blinder!

    If your going to replace you cartridge then me carefull. Althought the
    '3020 is a good amp the MM input stage does tend to clip when used with
    very ``dynamic'' (I hate that word) cartridges eg Rega R100, Bias, any
    thin by Linn. So be careful and take you amp along to any demos you go to.
    
    The cartridge I use to recommend for the NAD was the A&R C77 but I don't
    know if they still make it and there must be someting better these
    days.
    
    As for your amp problems it sounds like a wierd case of earth loop ...
    but I'm baffled by the time delay ???

    After re-reading my last reply I really should learn to read / type ...

    Re:.4
    >Ok we have proved what it isnt :-)
    > 
    >If all other inputs are ok I would think that Its the deck or cartridge
    >that is at fault. Or the suggestions in .2. Can you borrow another deck
    
    It could still be the NAD! Phono input has a separate preamp to get the
    signal (couple of mV) up to line/aux/tape level (couple of 100mV) and
    apply RIAA frequency correction. This makes Phono inputs more sensitive
    to problems like ground loops or bad ground.
    
    When you say it still appears when the record player is disconnected
    from the amp, it's clear that it MUST be in the amp, and only in the
    phono preamp to add. (You've disconnected all cables from the player to
    the amp, including the ground wire?). 
    The sound you describe as a truck passing has to do with a phase shift
    in the amp feedback loop (which controls the gain the amp delivers)
    that causes the amp to go unstable at a low frequency. When, as in your
    case, it is absent at first but appears over time, it seems to indicate
    temperature sensitivity of one component or the other. Most likely are
    electrolytic caps, but a bad soldering joint can do it as well. If it
    appears in both channels at the same time it's a component that's
    common for both channels, like supply stabilisation. Try getting hold
    of a can of freezer spray (mind the ozone layer!), and chill the
    various components near the phono input one by one. I can't give you
    details how things look like inside the amp, but this must be easy
    enough. 
    The problem itself is rather nasty to eliminate if it's a design fault,
    but since it shouldn't be (just a matter of component aging, tolerances
    going off), it should be fixable by replacing a component or two.
    
    Good luck.
    
                                                             - Rik -

    
    
    Another problem with a NAD amp unfortunately, maybe one of you lot
    could help me out a bit please...?!!
    
    
    I switched the amp on yesterday morning to listen to the radio and one
    of the channels was dead.  I did the obligatory checking of terminals
    round the back and the speaker cables.  After all this stuff I decided
    to switch the thing off for a while and check it later, when I switched
    it back on a few hours later, the dead channel did a bit of a
    re-incarnation job and was sort of working!  It seems to pack up when I
    turn the amp up slightly, but if I switch it off and leave it for a few
    seconds, it comes back working.  I haven't been running it heavily
    really.
    
    All that I can think of is there's a temperature related problem with
    one of the output (can't think of the component name, but think it's a
    round thing with an oval bit on the end! =;*)  )...
    
    Any help appreciated.
    Chris.

    re .10....
    
    If it happens with all sources, swap over the speaker connections, as
    a pair, between L & R. If the problem moves to the other side, the
    fault is with the amp; if it stays oon the same  side then it's the
    speaker or cable/plugs.
    
    If it happens on one source only, swap over the L & R inputs for the
    failing source. If the problem remains with the same side, the amp
    input for that source is faulty; if the problem moves, it's the source
    component or cable/plugs at fault.
    
    Dave

               
    
    The problem with the amp appears to have gone for the moment!  It was
    definitely a problem with the components in the amp itself as I'd
    swapped over the speaker cables, speakers, and the sources.
    
    Ta for the help.
    Chris.