Dual-mono chipamp (3886) problem - possible grounding issue

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phresh

Member
2005-11-28 11:51 pm
Hampshire
  • #1
    I've just been converting my stereo gainclone to a dual mono gainclone while building it into a new case and when turned on the transformer 'growls' (the best way I can describe it). I have to say i'm no expert on transformer noises so it could just be a (very) loud hum, although it does sound quite deep hence the growl.

    The reason I suspect it is the grounding is because both channels work perfectly fine if the other is disconnected. I just want to make sure this is it before I do some kind of other damage or change things that are perfectly ok!

    A few bits of info that may help;
    -It's the chipamp.com 3886 kits
    -240V primary, dual 22V secondaries. I think it is around 200VA
    -All in a single case with single ground for chassis and two amplifier boards
    -Mains passes through fuse (hot only) and dpst switch

    Thanks in advance
     
    xiphmont

    xiphmont

    Member
    2007-04-29 5:28 am
    www.xiph.org
  • #3
    he didn't say the output was humming, he said the *transformer* was humming. And only with both channels connected. A toroid is only going to 'growl' if it's under serious stress like a current draw way in excess of its rating... does it 'growl' even when idle so long as both channels are hooked up?

    So... dual mono, two rectifier boards, one tranny... got any pics? Sounds like the rectifier boards are not wired exactly the same way or something is grounded that *shouldn't* be...

    Monty
     
    P

    phresh

    Member
    2005-11-28 11:51 pm
    Hampshire
  • #4
    Yes, it growls when idle too. Could adding another rectifier increase the load on the tranny enough to make it do that? It was a transformer recommended for the kit so i'm inclined to say that unless faulty the transformer is fit for purpose.

    Won't be able to get any pics up until tomorrow night so i'll describe the wiring best I can. The black/red secondaries go to AC1 on each board (red in the front hole) and the orange/yellow secondaries go to AC2 on each board (yellow in front hole).

    If something was grounded that shouldn't be then I would expect one of the channels to not work (or not work properly at least) and there was little-to-no audible hum (in the speakers) when listening to each half of the amp individually.

    I think it's going to be a case of trying a different transformer and ruling that out.
     
    Nuuk

    Nuuk

    Member
    2003-02-19 6:20 pm
    Somerset, SW England
    www.decdun.me.uk
  • #5
    I think it's going to be a case of trying a different transformer and ruling that out.
    Click to expand...

    If it sounds OK with one channel working then it is almost certain the traffo is fine!

    Not to be pedantic but what you have is not dual-mono. But if you want to try a second traffo, you could then build a true dual-mono GC.

    If you can show us exactly how this is wired up, I am sure somebody can show you how to fix the problem. ;)
     
    gfiandy

    gfiandy

    Member
    2003-07-02 10:39 am
    Cambridge UK
  • #6
    Since the amplifer works in a single configuration it is very unlikely to be a problem with the transformer. The idle load of two amps is not sufficient to cause it to hum.

    I suspect you have one or more of the diodes the wrong way round in the bridge rectifier. This is working OK when the amp is connected on its own (probably half wave rectifiying but these amps have good PSU rejection so you may not hear it) but is causing a short accros the windings when you connect the two bridges together.

    The other possibility is you don't have them wired to the same connections on both the PCBs so recheck the wiring again as well.

    If neither of these turn up a problem then I would measure the diodes in the bridge to make sure they are all working correctly.

    Regards,
    Andrew
     
    P

    phresh

    Member
    2005-11-28 11:51 pm
    Hampshire
  • #7
    Nuuk -

    I know it's not true dual mono but it seemed the easiest way to describe it in a topic title! As soon as money permits and other projects are finished then I do intend to put two smaller transformers in and make it true dual mono.

    gfiandy-

    Diodes are definitely in the right way and I tested them all for continuity to check they were working. Obvioulsy if there was a short somewhere then the continuity tests could be wrong. Wiring is identical on both boards, i've checked it enough times as that is the one bit that always makes me paranoid! I'm going to run with the idea of a short somewhere across the diodes and test that first.

    Phil
     
    ashok

    ashok

    Member
    2002-06-06 4:43 am
    3RS
  • #8
    If each board works fine then the chances are that there are no shorts anywhere.
    Do a "visual' check of each and every diode and compare against the other board to determine if its correct.

    look at this thread :
    http://www.diyaudio.com/forums/showthread.php?s=&threadid=115901

    The psu drawing is incorrect for the negative side of the supply as mentioned in the posts. The board is supposedly correct. BUT did you follow the ( incorrect ) circuit diagram ?
     
    xiphmont

    xiphmont

    Member
    2007-04-29 5:28 am
    www.xiph.org
  • #9
    Pictures please! The speculation is heading off into the twighlight zone.

    Most of the suggested miswirings 'check this, check that' don't fit with the evidence you've presented. One tranny and two PSU boards, transformer 'growls' but only if both amps are connected (but either alone is fine) suggests you're shorting the transformer inadvertantly somehow... pics of the wiring please! Someone, quick, mail this man a digital camera! :)

    Monty
     
    P

    phresh

    Member
    2005-11-28 11:51 pm
    Hampshire
  • #12
    OK, just waiting for my sister to get homes so I can borrow her camera but here are some continuity results that show both power supplies to not be equal! I'm just not sure which one is wrong....

    Left Channel-

    Both sides check for continuity from both +>- and ->+ (i'm using an old school analogue meter)

    Right Channel-

    V+ only shows continuity on +>- while V- only shows continuity on ->+.

    Obviously one is wrong but can anyone point out which? I assume it is the left as it was the right channel power supply I was using before and that worked fine. For V+ & V- to show continuity both ways I assume there must be a short rather than a defective diode?

    Lot of assuming there, sorry!

    Phil
     
    audio1st

    audio1st

    Member
    2005-05-13 1:31 pm
    Foxton, CAMBS..
  • #19
    phresh said:
    Tested all diodes for continuity and two of them on the right channel only check in one direction. I take it this means they are defective as they are only testing for the circuit connection and not through the actual diode. Will try and find some more to swap out.
    Click to expand...


    Why are they defective??? Something else has probably damaged them..Is it easy to tell the difference between the 1r0 and 2k2 resistor with these boards..? Have you put the 1r0 across the cap by mistake.? Are the caps the correct way round.?(V+ cap, neg to ground, V- cap, pos to ground).
    Regards.
     
    xiphmont

    xiphmont

    Member
    2007-04-29 5:28 am
    www.xiph.org
  • #20
    audio1st said:



    Why are they defective??? Something else has probably damaged them..Is it easy to tell the difference between the 1r0 and 2k2 resistor with these boards..? Have you put the 1r0 across the cap by mistake.? Are the caps the correct way round.?(V+ cap, neg to ground, V- cap, pos to ground).
    Regards.
    Click to expand...

    Agreed and echoed, it's been a *very long* time since I've seen a 'defective' electronic component from a reputable supplier. If something is broken... chances are something else in your circuit broke it.
     
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