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Little Dot MK8SE / MK6 Super Mods (All verified mods are on first page)

Discussion in 'Headphone Amps (full-size)' started by redge78, Sep 26, 2015.
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  1. baronbeehive
    Just a shot in the dark but it could be wiring. You could approach the problem from that end and check the XLR output jacks on the rear panel, check each pin to ground to see if there is a grounding problem. This can happen and be difficult to see. Only pin 1 of each output, left and right, should be connected to ground.
  2. klnglim
    I have same mistake as u mention picture above , can u tell me what went wrong?
  3. bloodhawk
    I wired the one on the right incorrectly - [​IMG]
  4. klnglim
    ok, now I got it. Will go back and check my amp later. Hopefully will work this time, thanks a lot anyway!
  5. baronbeehive
    Just quoting myself because I wanted to correct something I said: there was nothing wrong with the ammeters, it was the wrong value resistor that Sonic pointed out that was causing the problem. Both meters do display continuity over their terminals which reads as a low resistance on the ohmmeter as Maxx said, that's OK now that I've changed the resistor…... BUT.... I've switched on after upgrading the PSU resistors and others totalling 10 in all, plus a new Jupiter cap, diodes and transistors. Now there is no smoke like kinglim but one meter reads 60mA correctly and the other reads 30mA so something needs looking at again. At least the meters aren't behaving erratically like before!

    Anyway next step take some voltages etc etc…..

    Bit of a bummer that, I tried to be so careful, checked everything in triplicate but this amp just likes to bite back.
    Last edited: Jul 26, 2018
  6. klnglim
    Well, I have checked all connection, nothing's wrong. When turning on, the left side meter light up follow by right side meter warm up slowly, unfortunately reading '0' and totally silence.
    From what I have seen, the meter lights are too bright so I turn off the amp immediately to avoid component smoke again, I guess need to add small resistor to dim the meter light, by now
    still searching for the cause of silence
  7. baronbeehive
    That's pretty weird, I've just taken voltages and they're all good:

    power cathodes all read -80v/-104v give or take on respective sides of the cathode cap
    power anodes all read 76v/101v give or take on respective sides of the 330R
    driver cathodes all read 0.1mv/1v give or take on respective sides of the 377R
    driver anodes all read 140v/186v give or take on respective sides of the 33K

    ...… yet the right hand meter reads 30mA only. This meter was not the one that was changed.

    Only thing I've done on this circuit was to desolder the gain switch but the resistors all read good, and a desoldered switch is the same as a switch in place in the "off", (high gain), position, so it can't be that.

    My reasoning now is that if the driver and power stages are OK I need to look elsewhere, after the power stage in the circuitry, or check out the connections from the meter back, any bright ideas anyone?
    Last edited: Jul 29, 2018
  8. baronbeehive
    I don't know if these voltages mean anything to anyone If so say now before I take the PCB out again!

    1 & 2 are the red and white wires going to the left meter on the .68R
    3 & 4 are the red and white wires going to the right meter lighting circuit, marked "right meter" on board
    5 & 6 are the white and red wires going to the left meter lighting circuit
    7 & 8 is the resistor marked "left meter" on board


    I also measured 100v on each of the meter terminals.

    Both channels sound fine and equal volume so without taking the board out, I'm not sure of the problem. I'm speculating that a meter resistor could have fried when a leg broke on one of the Jupiter caps, or maybe something on the back of the board.

    Edit: Thanks Sonic for the pic, better quality than mine..... also his amp LOL.

    The lighting circuit terminals were difficult to get at but measure 3.0mV each in the meter with the correct reading of 60mV, whereas the other 2 terminals read 0.26v (white wire), 11.7v (red wire), so something not right here.

    I've made some corrections to this since first posting.
    Last edited: Aug 12, 2018
  9. baronbeehive
    OMG...…. just been listening to one of my fav tracks with the complete set of 421A's...… all I can say is simply stunning.... I'm speechless.... I'm going to have to have some time getting used to this!

    On a side note there doesn't seem to be anything wrong with the amp, must be just a meter issue.
  10. SonicTrance
    Do you have the 5998/421a bias mod in place? I can't remember. That also makes a huge difference in the LD! Great sounding tubes .
  11. SonicTrance
  12. baronbeehive
    Yes I have the bias mod in place..... but haven't used it yet as I'm concerned about the amp atm, it seems ok but until I sort the issue with the meter I won't use it, or the 421A's, the next step is to try the various cathode-anode settings on SQ.... and finishing with the CCS some way down the line!

    Sad indeed... I have considered this also, it makes sense for you though now that you have your other main amp.

    Stay on the thread though Sonic!
    Last edited: Aug 6, 2018
  13. Maxx134
  14. SonicTrance
    Thanks guys!
    I really love this amp but I'm having too much fun with my DIY amps. Can't keep everything sadly, too little time.
    klnglim likes this.
  15. baronbeehive
    I just wanted to mention a few observations about the amp with the 421A's in, I know one or two of you will be interested. I've tried it twice briefly now and so I'm able to say one or two things:

    I had two slight reservations about the amp before which were:

    - bass lacking slightly in articulation - the bass is now very lively, good definition and balance, just the right amount

    - some treble frequencies missing - these frequencies are back and the amp now performs at a very high level indeed. I had suspected all along that this was down to the RCA's which are a bit lacking in extension, I still like them however.

    Also I noticed straight away the resolution and transparency has gone up massively and contributes to the soundstage accuracy and intimacy which was good before and is now top notch. I can't say about the size of the soundstage because my HE500's don't do massive soundstage unfortunately.

    Everything else is spine tingling to the extent now that I can't imagine anything better. The only slight thing is that with a top level system the bass would be slightly tighter and snappier sounding, Maxx will know what I mean because I feel it's still better on the other amp. If the problem is down to the PSU and balance then the CCS mod could sort this, when I get round to it, but I suspect this is down to the amp design and probably the NFB. Possibly those massive film caps of coinmaster might do the trick - but I haven't got room in the house for them.

    These are brief impressions but I would say now I'm pretty well totally satisfied with the amp, it has certainly come alive for me!!

    Just a pity these WE's are so damn expensive and there's nothing really in between the RCA's and them sadly.
    Last edited: Aug 9, 2018
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