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MMKs in feedback

Discussion in 'd.i.y.' started by James Evans, Jun 14, 2007.

  1. James Evans

    James Evans Bedroom Bodger

    Fairly old ground I know, but it's taken me some time to get round to acquiring these sizeable little boxes.

    Firstly, must commend Elfa on their customer service - ordered 4 caps last friday and they arrived from sweden on wednesday - speedier service than some UK based companies I've dealt with.

    So, here they are, wife goes out to yoga wed eves and the nipper amazingly decides to have an early night. Can't believe my luck...

    Whip the OA21 out of the rack and dive inside (must say its a damn site easier tinkering inside this than the claymore and obelisk3). Out come the 2 100uf 6.3v caps, and after some wedging, in go the mmks. As a full update, the 4.7uf power amp coupling caps have been replaced with 3.3uf polyprops and psu caps are mundorf mlytics, which I managed to squeeze into the existing pcb slots in the end. Pre section is passive in the onix so all I've now got between the output opamps (lm4562) in my rcd1070 are 10uf polyprop coupling caps in the cdp, then the polys and mmks in the onix - no lytics anywhere, bonanza.

    It's been playing for an hour or so now running through some standard test tracks and initial impressions are very positive - no loss of bass at all and things have tightened up down there. Much much cleaner highs - instrument and voice separation is vastly improved. High hats sound more realistic, speedy snare jiggery pokery on cinematic orchestra's everyday is crystal clear - every beat can be distinguished. Harshness reduced as well - can go louder without fatigue.

    Happy camper thus far.

    I've got 2 more, so the claymore goes under the knife next to remove the 100uf cerafines currently on feedback duty.

    Then its off to order 4 more for the obelisk....

    edit:

    and inspired by PDs post on his power amp refurb I had to try out she's lost control - there is indeed a sub bass part that was previously indistinguishable, marvelous, now, off for more listening...
     
  2. Agent_Cooper79

    Agent_Cooper79 pfm Member

    Glad to hear everything worked out for you with the MMKs James! They are just perfect in there aren't they. On tuesday I am comparing my oscon decoupled mmk in feedback equipped 102 to a friends 82, will report back with that.

    Cheers Magnus

    P.s. We will also compare my MiniCapsc to his standard HiCap. d.s.
     
  3. fatmarley

    fatmarley Registered User

    Hi James, Have you got a schematic for the Onix? Had a quick look in mine the other day with a view to doing a few mods but changed my mind because i didnt want to bugger it up.
     
  4. fatmarley

    fatmarley Registered User

    . . . Also, are the feedback caps easy to find?
     
  5. James Evans

    James Evans Bedroom Bodger

    Cheers Magnus!

    Hi FM, I don't have a schematic for the onix - I read somewhere that it's based on the same hitachi design as the claymore was so made some assumptions based on that... There's only 4 electrolytics in the power amp - the 2 coupling caps and the feedback caps (if my assumptions are correct).

    The linked to image shows a photo with them highlighted - the red arrow and circle are the 4.7uf input coupling caps (I know this for sure as I traced the pcb tracks back...)

    The two small caps highlighted with the green arrows are the feedback caps if my assumptions are correct.

    http://www.growthconsulting.frost.c...744BACB15802572FA0045A60D/$File/onix-caps.jpg

    Hope that helps!
     
  6. PigletsDad

    PigletsDad pfm Member

    As it happens, the Maclaren is also based on the same Hitachi application note; it just use 3 pairs of output devices (can do 24amps continuous!) and a bigger power supply, with regulation to the driver board. The Maclaren also has tweaks to the compensation and Zobel network to improve stability with reactive loads.
     
  7. James Evans

    James Evans Bedroom Bodger

    Sounds like a beefier amp than any I have here then. If these are anything to go by I bet it sounds rather good!

    While I'm on the subject of replacing electrolytic caps, below is the obelisk power amp schematic. Other than the input and output coupling caps, which would be worth replacing? I'm thinking C5 (100uf/25V) is a definite, but any others?

    http://www.growthconsulting.frost.c...F8025722800385D59/$File/Obelisk_power_amp.gif

    ta

    James
     
  8. PigletsDad

    PigletsDad pfm Member

    C18 is the bugbear - you have to use a 'lytic, but the whole signal passes through it, and all the nonlinear effects are present big time. Use the best, fresh one you can find, but the single rail circuit holds performance back horribly.

    Marginally worth swapping C5 and C1 for films, but the circuit is so nasty that you are basically gold-plating a turd.
     
  9. fatmarley

    fatmarley Registered User

    James, Your a star! Will have a bash at it after my tea. Ive got some MMK's for feedback but im not sure what to use for coupling. I like BC128's in my Naim kit but i have got some rather large 10uF polypops (Mundorf Mcap MKP) i could try. . . Cheers and sorry to hear your other amp is a turd!
     
  10. James Evans

    James Evans Bedroom Bodger

    I know, bit harsh that... ;)

    It actually sounds ok, although is resigned to living room/tv duty.

    You may find you'll have to remove the soap link to be able to fit everything in - I did - not a problem as I don't have one anyway... The polyprops may be a bit big as well as the MMKs... BC128s are well worth a go and will fit in the available space allowing easier fitting of the MMKs.

    Enjoy your tea!
     
  11. a.palfreyman

    a.palfreyman pfm Member

    Thats funny, my turd's not even gold-plated!
     
  12. James Evans

    James Evans Bedroom Bodger

    Hi Andrew, managed to completely blow up your obelisk yet?
     
  13. fatmarley

    fatmarley Registered User

    I fitted the poly's in the end because the BC128's are only 25v, the original caps were 63v. Any idea what the voltage measures in this position?. . . Its hard for me to comment on the sound because i only use the amp for the tv but will compare it to my modded Naim kit sometime.
     
  14. James Evans

    James Evans Bedroom Bodger

    you should be fine using the bc128s as coupling caps I would have thought, having said that, the polys are probably superior to the 128s anyway...
     
  15. fatmarley

    fatmarley Registered User

    Bought some BHC T-net caps from patrick dixon and fitted them saturday. First impressions were not good, with bugger all bass and splashy treble. Had a listen today - good cd's sound better (more detail, smoother and better bass resolution). A cd that i thought was a bad recording (thin/lightweight) actually sounds rather good now. They're a pain in the arse to fit but well worth the effort.
     
  16. James Evans

    James Evans Bedroom Bodger

    took out the 3.3uf polyprops I was using as coupling and put in some bc128s and after a few days I've got a lot more bounce back, without losing the "air". Got MMKs in 2 amps now and they're sounding great.
     
  17. fatmarley

    fatmarley Registered User

    Interesting! I hav'nt tried BC128's yet. You should try bypassing it with a 100nF polypropylene, they work well with BC128's - they give smoother, more detailed high frequencies. My Cambridge audio 640c has, what looks like a 100nF polyester cap, bypassing the electrolytic coupling cap, as standard.
     
  18. James Evans

    James Evans Bedroom Bodger

    cunning. I've got a couple of 100nf polyprops here so I'll give that a go when I get the chance. It's a bit fiddly soldering under the pcb in that position as the L-bend of the backplate overhangs that part of the pcb - will play around though.

    It's still not got the dynamic punch of my claymore, but has a lot more open feel to the top end, so for the moment I'll just enjoy listening to one for a few months and then swapping back!
     
  19. James Evans

    James Evans Bedroom Bodger

    right then, had a bit of time to myself tonight, so decided to drop in these 0.1uf polyprops. They were too chunky to fit under the pcb, so had to take out the bc128s, put the polys in their place and then whack the 128s under the pcb, which was a damn fiddly tight squeeze as well.

    Only just finished and first impressions are.... it all still works.... which is a bonus

    listening to be done tomorrow...
     
  20. fatmarley

    fatmarley Registered User

    Come on James! Whats the verdict? Are you happily listening to music or cursing me?
     

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