Mf X-pre Psu

Discussion in 'DIY Discussion' started by T-bone Sanchez, Jul 8, 2005.

  1. T-bone Sanchez

    T-bone Sanchez

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    Ive got one of these and whilst it does add something to my EL34 amp Im sure much more performance is available. The prob is its very crowded in that little can so I could do with making some room. An idea Ive had is for an external psu and do away with the majority of the internal pcb for this. Ive looked at it and it is a simple wiring up job. Hopefully Ive attached a copy of the psu schem as it is. Any suggestions on what you think would be an improvement would be great.
     
    T-bone Sanchez, Jul 8, 2005
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  2. T-bone Sanchez

    zanash

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    sorry for the delay !

    been on holiday.........too hot and I only went to suffolk!

    Yes theirs loads you can do .....

    swap the 1n4007 diodes for 11dq10's

    bypass the big resovoir caps with values decreasing by 10 % ie if 1000 use 100uf and 10uf

    or swap the caps for better ones stagets or silmics I don't like black gates

    if your still using the wall wart replace with one of at least 10va [230vac to 12ac]

    that should do you for starters
     
    zanash, Jul 17, 2005
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  3. T-bone Sanchez

    T-bone Sanchez

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    Hi Z, I did wonder if you'd been away, this is usually your sort of thing.

    First of all, I chose to keep the psu in place, wasnt worth the hassle to change. The main problem I had is that its very tight in that little tube so I had to go carefully. I swapped all the caps for either Panny FC's or Elan Stagents depending on size. I managed to double the capacitance of most caps. I really dont have any room at all to bypass them, it really was a struggle getting everything in. I thought about the diodes but decided to go with the caps first, I might be able to change them I'll have a look.

    I made a new psu over the weekend using a 80Va toroidal I had in a phono psu, this is by far the best upgrade so far. The difference in performance is night and day. I do have a 30Va toroidal which I might swap so i can put the 80Va back in its phono psu, I'll see. Thanks for your input.

    By the way, Im going to bypass the psu caps in my valve amp, where do I take them from??
     
    T-bone Sanchez, Jul 18, 2005
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  4. T-bone Sanchez

    zanash

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    The diodes make a big difference too......

    you can normally place 1uf wima bypass caps on the underside of the pcb...

    not certain I would have doubled the capacitance though....can make things sound ponderous.......I prefer fast and light with dynamics.

    Swapping the output sockets is a good one and wiring with silver jumpers from the pcb.....in fact if you can trace the signal path silver wiring its route could be a good one ....fiddley though.

    Swap out the cheap and nasty resistors for vishay types...bulk foils or the mil spec RNC55 series.

    Had some intresting results swapping one on a copland CDP [ Rsand's] just the one in the signal path ....Rsand says made a huge difference..." now almost holographic presentation " and that was after doing a few mods which had significantly raised the performance of the cdp.

    Do you have the whole schematic ?
     
    zanash, Jul 18, 2005
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  5. T-bone Sanchez

    T-bone Sanchez

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    No, I only have the tube part. I couldnt even fit wima cap on the underside, it really is that tight. The reason I doubled the capacitance in the psu is that everything I could find on MF psu's recommended this. It was also comfirmed by MF technical department who recommend this as the first upgrade.

    Ive bypassed all signal paths on the pcb and hardwired with silver plated ptfe. I looked at the sockets but decided to adapt what was there by cutting alot of that horrible copper away and soldering direct to the pins.

    Ive managed to change most of the caps on the main board itself for decent polyprops (again a major squeeze), Ive changed those awful little ceramic discs for silver mica and any elec's in the signal path are now using Elna's. I played around with removing some caps around the 'pot and input selector but decided to go with the original.

    Needless to say the crap tubes that came with it are gone.

    One point I have considered is scrapping the iput selector and having one input wired direct to the pot. This would cut-out quite abit of messy pcb work and I would lose a number of caps. On the downside I would have to resort back to the input selector so I dont know.

    The resistors are next on the list I just need to take a breather, a few hours of soldering and Ive had enough!
     
    T-bone Sanchez, Jul 18, 2005
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  6. T-bone Sanchez

    rsand I can't feel my toes

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    Can confirm that one resistor on each chanel has improved the sound no end, strange thing is the volume has gone up too, even though the resistor is the same value as the old one?
     
    rsand, Jul 18, 2005
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  7. T-bone Sanchez

    T-bone Sanchez

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    This is basically the schem for the tube.
     
    T-bone Sanchez, Jul 18, 2005
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  8. T-bone Sanchez

    zanash

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    Not much to it !.....
    if you've done the coupling caps and the psu, it doesn't leave a lot left.

    Though any resistor in the signal path and to ground will affect the sound, so use the best you can ...if your intrested I've a selection of vishay dale and holco's in 1, 0.5, 0.25watts. The vishays are not bulk foils but as Rsand said sound fine !!
     
    zanash, Jul 19, 2005
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  9. T-bone Sanchez

    T-bone Sanchez

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    Thanks Z,

    Ive just had a quick look and I think I'll need values of 1M, 1.5 & 470K. There will also be a few much smaller values but Id have to check. Any of these?
     
    T-bone Sanchez, Jul 19, 2005
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  10. T-bone Sanchez

    zanash

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    Typical..... you have avoided all the values I have ! I'll check what I have and make a list.
     
    zanash, Jul 19, 2005
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