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Vintage Musical Fidelity Mosfet amplifier guide

Discussion in 'classic' started by Arkless Electronics, May 25, 2019.

  1. Spike

    Spike pfm Member

    Thanks Jez,

    I’ll keep my eye out for one in nice condition case wise. If you see any drop me a line.
     
    Arkless Electronics likes this.
  2. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Avoid! Monoblock power amps based on A1 and A100 respectively which run even hotter than the models they were based on and in the case of the MA50 used to regularly melt their mains transformers.
     
  3. Chops54

    Chops54 pfm Member


    Thanks Ark, yes they do sound pretty good, maybe just a little flabby in the bass. I understand how the mono button works and I used this when vertical biamping with these when they were mine. I did have a wiring diagram somewhere when running as monos but I’m buggered if I can find it. I would be interesting to get these out of hibernation and connected up
    to my TLS80s. Running them as monos might just cure that touch of bass flab but I must say they never sounded harsh or strident ever.
     
  4. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    It does! I mentioned this earlier up thread:)
     
  5. Chops54

    Chops54 pfm Member

    Indeed and thank you.

    When I overhauled them I made a temporary heatsink out a large L shaped piece of aluminium so I could take my time setting the bias. With the lid off these things get hot bloody quick. Tbh I think the lid/heatsink is a bit marginal. I’ve had five P140s and two had split lids and the other three had one piece lids. I do prefer the single lids.
     
  6. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yep it is fairly marginal but not quite as hot as an A1. With good quality caps though they are reliable. I don't think the one or two piece lid makes any difference to heatsinking but the one piece is a tad easier to work with.
     
  7. russel

    russel ./_dazed_and_confused

    If anyone wants to build a P170 type amplifier then I have a design/layout using discrete transistors, if anyone wants it modified to use an op amp then I can do it if you ask nicely, promise to actually build it and give a donation you can afford to charity. The design uses to3 transistors and is quite listenable, I use it upstairs where it replaced a 303, there is a plastic transistor version that I haven’t built but will almost certainly work.
     
  8. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Ah! Glen at Alchemist made a bipolar output stage version as an experiment, it still used the op amp. Sounded great but never got to directly compare it with mosfet version.
     
    Alan Chung likes this.
  9. russel

    russel ./_dazed_and_confused

    Sorry, didn’t make it clear plastic packaged MOSFETS, easier for the diyer.
     
  10. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Thought you meant bipolar OPS... as you were:)
     
  11. JensenHealey

    JensenHealey pfm Member

    So - can one of tell me what the X-A1 that I had some years ago was? Is it an A1 in a different case? or something else.

    It blew up the power supply unit and the repair guy I used could not find out what transformer was required, so it got left unrepaired. Shame, I rather liked it.
     
  12. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Can't say without seeing the innards. As I reported a few years ago, MF sold many B200's as A1's!! Same casework so whilst I can only guess at reasons.... order for batch of A1's in and no PCB's available maybe? B200 would have actually cost more to make. A1 transformer was used and a couple of resistor values changed on B200 to allow the bias to be turned up.

    That would have been a dead easy repair BTW!
     
  13. Jim Audiomisc

    Jim Audiomisc pfm Member

    Just popping up on this thread to mention that UKHHSoc have almost no details of the circuits, etc, of the MF designs. So if anyone has diagrams, manuals, etc, I'd be interested... :)
     
  14. cjarchez

    cjarchez pfm Member

    Don't get the X-A1 and the A1-X mixed up.
    The latter is a close relation to the A1 in the same griddle casework.
    The former is from the later, wider extruded sleeve and ally front panel family, with external PSU. This ran cool with just an aluminium billet sinking out to the half width casework and ran cool.
     
  15. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Good point!
     
  16. kempeu

    kempeu New Member

    How’s sound quality of P180 compares to A270/A370? If can manage to get rid of those heat issues..
     
  17. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    I couldn't really say. A370 was MUCH more expensive than P180. I've had both but not at the same time and they were in systems at opposite ends of the country. I liked both but would expect the 370 to be better. The heatsinking was totally inadequate on the P180 and nothing can be done about this really. It uses those tiny individual TO3 finned heatsinks for each mosfet and if driven hard they would get so hot (probably like 150C plus!!) that the mosfets would start to "shut down" due to the heat! This is not any sort of thermal shut down system but the mosfets getting SO hot that they basically cease to amplify until they cool down.
    A270, P270 and P180 have identical circuitry apart from 5 pairs of mosfets used in 370 and 180 and 3 pairs in 270. The quiescent current is obviously turned down to bugger all in the P180 to avoid them overheating whilst just idling and so it has very little class A output.
     
    kempeu likes this.
  18. Isobarik

    Isobarik pfm Member

    I use to own the A200. Loved the sound but got a bit fed up with the sound cutting out every 30 mins, although it was never pushed hard! just let it cool down for a few minutes and the sound was restored. Never heard a CD play all the way through whilst I was using it!:D ' probably a blessing in disguise'.
    Ray
     
  19. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Sounds like it had far too much class A bias and was heating up until the bimetallic strip thermal cut-out was operating! Terrible QC! The quiescent current in this range (A1 etc) was set by a pair of high value fixed resistors and it was just assumed that all would be correct whereas in practice it would vary quite a bit depending on the transistors used ie the batch they were from etc...
     
  20. Isobarik

    Isobarik pfm Member

    Cheers Jez.
    I should have done a bit of research when it was in my possession and had it looked at.
    Crimson Audio who were based in Longton would have had a look at it and no doubt sorted it out for me .
    Can't remember the name of the engineer who helped me sort a few amps out. He was a partner in the business at the time,lovely helpful guy. He eventually left and went on to teach IT in a college somewhere.
    Apart from the fault it really did sound lovely.
    Regards
     

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