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Vintage Musical Fidelity Mosfet amplifier guide

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

  1. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    This started as a reply to a PM.... then I posted it as an addendum to another thread where the subject had come up... and ended up adding further to it... Eventually it occurred to me that it could do with its own thread.

    This range of amps, designed by Tim de Paravicini, rarely gets mentioned and has an often deserved reputation for unreliability. However, I put it to fishies that when suitably fettled they are not only very reliable but are more than a match for the usual forum fodder of Quad, Naim and Rega, which one could be forgiven for thinking are the only second hand amplifiers in existence!

    The same basic design of power amp was used in everything from the B200 integrated to the SA470 beast and basically the more money one spent on this range the more output mosfets were used, the bigger the power supply and the more biased into class A they got... in rough order of expense and "goodness" it went:

    B200 integrated (B200, P140 and P150 basically the same power amp section but you get a pre thrown in with B200)

    P140 (power amp section only of B200)

    P150 (identical to P140 but has "MkII" mod to reduce hiss. I'll come back to this)

    All the above have a rather small mains transformer for the output power but quite a lot of reservoir capacitance so they are a case like the one described by "JimAudiomisc" the other day in that whilst rated at 75WPC they can give more like 200W for very short peaks... this also means that when used as paralleled monoblocks (see below) RMS power goes up to about 110W rather than staying the same (75W). The doubled up mosfets mod adds to this and about 120W then available as paralleled monoblocks.

    P170 older version without a couple of minor "improvements" but bigger power supply and more output devices.

    Studio T Similar to P170 but IIRC even bigger PSU

    P180 A bit of an odd one this... Basically an A370 built down to a budget and without much class A bias (or heatsinking!) but you get the 5 pairs of mosfets per channel. Has a separate power supply and there was an extra "choke regulated PSU" available which if anything held it back a bit. In effect the £3K A370 with much smaller PSU, very little class A bias and vastly under heatsinked but was more like £900 IIRC. Bit of a curates egg... can sound fantastic but if thrashed can have overheating issues....

    P270 An A370 with 3 pairs of mosfets rather than 5 and smaller PSU but still quite a beast.

    A370 5 pairs of mosfets, huge beast of a thing, 2 x 1KW mains transformers,
    big heatsinks 220WPC class A to 18WPC

    SA470 11 pairs of mosfets, two man lift etc.... Very few made.

    There were P140 X and P150 X later on IIRC but I don't know what precisely the differences are to the earlier versions... probably a last run with left over parts and they added the x for marketing reasons! I had left MF when these were around and have never worked on one.

    In original form they share a foible of being a bit hissy. It wouldn't bother me personally as it's not loud and even very quiet listening masks it but with nothing playing it's certainly noticeable. If you have 103dB/W horns I guess it would be a problem! A modification with a couple of transistors solved it and it was gradually incorporated into the range. B200 I don't recall its name changing at all, P140 became P150, P270 and A370 became "MkII" versions. P170 and Studio T are older and had finished production by this time.
    A generic version of the topology, most similar to the P170, was made for Bose and sold as a Bose Professional product but I suspect only small quantities were made and in fact it may never have been taken up by Bose after say 20 evaluation units had been shipped... More vague on this but some Studio T's may have been sold as Bose also (virtually the same inside but very different styling).

    The problem with all of them is they run hot and were built with basic bog stock electrolytics that were only just up to the fact not quite even up to the job as they used 63V rated caps and put 70V across them!!:eek: so even if you find any of the above amps actually still working they will be on their last legs... Don't let this put you off though because....

    A rebuild with 105C rated top quality electrolytics and general service makes them reliable Quad and Naim etc slayers! They really are considerably better sounding than the forum regular fave rave power amps at and around this price bracket:)

    Other mods are available such as doubling up output mosfets. P140/150 also can be used as monoblocks and have a mono button for this. When used as monoblocks they are paralleled, not bridged, so power goes up only a little but real world load driving ability is much increased (doubled). A pair of P140/150's used as monoblocks is quite formidable!
    A mate of mine has been using a pair (with full mods by myself including doubled up mosfets) for about 20 years plus of daily use without any issues and is presently using them to drive his ATC SCM40's.
    torstoi likes this.
  2. Woodface

    Woodface pfm Member

    The main problem I felt with this brand was that they introduced too many models too frequently. If they had reduced the model range & developed them properly they would probably have a better reputation. They always reviewed well, new models is the lifeblood of the hifi press though. I had heard rumours that the models sent for review were not always representative of what was actually retailed; you will know far better than most on that score. I don't think I have ever heard any of their kit to be honest. Interesting & useful post though.
    crimsondonkey likes this.
  3. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Too many models yes! Especially pre amps at the time I was there... must have been about 8 different pres!

    They tended to try and out do all the competition on "high end-ness per pound" (eg class A budget amp) and sound quality for the money, and largely succeeded I would say BUT the BIG problem was that rather than say "you can't put a 20WPC class A amp in the shops at only £199... it can't be done" they would save money anywhere they possibly could, and usually on the component quality (and even rating!) rather than anything that would spoil sales room appeal such as cheap casework.... So you ended up with amazing sounding best in class amps that unfortunately blew up after 2 -5 years.... The basic circuit design was fantastic and many amps were TdP designs. Rebuild them with parts that are up to the job and they can be as reliable as they are good sounding;)

    I checked over a few amps that were going for review (A valved Chronos pre, power and PSU was memorable as few were made and they cost a fortune) to be sure they were not "Friday afternoon" ones and met all specs but IME there was no "souping up" of review samples...
    Woodface likes this.
  4. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    For those wanting to know what they actually sound like, well, all IMHO of course... All have a notable lack of the "greyness", "flatness" etc that can afflict many amps. They have a nice liquidity/fluidity to them. The smaller ones especially sound a touch valvey and as more mosfets and bigger power supplies come in further up the range they gain more authority, grip, large scale dynamics ability and neutrality. The treble is very good and has a notable "sparkle" and "life" to it without any grain.... Bass is extended, properly "filled out" and powerful on all models.

    Downsides? To Nth degree level some may find them just a tad bright but if so it's in an "impressive colouration" way.
    Also, with the smaller ones, although the bass is I would say tight enough to satisfy most, it gets tighter still as more mosfets and bigger PSU's come in.

    Overall I'd say they sound more "valve" than "SS" but in an EAR or Audio Research kind of way rather than the Quad II extreme...
    ToTo Man likes this.
  5. suzywong

    suzywong Wot, no electrons?

    A most interesting thread.

    So Jez, IYHO, how did the 370 & 470 stack up against the “equivalent” Krells of that era? Or as SWMBO commented, “Will it drive Scintillas?”.

    The number 470 amused me (being a “computer engineer”) because 470 was the model number of the Amdahl “IBM Killer” mainframe, and I always wondered whether MF were trying to be the “Krell Killer” of the audio world.
    torstoi likes this.
  6. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    A good question! I'm afraid I can't really answer it very satisfactorily as whilst I had an A370 for years I only briefly heard the 470 as only a very few were made, but I can say that AM had a pair of Scintillas which were used with an SA470... I heard that the Scintillas were blown up by the SA470 when staff decided to play dub reggae at high levels through them whilst AM was on holiday! This was apparently a few months before I joined them.

    Also there are many years between my hearing Krell's and the big MF's and on very different systems... I would say they are in a similar ballpark I guess!

    There was an ultimate version... the SA570... which was a pair of monoblock SA470's with the channels of each paralleled. I did final build and setting up on a pair of these but they were only made to order and maybe 2 or 3 pairs were made...

    Krell were better made mechanically and used better, more expensive, parts. They were also rather better designed in terms of things like keeping heat away from electrolytic caps etc. The top MF models were much better made than the cheaper stuff though and no slouches in build quality. The big MF's tended to use even bigger power supplies than krell, 2 x 1KVA mains transformers and 5 pairs per channel of Elna 22,000uF smoothing caps! SA470 probably 1500VA transformers and (IIRC) about 8-10 pairs of the same caps per channel AND huge smoothing chokes...
  7. Grenadier_45

    Grenadier_45 pfm Member

    I miss my A100 even after 20+ years which I believe was a Tim de P circuit too. It’s irritating habit of randomly turning into an amplifire every few years is why I didn’t repair it the last time. I’d love to hear it back in proper fettle as it once was, but I fear the circuit board in mine probably wouldn’t support a permanent repair, and I doubt new ones are available. Always fancied an A370, but don’t really need that much power, and couldn’t afford one at the time.
  8. cooky1257

    cooky1257 pfm Member

    The Dr Thomas was used by a subsidiary of Bose: Studiocraft IIRC.
    I used to own the F22/F18 pre power-the F18 power amp was a 60kg monster.
  9. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    There's another one then:) I more recall Studio T's with "Bose" on them and like I say there was at least a pre-production batch of a kind of cheapened P270 made....

    I left just as the "F range" was coming out and only worked on a handful and never got to hear them so can't comment on how these compare to the mosfet models.
    cooky1257 likes this.
  10. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    It was indeed a TdP circuit... same as A1 but bigger power supply at higher voltage meant 50WPC rather than 20WPC but required cooling fans in an attempt to stop them self immolating... It's probably repairable.
  11. Rana

    Rana pfm Member

    I have a B200 in the garage somewhere - need to get it out and serviced. I use it as a stop gap amp, once pressing it into service between having an Supernait (inc PS) and a Bedini running Super Elves. The B200 gave the SN1 a real run for its money. Was not far behind the Bedini. Not as good as my Linx Mosfets (had a Vega power amp and now have Nebula integrated and Stratos pre/monos) but could be down to needing a service.
  12. Chops54

    Chops54 pfm Member

    I rebuilt my son's P140s a while ago. I replaced the smoothing caps with 100v 105 degree Panasonics and I had Terry at Canterbury wind me two new transformers. Never tried them bridged. That would be interesting.
  13. cjarchez

    cjarchez pfm Member

    I do have a soft spot for a selection of the MF stuff, some of the models were terrific.
    The A1000, E100, F-series, Digilog, early X-Can series (I still have a XLPS, cheapy components swapped out, voltage splitter removed and proper psu added) but I'm not an A1/A100 fan, I only remember them as chuffing hot, sharp edged, nasty things with power switch button-caps that had a propensity for pinging off across the room. Maybe a model or two I've missed there.

    We had an A370 go into meltdown in the demo room taking a customer's SL600s, IIRC, with it. The smoke and smell was the most impressive magic-moment I've experienced.

    It was the B1, again IIRC, that caused the review sample different to production scandal.

    One positive thing about MFs chequered reputation is that you can find some of their offerings for peanuts second hand, then being worth some TLC to obtain decent equipment on the cheap.
  14. david ellwood

    david ellwood Kirabosi Kognoscente

    The big musical fidelity power amps are incredibly similar sounding to the Krells and a perfect match for ATC loudspeakers.
  15. stuwils

    stuwils pfm Member

    I still remember the lights dimming when I switched on my Doc Thomas, and it playing for around 30 seconds after being switched off.
  16. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    I bet they sound great! Just to be clear, the mono switch on these does NOT put them in bridge mode. They have no bridge mode. It connects the two channels inputs together so it works in mono. IIRC you have to add shorting links between the two red output terminals and between the two black but that's just IIRC, I'll have to check. It then works with the channels in parallel to give twice the current and twice the damping factor.
  17. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yep that's my thinking too. Some superb amps going pretty cheap cos of reputation but when sorted with decent components.... bingo!

    That sounds unusual for an A370.... the main fault that happens with these is the soft start resistor, a big metal clad wirewound, can go short to mains live! It is bolted to the case so immediately blows the fuse as an earth fault... Easily fixed.
  18. Spike

    Spike pfm Member

    I loved my A1. Never had a problem with it. Paired with Gale GS301 speakers sounded great. I fancy another. Can you do much with them Jez?
  19. foxwelljsly

    foxwelljsly Hawkwind and Fire

    What's the take on the MA 50 and the MA 100 power amps?
  20. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yep I can rebuild with better parts, newer op amps, bigger smoothing caps etc and both improve the sound and make it decently reliable.

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