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Musical Fidelity P140

Discussion in 'd.i.y.' started by Chops54, Jul 17, 2015.

  1. Chops54

    Chops54 pfm Member

    Help, does anyone know the bias setting and where to take the measurements from for the MF P140 power amplifier?

  2. df_genius

    df_genius Solder slinger

    This might provide some useful info:

    but I can't find mention of bias levels. They should be measured across one of the emitter resistors. Do you have one channel producing more heat than the other? Does it get particularly hot?

    I think Arkless Repair might be able to help you with bias levels from looking at an old thread on here. It seems to be a MOSFET output stage, these usually bias higher than BJT
  3. russel

    russel ./_dazed_and_confused

    IIRC its got 0.22ohm source resistors, lateral FETs need 100-150mA bias current, so 22 to 33mV across the resistor.
  4. Chops54

    Chops54 pfm Member

    Thanks chaps. I've just received a PM from Jez and he kindly confirmed 100mA, so across those source resistors I should see 22mV. I've recently serviced a pair of these stereo amps for my son and thought it wise to check the bias. One of the mains transformers had the primary winding go short so I sent them both to Canterbury Windings for rebuilding and I have to say that Terry did a first class job for very reasonable money.
  5. Robert

    Robert Tapehead

    I ran a pair of these some years ago into my then Impulse H2s.
    Nice amplifiers IMO and I think the power amp section of the integrated B200, which is also a nice amp with iconic A1 styling.
    The power amp design is simple and unique but works surprisingly effectively.

    It looks like a TdP back of a fag packet design....'here you go <scrible scrible> this should work' :D
  6. Chops54

    Chops54 pfm Member

    They used to be mine until I handed them over to my lad some years ago. I always liked their gutsy presentation and I've recently had them driving my IMFs and they didn't disappoint. They can be picked up for next to nothing too. I've just bought another pair in need of some tlc for 140 quid :)
  7. Rana

    Rana pfm Member

    B200 is a surprisingly capable amp which I have stashed in my attic as a substitute amp. It last came out when I sold my Supernait / TPX combo and was scarily close.
  8. bugbear

    bugbear pfm Member

    Zombie thread!

    I have a "chance" at a pair of P140s, one of which works and the other "used to work".

    Cosmetic quality is good; I assume (since it's a pair) they were used in bridge/parallel mode.

    But (google!) I have read good things about their sound.

    And truly terrible things about component selection and build quality.

    So - what's a "sane" price for a speculative pair?

    (comments from Jez especially welcome!)

  9. Gervais Cote

    Gervais Cote Predator

    Here in Canada, aP170 in a used to work condition has been sold for £290.00
  10. bugbear

    bugbear pfm Member

  11. Chops54

    Chops54 pfm Member

    Nothing wrong with P140 build quality. The voltage rating for the smoothing caps is questionable but if you’re going to recap it’s no problem to address. I did my son’s pair of 140s a few years ago.
  12. bugbear

    bugbear pfm Member

    Hmm. Since the heatsink is on top, you can't stack a pair of PXXX amps. They're already prone to over heating. I suppose since they would be mono-blocks, I could "floor mount" them near the speakers.
  13. Chops54

    Chops54 pfm Member

    I never had any over heating issues with mine. I think there’s a temp sensor on the output device mounting rail but I can’t remember. I do have a spare P140 somewhere with the lid off. I made a heatsink from a large piece of thick L shaped aluminium so I could set the bias with the lid off. The output devices get very hot very fast without it :eek:
  14. DoctorRad

    DoctorRad pfm Member

    If your preamp can drive long interconnects okay, mount the power amps vertically behind the speakers for better convection ventilation, maybe?
  15. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Very good amps (will thrash Quad 405 etc etc, Better than most Naim amps and better than eg Exposure VIII). P150 basically the same but had a mod to remove the background hiss which is present with the P140 (unlikely to be objectionable unless >100dB/W speakers).

    As with pretty much any amp of such age, will need a complete rebuild now.

    Best used in pairs with channels paralleled (NOT bridged! Which seems to be what many think goes on with these and other similar MF amps).

    I have mods which can transform these to another level but very expensive.

    I can't fit in any such work at the moment.
  16. Gervais Cote

    Gervais Cote Predator

    Is the P170 in the same league ?
  17. Arkless Electronics

    Arkless Electronics Trade: Amp design and repairs.

    Yes. It's pretty much the same design with bigger power supply.

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