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Musical Fidelity P140

Chops54

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

Cheers.
 
This might provide some useful info:

http://www.diyaudio.com/forums/solid-state/24723-schematic-a370.html

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

Cheers.



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

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.
 
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!)

BugBear
 
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.
 
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.
 
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:
 
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.
If your preamp can drive long interconnects okay, mount the power amps vertically behind the speakers for better convection ventilation, maybe?
 
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.
 
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.
Is the P170 in the same league ?
 
Hi there, I am resuming this post as I just purchased a second P140 for my kitchen setup and I was curious how to connect both the amps to the 3a preamp in parallel mode.
I assume the "mono" button won't do the job so easily. Right?
cheers,
 
Hi there, I am resuming this post as I just purchased a second P140 for my kitchen setup and I was curious how to connect both the amps to the 3a preamp in parallel mode.
I assume the "mono" button won't do the job so easily. Right?
cheers,

Press the mono button to connect all the input sockets together, hence any one of them is used as the input for that monoblock. Next connect the two red speaker outputs together and the two black ones together using short pieces of wire through the holes in the binding posts (DO NOT CONNECT RED TO BLACK ETC!!!) and use either red socket with either black socket as the speaker output. Both channels now work in parallel and although you will get only about 5W more power you will get twice the current ability and twice the damping factor.

If they have not been fully recapped and serviced they will desperately need it by now!!
 
Thank you! This is very clear now!
Mine 140 was recapped 2 years ago. The one I just purchased seems to have been thoroughly checked by the owner who is a technician himself but I will have it checked by mine as soon as it arrives.
Can't wait to have them powering my Snells
 


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