anyone know the musical fidelity a1 -x amplifier

[tube shop]

anyone know the old musical fidelity a1 -x amplifier, what is the didderent compare to mucical fidelity a1 model ???

[MikeB]

Hi !
If i remember correct, the -x is about the powersupply...
I also have one, was a quite good amplifier !

Mike

[tube shop]

do you think it is a 100% class a?

i want to mod it and replace the cap, i will send you the picture soon.

[jacco vermeulen]

Not fully class A.
Not enough cooling surface of the heatsink/case top for that much power, the A1 ran plenty hot as is.
if you do a search you'll find a link to a homepage dedicated to MF.
The X version is said to have had a larger powersupply, bigger capacitors, but the other version was that it was just the Euro name. X for export, i've only seen X versions here in Holland.
A modification could also be to exchange the output devices.

Dont bother with searching, still had it on my fav list:
http://www.mhennessy1.f9.co.uk/mf_a1/introduction.htm

You'll find the schematic under the tech notes

[jez]

Yes, it is totally class A. I once worked for musical Fidelity so I know!. Non of their other amps are truly class A.

[lineup]

Quote from the above website

What sets the quiescent current? Well, I think you need to look to R30/31 for the answer. There is a small voltage across these resistors, and assuming that R30/R31 are accurate in value, you can determine that the standing current is 0.69A, resulting in 7.6W of Class A. Two factors influence this. Firstly, each half will have a small dc offset, due to the unbalanced LTP and other circuit conditions. In production, this is bound to be rather variable from unit to unit, so the 3M7 resistors (R6/11) appear to have been added to introduce a defined dc offset in the LTP, presumably swamping the other (poorly-defined) offsets. This will generate the voltage across R30/31, and the resulting standing current.

and

A note about class A operation:

The claim of class A operation has always been a controversial matter. It's worth pointing out the Musical Fidelity say that the amplifier is "strongly biased into class A" - apart from the lettering on the front panel (!), they have never claimed that the amplifier is a pure class A design as far as I can establish.

In truth the A1 is, like most other power amplifiers, a class AB push-pull design. The quiescent current is simply set high enough to ensure class A operation up to a certain power level with a particular load impedance - around 8 watts into 8 ohms. Outside of this, the amplifier reverts to class AB. This is absolutely the right way to do it, otherwise low impedance speakers will cause clipping at artificially low levels. Classic Krell amplifiers like the KSA-50 will have enough standing current to provide 50W of class A operation to 8 ohm speakers (around 1.75A), however when driving 2 ohm speakers, it will revert to class AB above 12 watts. The prodigious low-impedance capability of this amp meant that it was frequently used to drive Apogee ribbon speakers...

and

Finally, I'd like to quote from a Hi-Fi News review by Dave Berriman, published in the January 1994 issue. This review was admittedly of a mark 3 model (see below), but it's useful information anyway:

Out of curiosity, we checked the quiescent current through the output transistors. At 0.71A, this is enough for up to about 8W of pure Class A, which implies that over 8W the A1 runs into Class A/B.

I can think of a possible explanation for this - I wonder if models destined for hot climates like Australia were different to European examples? Within a week of posting this, I received an email suggesting that doesn't seem to be the case, but I'd love to explore this further minus the "Usenet vitriol". Please get in touch if you have some experience of these amplifiers "down under".

Power supply is +-24 V

[jez]

During my time working for musical fidelity (i ran the service dept.) I was asked to check the power consumption of the various models to see if it was anywhere near the rating on the rear panels. It was whilst doing this that I got curious as to the class A rating of the various models that claimed to be class A. The A1 was the only model that passed the test. The A370 actually made less class A watts than the A1!

[jez]

Oh yea....and it was me that supplied the circuit diagram for the above web site....

[Zen Mod]

Originally posted by jez
Oh yea....and it was me that supplied the circuit diagram for the above web site....

I back-engineered that some 15 yrs ago,and maded DIY of just power amp,because I have thoughts that preamp is weakest part;
I didn't know then what I know now (primary I know more how much I don't know ) and that was before internet era (at least -mine).

it was interesting that ,in my quest for type of original transistors,I wrote few letters to MF,with story that I bought that amp with broken channel;
after secod letter they send me output transistors for both channels,along with BDs for drivers.......without any further info..

then I realized that I allready finded exact replacement transistors -2N3055 and BDX18 ,just because they have exactly same parameters in circ as originals from factory


interesting-when I first time powered my DIY A1,Iq was above 1A !
after few hours ,it was settled down to more regular ~ 700mA....
I was never able to figure how that happend.......

[jez]

The outputs were (as you say with the NPN) 2n3055 and MJ2955 but marked up as A1p and A1n.
As you found, the quiescent current could vary a fair bit!, I seem to recall the example I tested makeing 14Wrms in class A which was pretty good I thought.
I sometimes had to change the value of the resistors that set the standing current on these amps in order to get one to run cool enough to not self distruct during an overnight soak test. They would sometimes go through 3 mains transformers in 3 nights!, with a reduction in quiescent current each time, before we could get one to last a night...it would then go back to the customer...
The MA50 was much the same (it's an A1 with no preamp section and the channels in parallel to make a monoblock, a pair of MA50's cost much more than two A1's though!)