Power amp running Hot or Not???

[RPM-MACHINE]

Hi All,

I recenty sold my Musical Fidelity F19 Power amp and F25 Pre to a forum member. He has said that the amp is running VERY hot and I told him that if he is not happy with it I will refund him.

Problem is that I know that the Power amp runs VERY hot and the previous owner said that it is like that and is very heavily biased into class A with alot of power consumption even during idle. I have had no problems using the amp for hours running my Magnaplanars.

It is a hybrid amp using 2 Tubes. The output is around 75w Class A or 300w class A/B 8 ohms. Doubles into 4.

The Thing is.......Some people tell me that Musical Fidelity amps in general operate at very high temps while some people say that the bias needs calibiration. What is "NORMAL" operating temps for these amps?

Another forum member wants this amp, but I am curious about this now.

Your views / experiences with this sort of issue please.


Thanks

[LAV]

AFAIK Musical Fidelity amps did always have a reputation for running hot, and a heavy bias towards Class A would account for the versatility as a room heater 

I would also make sure it is located where there are no obstructions or even shelves / racks or anything above it, i.e. pref. located on the floor (on its own) or on a separate plinth.

IMO unless there has been replacement of output devices at some point, I would not think that there should be any bias misadjustment problems unless something has drifted severely out of spec due to component ageing (unlikely) but of course it would do no harm to have the bias of the output stages checked.

[d0dja]

Like really on it's own, no shelves above it or close to it or behind it.

[fdlsys]

F19 and all of the "F" series are biased very high and supposed to run VERY hot. Dangerous to touch hot.
It would be rather unusual (especially for cost consciences Musical Fidelity) having those amazing(ly expensive) heatsinks for no reason, hmm?

As far as placement, floor is actually not the best idea because it does obstruct the air flow, but it will be pulling dust through and deposit it on top and inside the amp.

The best is a separate stand that does not obstruct the airflow, all the way through the fins.

[JimGore]

Agreed!

Any class A amplifier will run hot to the point of near-burning yourself on a heatsink.  It's how it was designed.

Ian.

[GearSlave]

Guys,

I've had a look at this amp and raised the initial concern. This specific unit has two modes of operation, Class A and High Power - which is Class A/B. I had the amp on my desk and ran it in Class A/B mode - the heatsinks got to 72deg/C within 5 minutes. Now I can accept that this may happen in Class A mode, but I can't think this should be normal for Class A/B operation. Subsequently, I didn't have the guts to test it in Class A.

Because I don't know these amp at all, I'm hardly qualified to comment on if the amp is operating within specifications. Logic does tell me that an amp running in Class A/B mode wouldn't get so hot so quick. Also, much Googling hasn't shown any hits with people commenting on how hot it gets. So I can only guess here - I suggested to the OP that he fires off an Email to the MF factory directly for them to comment. My suspicion is that the 'sliding bias' scheme has failed and this puts the amp into heavy Class A operation all the time, irrespective of the mode switch setting. And at the price of those output Mosfets I was quite frankly too scared to start messing about with it.

[Atjan]

I don't know much about amps, but I can comment purely from a quality assurance point of view.
I agree with Gearslave, get the factory specification for testing the amp's operating temperature and compare that to what you observe here.

They will typically specify room temperature, class A or A/B, wall distance from the sides of the amp, idle or full power, temperature on the cooling fins or on the output transistors.
Without those specific conditions being present and measurements being taken with the correct devices, they will not be able to tell you if its in or out of specification.

Sounds dangerously hot though.

[LAV]

72 degrees within 5 minutes? Mmmm, if Gert is worried, I'd be worried. Fully agree with his reply.

I agree that the floor could be a problem, but a solid plinth would probably be fine.

[Timber_MG]

the heatsinks got to 72deg/C within 5 minutes.

Good heavens. Junction temps would be a lot higher than that and those temps don't seem right. Factory spec would be the best option with a known temperature. You might also get an idle current spec for Class A or A/B idle operation that might be more telling.

The difference in natural convection won' t be enough to warrant such temperatures which will dramatically reduce life expectancy of the amp.

[GearSlave]

You might also get an idle current spec for Class A or A/B idle operation that might be more telling.

The temp I quoted was for the idle state in Class A/B, without any speakers connected - hence my concern.

[Timber_MG]

Sure, I'm just stating that if the biasing is running too hot then that would reflect in the idle current at the supply side and is less involved to measure than a temperature gradient.

[Atjan]

I don't know much about amps, but I can comment purely from a quality assurance point of view.
I agree with Gearslave, get the factory specification for testing the amp's operating temperature and compare that to what you observe here.

They will typically specify room temperature, class A or A/B, wall distance from the sides of the amp, idle or full power, temperature on the cooling fins or on the output transistors.
Without those specific conditions being present and measurements being taken with the correct devices, they will not be able to tell you if its in or out of specification.

Sounds dangerously hot though.

[fdlsys]

The proverbial "other side of the story" 

No brainer - in conventional class-AB, the heatsinks should be room temp without input signal.

In class-A, as much as 72 deg. sounds like a madness, it is hardly unusual. No input signal, no shorts on the input, no speakers = max. Current to Heat conversion. (getting there in 5 min - that could be a bit unusual)

BTW, I thought you hated the MF? So, what's this thing doing on your desk?

[GearSlave]

BTW, I thought you hated the MF? So, what's this thing doing on your desk?

Yes, I do. Immensely! Probably the reason why I should be the last person to have a look at it. And if this is in fact 'normal' for this amp, I've got another reason not to like them.

[fdlsys]

Agh, this particular amp aside, 100% working or not - but people really must understand what is it they are getting into whenever they come close to anything that resembles the Class-A.

The spec sheet for this beast says power draw = 600W in Class-A. Class-A, if biased high (and this one IS, very much so) runs the full current (supply potential) across the output devices the whole time. 600W of power consumption HAS to be dissipated somewhere... no?

I have a modest MF A120 which is referred to as class-A; quite possibly only up to 5-10W, maybe, the rest (40-50w - not sure what the rated power is) switches over to class AB operation.
Now, try putting your palm on top of that heatsink after 1hr of play time... I dare you. And this one has been running since ... early 90's? Ian has (or had) a A120 also - I'll bet you his experience with A120 heat would be the same. And no, there's nothing wrong with either of these.

Don't get me wrong Gert - I know that you know this stuff and that your dislike of the MF is part serious part a long running joke, but a (large?) number of people doesn't, so they react (differently) when they experience the class-A heater in operation...

Anyone ever dared to touch the JLH (10W/channel) or Hiraga (20W) or Lang (22W) or Pass A40 (30W) or even an F5 after an hour of warm-up