MOSFET Temperature Vs. Quality

[promitheus]

I would like to aks Mr. Pass and anybody else who knows anything on this subject.

Does the temperature of the Mosfets in preamplifiers and amplifiers have anything to do with the music reproduction quality?
Should the Mosfet be hot or cold?

I think I have heard that mosfets "sound" better when warmed up to 50-60 °C. I came up with this question because a friend of mine had a very good and practicle idea. We were thinking of making a new chassis design and mounting the mosfets on the bottom of an all aluminium box. Using in this way the whole chassis as a heatsink. Some people are actually afraid of the MOSFETs getting so hot and concerned of what could happen when they forget the preamp powered on for days. In the balanced line stage with the small heatsinks they get very hot when the circuit is using +/- 60 Volt rails.

I didn´t have such problem of overheating after several hours. The temperature stays constant.

 

[Nelson Pass]

50-60 deg C is about where I run my case temps, and my
stuff has been optimized around that. It's hard to say
which temp is best, but Mosfets like lots of current and
voltage, so I always run them as hot as I can safely get
away with.

 

[promitheus]

Thanks for the reply Mr. Pass.

In the Amplifiers the mosfets are hot anyway so there is no problem with that.

In the Balanced line stage then it wouldn´t be such a good idea to totaly cool the Mosfets. I should use a small heatsink.

 

[grataku]

Promitheus,
I don't think that was what NP meant. I see no reason to undersize the heatsinks in the BLS. The best is to keep the operating point temp. as low as possible. Realistically that's hard to do.

 

[promitheus]

From what I understood is that NP optimises his circuits so the mosfets work around 50 °C.

It´s not at all hard to keep the temperature low. I could easily keep their temperatur under 30°C in the balanced line stage.
They run at 1.2 Watts. With a 1K/W heatsink or less you keep the temperature only1-2 °C above ambient that means around 25°C.
With a normal 5K/W small power heatsink that would be 30°C.

My idea was to mount the mosfets on the chassis bottom or side. The whole aluminium box is a very big heatsink for the BLS.
But the question is, is that good for quality?
I think no.
That means it is a good idea to downsize the heatsink.
Or not?

 

[Henrik]

Hi all

Does any of you have some comments to this post, and the discussion on heatsinks in this thread.

http://www.diyaudio.com/forums/showthread.php?postid=285804#post285804

I have mounted the fets i my xbsoz to the bottom at the case, and just besides them i measures 45-50 degree C. This works verry well, sound is really good, but might be heatsink-overkill.

Regards

 

[promitheus]

What kind of box did you use?
That´s a bit hot.
Maybe then I should use your method.

 

[Henrik]

Hi promiteus
The measurement was closte to the fets, so in distance of 10cm the tempereture is around 35-40 and the whole case is around 35 at ambient temp. at 20 deree C, as I recall.
Regaeds

 

[Henrik]

The bottomplate is 3mm aluminium.

 

[grataku]

The important thing is how many watts the device dissipate and how constant is the temperature not how hot do they get. It saves room to go with small HS but that's seldom "better".

 

[jackinnj]

My idea was to mount the mosfets on the chassis bottom or side. <SNIP>
That means it is a good idea to downsize the heatsink.
Or not?

The new Linn amplifiers have the transistor heat sink on the bottom. They also use a switching power supply.

Jack

 

[promitheus]

It doesn´t tell me much if Linn cools the transistors on the chassis bottom.
Different companies have different design philosophies.