MOSFET problems (need a hi-fi techy!)

[bojangle]

I have a Arcam Alpha 5 amp that i 'cooked' and had repaired, i was told that where it got so hot the MOSFET internals had melted together and caused a short circuit. But..........it has happened again, (i know i should not have it that loud!!) I have now had it apart and can see what was replaced so i know i can now do it myself this time.

my question is this...Can i simply replace the MOSFET's with higher rated ones or would that mess everything up!

The main cause of the problem is the amp is not big enough to run my speakers and gets too hot on full whack, but don't want to fork out for a new amp just yet! ( replacement MOSFET's are £1.19 each!)

So can anyone help?

 

[FluffyChicken]

If it's getting hot, can you not add or increase or improve a heat-sinking & cooling solution ?

 

[WD Pro]

Do you have it loaded correctly ? (Ohm's)

WD

 

[gump]

Its pretty tough to kill FETs like this, as they have neg gain with temp (hence why used in many cheap/simple designs)

Maybe fault elsewhere?

 

[gbsimpsa]

The answer to your question is 'possibly it depends'.

To kill a MOSFET usually means it cooked due to an over-current condition.

To replace it with a higher rated device would possibly mean a different package size, and possibly different overall sound due to differing threshold characteristics.

I would expect most consumer amplifiers have over-voltage, over-current and possibly over-temperature protection on them nowadays. Don't know your amp but perhaps the protection circuits 'if any' are damaged too.

If you simply put in a higher rated IDS device another part of the output stage might fuse instead next time and it may not be pretty!!

WD Pro is right, you should check what speaker loading impedance you are connecting to your amp first. Does the amp have a minimum loading impedance printed on it?

 

[bojangle]

If it's getting hot, can you not add or increase or improve a heat-sinking & cooling solution ?

that was my next plan!

In regards to the speakers, i'm running two small shelf speakers and two large floor standing. Both sets of speakers are rated 4-8 ohms!

The guy who fixed it for me is very techy and does fault finding/testing for some huge blue chip companies, so i kinda trust what he says!

 

[WD Pro]

How many channel is the amp ? You need to work out how the speakers are loading it (which is altered depending on how you wire them if you have multiple speakers per channel) and then check / compare to the amp spec

Where is the amp mounted / fitted ? Can convection work as it should to remove the heat ?

WD

 

[gbsimpsa]

In regards to the speakers, i'm running two small shelf speakers and two large floor standing. Both sets of speakers are rated 4-8 ohms!

That's the root cause failure mechanism right there!!

You need to make sure the total combined load impedance goes no lower than 4 Ohms or it will cook itself. Based on the speakers you are using your current combined load impedance is between 2 and 4 Ohms.

My advice is to disconnect one pair of speakers!! Failing that if you want to drive 2 pairs of speakers find speakers rated between 8 and 16 Ohms each.

 

[Skeelsie]

Exactly as above.

You're running the speakers in parallel showing the amplifier a very low nominal load, when you get to certain frequencies the load drops even further causing the amp to pretty much puke its guts out through the speakers causing way to much heat and a near dead short in the outputs.

You'd get away with doing this for a short time at low volumes but hi-fi amps are never designed to be 2ohm stable as purists doubt the damping factor qualities of small loads.

 

[bojangle]

Can you get speakers with lower impedance? :?

i've looked everywhere for some and the best i can find is 6ohms!

The amp is a four channel amp, so is designed for two sets of speakers.

The amp has plenty of air movement around it, and is not under or on top of anything else hot! I might be being dumb/ignorant/uneducated (delete as applicable) but isn't it just a case of matching the output of the amp to the speakers e.g if if output of speakers is 100w then the amp should run at least this?