Aleph 5 IRFP244 mosfets cold as ice

[Ully]

Hey to everyone.

I burned all my mosfets when putting amp on heatsink without isolating tape. So i replaced my irfp244 mosfets and turned on the amp, and when i try to play some music the sound is not like a class amp. So i desoldered my irfp9610 and measured they are also dead. I solder new ones and try to play music again and its a little better than before but not good at all. And the mosfets irfp244 are cold as ice after 20min of running time. Maybe any ideas what can be wrong?

 

[Zen Mod]

pics, reference schmtc?

 

[Ully]

Schematic is the same like here PASS LABS ALEPH-5 - Class-A DIY amplifier schematic & PCB
I measured the Voltages on all 1ohm rezistors everywhere is almost 0V. Also measure on MPSA18 between E and C also almost 0V.

 

[Zen Mod]

it's hard communicating without parts nomenclature

ok, schm enclosed

what's voltage across 9V1 on top , in upper IRF9610 gate?

what's voltage across 220R on top , in upper IRF9610 source?

what's voltage across 390R on bottom , in left IRF9610 drain?

what's DC offset on output?


inputs grounded, output not loaded

 

[Ully]

what's voltage across 9V1 on top , in upper IRF9610 gate? 9V

what's voltage across 220R on top , in upper IRF9610 source? 5.27V

what's voltage across 390R on bottom , in left IRF9610 drain? 3.3V

what's DC offset on output? 0.1V (if i measure it right)

 

[Ully]

what's voltage across 9V1 on top , in upper IRF9610 gate? 9V

what's voltage across 220R on top , in upper IRF9610 source? 5.27V

what's voltage across 390R on bottom , in left IRF9610 drain? 3.3V

what's DC offset on output? 0.1V

 

[Dennis Hui]

Did you match your parts?

 

[Ully]

Yes i buy everything as a kit, all the parts the same as in schematic

 

[Zen Mod]

your input mosfets are not matched

they are cheap - buy 15-20 pcs and match them by your self, then you can go further

are output mosfets matched?

you need matched triplets , one goes to upper rail , second to lower rail

no need that they are all (6) matched

 

[Ully]

Okay, thank you for the answer, but how to know when the mosfets are matched? And what about heat as i know this amp should be a hot when its turned on. Sorry if the questions is stupid, but this is my first A class amp.

 

[nigelwright7557]

A cold class a amp sounds like incorrect bias current.
Even on my 10 watt class a JLH1996 I can just hold my hand on the heat sink.
Surprised to see no bias pot on the schematic.
No offset pot either.

 

[Zen Mod]

we'll teach him how to do that, when he acquire matched parts, what's necessary

 

[Ully]

Maybe u know some online shop? Where i can found, these matched triplets as you say before. Because in my country i only can buy seperate parts.

 

[Ully]

I speak with the guy who sell me that diy amp kit and asked about matched parts. he write me (in that case you will not have sound problems... it means that some mosfet will be more heat than others. Your mosfet do not heat at all and sound is bad so problem is something other. Try to change white cap 680pf . It is good , but try to use any from 680 to 1nf).

 

[Dennis Hui]

My guess is your output mosfets aren't really 'on' because the front end mosfets
are not matched so the voltage across the 392R resistor is too low.

Unmatched output devices can lead to uneven current sharing, which is not good
for reliability.

 

[Ully]

Found the problem it was two 1ohm 5w resistors with open line. Replaced resistors and now enjoying the music of A class amp. Thank you guys for the help and advices.

 

[Zen Mod]

good