Mostet problem

[ivan-UTN-ARG]

hello i built the 25W Mosfet amp of "red circuits" ... and i have some problems...

the amplifier works fine with 35ma of quiescent current, but when i set the quiescent current at 100mA the output transistos blows

i tried with IRF530 & IRF9530, and with IRF540 & IRF9540 and they blow...

with 35ma of quiescent current the amp works, you can play music at max volume, but the crossover distortion is horrible at low volume.
when i set the quiescent current at 100ma only the output transistors blow with a low input singal.

if someone can help me...

my ps is a toroid 300VA, and the output volage unweight is +39 and - 38.9.
the voltages weight (With 2 x LM3886 with 8ohm load each) is +/-36.5

I think that the problem is in the output transistors...


Parts:

R1,R4_________47K 1/4W Resistors
R2____________4K7 1/4W Resistor
R3____________1K5 1/4W Resistor
R5__________390R 1/4W Resistor
R6__________470R 1/4W Resistor
R7___________33K 1/4W Resistor
R8__________150K 1/4W Resistor
R9___________15K 1/4W Resistor
R10__________27R 1/4W Resistor
R11_________500R 1/2W Trimmer Cermet
R12,R13,R16__10R 1/4W Resistors
R14,R15_____220R 1/4W Resistors
R17___________8R2 2W Resistor
R18____________R22 4W Resistor (wirewound)

C1___________470nF 63V Polyester Capacitor
C2___________330pF 63V Polystyrene Capacitor
C3,C5________470µF 63V Electrolytic Capacitors
C4,C6,C8,C11_100nF 63V Polyester Capacitors
C7___________100µF 25V Electrolytic Capacitor
C9____________10pF 63V Polystyrene Capacitor
C10____________1µF 63V Polyester Capacitor

Q1-Q5______BC560C 45V 100mA Low noise High gain PNP Transistors
Q6_________BD140 80V 1.5A PNP Transistor
Q7_________BD139 80V 1.5A NPN Transistor
Q8_________IRF530 100V 14A N-Channel Hexfet Transistor
Q9_________IRF9530 100V 12A P-Channel Hexfet Transistor

this is the schematic...

[alex mm]

... Hi , are u tried to change output mosfets with lateral mosfets?
regards alex mm

[unclejed613]

.....or maybe you don't have the transistors on a heat sink? or no heat sink grease or other heat conductive substance like sil-pad.

[ivan-UTN-ARG]

i was thinking to replace the IRF530 & IRF9530 for the IRFP240

and IRFP9240 . . . or any other, i don'k know wich one ...

here ( in Santa Fe and Paraná) is very dificult to find good quality components...mosfets are

expensive and dificult to find for example

the IRF530 is about U$s 4
and i burn too many of them...( about 20 )

[Tube_Dude]

Hi

With vertical Mosfets, you need a biasing arrangement with a transistor (Vbe ) or diodes , in direct thermal connection with the heatsink and you must use source resistors.

For your simple biasing circuit ,you must use ,as Alex have already told you, lateral ones.

Only lateral mosfets have negative temperature coefficient at about 100 mA.

[ivan-UTN-ARG]

ok ... thanks .... i'll try and then i'll tell us what happend

[Don S]

Ivan, I agree with tube dude about needing thermal compensation. Even one diode in series with R11 and mounted on the output heatsink might work. You didn't say whether the bias on the outputs ran away or stayed at 100mA. How long does it take at 100mA bias for the outputs to blow? If they blow immediately it might be oscillation. You might need to change the value of C9 to stop it if this is the problem. I had this problem with my amp when I built it. If I increased the bias above a certain level it would oscillate. If I didn't have a light bulb in series with the transformer primary I am pretty sure I would have lost the outputs too.

It will probably help to use source resistors on the outputs for improved bias stability also.

Thanks, Don

[ivan-UTN-ARG]

Hi ...
i set the bias current with the input shorted and everything seems ok, ... the transistors blow with a small input sing ...

with 35ma the amp works very very weel, exept for the distortion at low volume...

[Don S]

Ivan, that sounds like oscillation to me. If it will idle at 100mA's steady for a period of time until you apply a signal your bias is stable. The distortion you hear at lower bias levels is probably caused by oscillation not crossover distortion. Try changing the value of the FB cap.

Thanks, Don

[ivan-UTN-ARG]

Ok i will try ....

thanks Don