MOSFET Amplifier IRFP240/IRFP9240

[clocktower]

very good looking amp, sir where i can find PCB layout for this?

 

[clocktower]

hiddien post?

where is my post?

 

[tinitus]

man, kids have no patience

 

[clocktower]

man, kids have no patience

Oh, sorry


Just recognized what going on!

Thanks for help.

 

[apexaudio]

I understand that it is not normal to heat up as fast as for me.
Tomorrow first task will be to change thermal isolation.
Apex do you have any idea what is wrong with my amp? Could you advise me anything?

Thermal isolation, 200mA bias can't heat up that fast your heatsink.

 

[harizhent]

Thermal isolation, 200mA bias can't heat up that fast your heatsink.

heat sink wasn't hot, but mosfets were very hot, i think thermal isolation was not able to transfer that amount of heat this fast. but as i understand mosfets shouldn't heat up as this without load and thermal isolation is not the main problem in my amp.

 

[Tekko]

Like i said above, those thermal pads you used are useless!! They are made for computer harddrive coolers.

 

[AndrewT]

I see a resistor bias for the output stage and I see Vertical mosFETs in the output stage.

This is class AB amplifier, bias is overcompensated and always drop when amp get warm. ............

Apex,
please describe the operation of the overcompensated Bias Tempco and where to find that bias voltage circuit in the schematic.

 

[apexaudio]

heat sink wasn't hot, but mosfets were very hot, i think thermal isolation was not able to transfer that amount of heat this fast. but as i understand mosfets shouldn't heat up as this without load and thermal isolation is not the main problem in my amp.

30mV on 0,47R is 5W per mosfet and without heatsink it's shoud be hot

 

[apexaudio]

Apex,
please describe the operation of the overcompensated Bias Tempco and where to find that bias voltage circuit in the schematic.

Bias resistor is about 230ohm (P1+R20 in parallel with R19) X 30mA (VAS current) = 7V (bias voltage). VAS current drop with heat (Q9 and Q10 are on main heatsink) and compensated bias (bias voltage drop).
Bias 200mA per mosfet on start (cold amp heatsink) drop to 120mA with hot heatsink and it's overcompensated but very safe for PA use.

 

[AndrewT]

Which of Q7, 8, 9 or 10 are located on the main heatsink?

 

[Tekko]

Apex states clearly (Q9 and Q10 are on the main heatsink)

 

[AndrewT]

Thanks, I have found it in post3.

But, it is not on the schematic nor is it in post1.

 

[apexaudio]

Q9 and Q10 placed on pcb for main heatsink montage with outputs.

 

[harizhent]

I have changed thermal insulator to mica and again irfps get hot pretty quick (few seconds) and i cannot adjust bias in that time. And again... one pair down one pair left
With one pair and with no load i have adjusted bias to 20mV but when i connect load protection lamp lights up... and from +75/0/+75v left +13/0/-13v.

 

[apexaudio]

I have changed thermal insulator to mica and again irfps get hot pretty quick (few seconds) and i cannot adjust bias in that time. And again... one pair down one pair left
With one pair and with no load i have adjusted bias to 20mV but when i connect load protection lamp lights up... and from +75/0/+75v left +13/0/-13v.

What is DC offset on amp out without load?

 

[harizhent]

What is DC offset on amp out without load?

From +75/0/-75 to +61/0/-61.

 

[apexaudio]

From +75/0/-75 to +61/0/-61.

This is rail voltage, what is DC voltage on amp out to gnd?

 

[harizhent]

This is rail voltage, what is DC voltage on amp out to gnd?

~140v

 

[Deleted member 148505]

Simple Soft-start, DC, Thermal and Overload Protect with NE555. Use single 12vac for suply.

Hi,

Is this circuit fast enough to protect the output transistors in case of short circuit in speaker?

Thanks