...okay- then it is good practice
R4 should be around 0.65V ( the EB diode in TR1) which gives around 0.8mA
R5 (Input stage) should show something like 3,7V ~ 0.4mA.
R8 and R12 should give you something between 20 and 25mA, and the current needs to be equal to make the smallest amount of DC offset.
R14 and R15 should have 22mV at 100mA idle-current. Adjust R20 until this is true.
Make sure you have fuses or 100ohm/1W resistors in the supply lines in case of the output MOSFETs going shortcircuit. ( I guess you know this, or maybe you allready made other precautions since you allready came this far - but if not then do it, it will save you money and time.)
I haven't done my homework, yet, but I rememeber I through R4 was about 0.8 mA, and that's really low, I think. I through R12 is 24 mA, and V6k8 was about .66mV...
I'm gonna replace Tr6, if it is damaged and I haven't noticed, I'll place the sucker face cartoon as my profile picture!
Well, I've measured something more: I don't have any voltage across .22 ohm resistors, but reached 9mV in N-Channel ones with some signal, nothing happened with P-Channel ones... everything else seems to be ok, replaced Tr6, but I'm gonna replace Tr7 too and maybe P-Channel output mosfets... I'm almost convinced I do need audio mosfets... sure gonna buy them next week.
Good.
You could try to take out the output transistors, connect the feedback to tr6/tr7 and then try to listen if the signal is distrotet here ( my guess is that it is not!)
You can do this by adding a capacitor and a 220 ohm resistor and a headphone.
Or a capacitior, a potmeter and then listen to the output through a line input of another amp.
You could try to take out the output transistors, connect the feedback to tr6/tr7 and then try to listen if the signal is distrotet here ( my guess is that it is not!)
You can do this by adding a capacitor and a 220 ohm resistor and a headphone.
Or a capacitior, a potmeter and then listen to the output through a line input of another amp.
So the IRFP**** amps will work without a Vbe multiplier?
Is there a possibility that they'll blow up from the heat you were talking about?
Thanks
If you run the bias really low (minimal) then you dont need a vbe multiplier just some way of setting the bias voltage to around 6v8.
I run my amps with a Vbe multiplier but its not thermally attached to the heatsink. I get away with about 10mA of bias current and no crossover distortion.
Actually I recently build an amp with IRF530/9530 and it had no temp drift. I ran it for 3 months up til 5-6hours a day. It was without thermal compensation, but then it dismantle it to change it.
The secret is: If the heatsink is large enough, there is a lot of thermal inertia, but of course it is always a risk, and you should always have thermal compensation with verical fets.
The secret is: If the heatsink is large enough, there is a lot of thermal inertia, but of course it is always a risk, and you should always have thermal compensation with verical fets.
I'm doing a project with the IRFP9240/240 and the heatsinks that I'm planning to order are 0,65K/W, will this be enough, I use 1 heatsink for 1 MOSFET. I don't want to Hijack this thread.
>http://www.diyaudio.com/forums/solid-state/169348-mosfet-irfp240-9240-a-3.html
>http://www.diyaudio.com/forums/solid-state/169348-mosfet-irfp240-9240-a-3.html
Just use a big heatsink... and place a 80 x 80 mm fan inside the chassis if you have space... can't fail!
BTW... I'd forgotten to tell everybody, I made it work! I don't know why... I changed all mosfets, could have been a bad device...
Increasing dynamic range in JLH's all-mosfet PA
I had to change differential pair from Bss92 to IRF9530 (but i can't adjust 0V at output) because the preamp I used made the amp break a bit, I'm using it as a bass guitar amplifier. I've already placed a PA gain pot to minimize PA's overall gain, but, can I increase dynamic range of the input stage?
I had to change differential pair from Bss92 to IRF9530 (but i can't adjust 0V at output) because the preamp I used made the amp break a bit, I'm using it as a bass guitar amplifier. I've already placed a PA gain pot to minimize PA's overall gain, but, can I increase dynamic range of the input stage?
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