Problems never end - quietscent current!

[Mlaen]

With this new design of amp I have problems with seting the quitescent current - if I increase her a bit she'll keep increasing and if I decrease her a bit she'll keep on decreasing... what could be the cause of that??? Also when I connect the speaker with no input singal I hear a noise on the output! Could these two things be connected?
NOTE.
with 'first version' of the amp I didn't have these problems but afetr I put 0.33R resistors in series with output mosfets drain I got them!

[edl]

Sorry, but what/which amp do you talking about?!

[Nelson Pass]

I think you want those resistors in series with Mosfet Source.

[Mlaen]

My mistake, crap! I ment resistors on source!
Here's the ola amp: http://www.electronics-lab.com/proje...001/index.html
And 'new' design' (just the output stage since rest is all the same) is in attachment.

[DarkOne]

All you need is a thermal compensation
Replace pot in the circuit with Vgs multiplier and attach that transistor to heatsink, thats all.

Or you can increase source resistors, so difference in Vgs during heat up does not change idle current so much.

[Mlaen]

Ok, thanx!
Which value should I choose for those resistors so that it doesn't effect the output signal?

[AKSA]

0R47 is a good value for a mosfet amp.

You won't really need a Vgs multiplier with this source resistor.

Cheers,

Hugh

[Mlaen]

Thanx for reply but it's still the same thing
Any other ideas? Perhaps a zener diode could help?

[Mlaen]

PS
I just short circuted the source resistors and I have the same problem...

[DarkOne]

Try this -
Set the bias to 0A (turn the pot little further bellow position where bias is 0A).
Put some test signal (generator, cd) to input and attach some load on output (some power resistors, or some not expensive loudspeaker driver which can handle enough power).
Wait 5 minutes, then check if bias current is still 0A (maybe up to 100mA or so).
If so, then let the amp heat up with normal load and maybe half power (remember to use some fast fuses, it will save you money if bias will jump higher).
Then set the bias to value you want (100-500mA).
Stop playing signal/music and let the amp to cool down.
Check bias.
If bias is in range 100-500mA you won.
If bias is less than 100mA, you need even more source resistance.
If bias is higher, then stop building that amp and find something better with normal temperature compensation (this amp does not have any).

I don't understand how can anybody post schematic like this, it had to be some teoretical expert, who don't know about thermal effect on output transistors.

BTW: What's the range of idle current? (cold/warm)