Simple MOSFET amp problem

[luixssj3]

No zener goes from emitter drivers to output, after the power resistor...

I know this zener is here for limiting the Vgs in case of short output because a Vgs higher than 20V destroy the glass that isolate the gate from the channel...

 

[luixssj3]

PS I've changed the 10k on the Vbe multiplier to 6.8k, changed the MJE350 to BD139 and mounted it distant from heatsink, now the bias start at 220mA and decrease rapidly to 100mA after one minute or less....

Many thanx to all expecially for the document posted!!!

bye

 

[jackinnj]

Hi Luix,
am I reading your layout correctly, the Zener goes from Driver emitter to Output rail?

Is this the best place for it? A genuine question not a comment.

The zener (in that position) prevent gate-source breakdown.

Here's the Vgs Multiplier which was described in the Thagaard/Pass A75 article -- it's pretty difficult to see the values from that which appeared in the TAA article of 1992:

An externally hosted image should be here but it was not working when we last tested it.

 

[GRollins]

The rule of thumb I always heard was to use the same sort of device for bias that you do for the ouputs. Yes, yes, yes, there are scads of counterexamples, but if you use the same sort of device, they will tend to behave similarly thermally.
I once started to ask Nelson a question about bias circuit impedances, but did not phrase myself clearly and he answered something along these same lines. I never got around to rephrasing the original question and now I've forgotten what it was.
Oh, well.

Grey

 

[luixssj3]

Thanx to all!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Now I've installed two fan, the temperature of the heatsink is very low!!! I can't immagine so low with only two 12V fan!!!

Some photos


http://xoomer.alice.it/luixssj3/index/v1.jpg

http://xoomer.alice.it/luixssj3/index/v2.jpg

http://xoomer.alice.it/luixssj3/index/v3.jpg

http://xoomer.alice.it/luixssj3/index/v4.jpg

 

[luixssj3]

PS copy and paste the link...

 

[Wavebourn]

I have some trouble with it, the Vbe multiplier is oversensitive, if I set the bias (100mA per MOSFET) this slowly decrease as themperature increase and if I rebias at 100mA when the amp restart at cool the bias is over 100mA and I risk some thermal runaway....

Now I set the bias at 70mA ad is OK for the start, no runaway but after the heatsink is hot the bias get 50mA or less...

In your experience is happened thing like this?
Is there any solution?

Sorry for my bad english and thanx in advance...

bye

Are you sure you have to worry?

Are you going to listen to the music, or to measure the bias?
I think, no difference in sound between 10 MA and 100 MA, right?