Simple Mosfet SE amp ?

For the three and more legged things
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pre65
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#1 Simple Mosfet SE amp ?

Post by pre65 »

I have long thought about playing with mosfets (will i go blind ?) and
found this "simple" circuit on the interweb.

http://diyaudioprojects.com/Solid/ZCA/ZCA.htm

Any comments from those who know ?
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#2

Post by pre65 »

As it is a very long time since I had any dealings with transistors there will be a few simple (I hope) questions.

The 2SK1058 mos-fet is in a TO-3P package. Does that "bolt" directly to the heat sink without an insulating washer ? Is thermal paste necessary ?

http://www.futurlec.com/Transistors/2SK1058.shtml
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#3

Post by Nick »

Needs insulating. There are insulators now that avoid the need for thermal paste.
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#4

Post by Mike H »

Yes suggest you get them grey greaseless insulator type ones. Much more robust than mica and no icky yucky slimey stuff involved.

Saw the sircwuit, looks most elegantly simple, which is usually a good sign. :D

Been absolute stonking ages since I messed with any power mozzys, couldn't get 'em to work without oscillating. However probably needs same sort of precautions you might use with power valves. The usual precautionary measure as I remember is a small capacitor between gate and source, but a 'grid stopper' would probably help as well. I hadn't thought of that at the time
 
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#5

Post by pre65 »

I have been exchanging e-mails with the amps designer (he be Australian,cobber) and he says hundreds have been built to this design all over the world (but not Yorkshire :shock: ).

I did find out that the 15R 40W load resistor is actually 4 X 15R 10W resistors in parallel,a bit misleading on the diagram.

Burns 30W at idle to get 5W out !
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#6

Post by Mike H »

The load resistor could be a 50W or 100W aluminium clad. With its (their?) own heatsink?
 
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#7

Post by Mike H »

We were obviously posting simultaneously! Yes saw about the 4 x resistors and the 30W power drain on his web page.

None yet in Yorkshire eh?
 
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#8

Post by Nick »

The F2 is very simmilar, its just using a active (CCS) load.

http://www.firstwatt.com/downloads/F2-s ... ual-sm.pdf
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#9

Post by pre65 »

Nick wrote:The F2 is very simmilar, its just using a active (CCS) load.

http://www.firstwatt.com/downloads/F2-s ... ual-sm.pdf
So it could be a half decent amp then ?
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#10

Post by ed »

I'm not sure if these weren't the originals that popped up when the F2 was all the rage. At the very least these threads are highly educational if you are interested in how and why.

sewa thread
http://www.diyaudio.com/forums/showthre ... adid=66822


ota thread
http://www.diyaudio.com/forums/showthre ... adid=73598

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#11

Post by pre65 »

Looking through my parts box to see what may be suitable for this amp I have found eight Berco 15R resistors (see photo) they are approx 2"x1"x1/4" with an open centre.

http://i111.photobucket.com/albums/n146 ... sistor.jpg

The design calls for four 15R 10W non-inductive resistors connected in parallel on each channel.

It would be a shame not to use these but I don't know the wattage (but would think must be at least 10W) or their inductive properties.

Anyone any comments ?

What would be the benefit of non-inductive resistors as the mos-fet load ?
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#12

Post by Mike H »

How big are they physically?

I can only imagine that since the resistance is very low, there's a risk of the wire wounds acting like coils so ends up like a RF oscillator (?)

You could contrive it in such a way that you've got pairs in series, but side by side, so the current goes up one side then down the other. If they're inductively coupled that should cancel out.

When you say '4 in parallel' I presume you mean 2 in parallel, then another 2 in parallel, then those 2 pairs in series? So ends up like an 'H'. Cos they're 15R each and you want a final value also of 15R
 
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#13

Post by pre65 »

Hi Mike-lost the glasses again ? :lol:

The size is 2" X 1" X 1/4" !

This business about the 40W 15R resistor has been doing my head in, but now the fog has cleared and I understand (possibly :oops: ).

I've done a little diagram. two 15R in parallel = 7.5R and two 7.5R in series = 15R. Simple innit ! :shock:
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15R 40W003.jpg
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#14

Post by Nick »

Or two 15R in series, 30R, and two 30R in parallel = 15R
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#15

Post by pre65 »

Nick wrote:Or two 15R in series, 30R, and two 30R in parallel = 15R
Yes,either way would do !
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