audio-talk

pre65 · Posted: Wed Feb 25, 2009 8:54 pm Post subject: Simple Mosfet SE amp ?

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 ?

pre65 · Posted: Thu Feb 26, 2009 11:33 am Post subject:

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

Nick · Posted: Thu Feb 26, 2009 11:42 am Post subject:

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

Mike H · Posted: Thu Feb 26, 2009 9:50 pm Post subject:

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. Very Happy

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

pre65 · Posted: Thu Feb 26, 2009 9:56 pm Post subject:

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 Shocked

).

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 !

Mike H · Posted: Thu Feb 26, 2009 9:56 pm Post subject:

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

Mike H · Posted: Fri Feb 27, 2009 1:49 am Post subject:

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?

Nick · Posted: Fri Feb 27, 2009 7:57 am Post subject:

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

http://www.firstwatt.com/downloads/F2-service-manual-sm.pdf

pre65 · Posted: Fri Feb 27, 2009 9:02 am Post subject:

ed · Posted: Fri Feb 27, 2009 9:18 am Post subject:

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/showthread.php?threadid=66822

ota thread
http://www.diyaudio.com/forums/showthread.php?threadid=73598

Ed

pre65 · Posted: Mon Mar 02, 2009 1:17 pm Post subject:

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/pre65/15Rresistor.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 ?

Mike H · Posted: Mon Mar 02, 2009 1:59 pm Post subject:

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

pre65 · Posted: Mon Mar 02, 2009 2:22 pm Post subject:

Hi Mike-lost the glasses again ? Laughing

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 Embarassed

).

I've done a little diagram. two 15R in parallel = 7.5R and two 7.5R in series = 15R. Simple innit ! Shocked

Nick · Posted: Mon Mar 02, 2009 2:27 pm Post subject:

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

pre65 · Posted: Mon Mar 02, 2009 2:34 pm Post subject: