F5 power amplifier

[ichiban]

Originally Posted by jacco vermeulen

Even the 20W Class A Lateral Mosfet amp by K. Lang did not have that feature.
That thingus was biased to 2A btw, aka 32W rms Full Class A in 4 Ohm.
AudioXpress should still sell boards for the amp, the design appeared in audio/radio mags all across the globe mid 1980 ....................
Wasn't it Borbely who put 300pF caps across each gate of the N-channels to level out the input capacitance on both sides ?

I have that issue of Audio Amateur #2 1986 page 7, 'The Lang 20W Class-A Mosfet Amplifier'. Pulled it out last Friday to see how high he biased it. He has two pair per channel at a quiescent current of 2 amps with a supply voltage of +-20volts.

I've got 330pFs on each 2sk176 (in the Borbely DC100). I still had oscillation, it drove me nuts, until while poking around with a grounded wire, I finally discovered that grounding the heatsink the driver mosfets on the PC board were mounted on killed the oscillations.

[ichiban]

Originally Posted by alazira

You aren't kidding, ampslab has the 2sj56/2sk176 for $60/pair!

So, you'll have a gate resistor on each mosfet plus one on the board being shared? Please post a picture when it's done.

Bought matched sets from Bear Labs N.Y. late 1980's. Don't recall what I paid.
Yes, the onboard resistor and one at the gate of each mosfet. I don't recall where I read it, someone who works with high frequency circuits wrote that it was used to prevent osc.

Will take some pics of the F5/DC100 cobble, when done.

Did more listening this evening and I'm finding very nice. I have it scabbed onto one of the F5s(dual mono) power supplies. I should rewire the F5 into a single ps then I could switch amps easily to compare. Audio memory being as they say not so reliable. But, I'd bet on the Hitachi output winning this one.

[bluewater]

Just in update (if anyone is curious) on my problem. It was most likely my output mosfets. I changed out everything that I thought could be the cause, with out any change. I'm guessing that it my heatsinks that I fabricated together. I mounted three
heatsinks (each 4X6 with 2inch fins) on a 1/2inch thick aluminium bar. I don't think they were completely flat causing pockets. So hopefully that was it. I also added some small heatsinks directly on top of the mosfets when mounting them. The little guys get hot.

[Nelson Pass]

Originally Posted by ichiban

I still had oscillation, it drove me nuts, until while poking around with a grounded wire, I finally discovered that grounding the heatsink the driver mosfets on the PC board were mounted on killed the oscillations.

A fine example of the value of poking around. When you
get oscillation, you arm yourself with some clip leads,
capacitors, resistors, and a couple hours.

[jleaman]

no one has any comments on these boards being fake or not ?

[Nelson Pass]

Do you mean like rip-offs of copies of clones?

[EUVL]

Uwe,

I would agree with your impression that there is a difference between floating X and grounded, but in my case the difference is small. It may have to do with our circuit differences. The drift figures are similar to mine.

I have already explained in detail before why I chose the Toshiba MOSFETs (with measurements to support). I am not going to repeat that again.




Patrick

[jleaman]

Originally Posted by Nelson Pass

Do you mean like rip-offs of copies of clones?

Looks like some clone!

[osscar]

hello for all,
I started to worry (little) about my heatsink size for F5 when looking at these large on pictures above . Your opinion on my radiators- ? The size is 12x5x1 inches or 29x12x2,5 in cm. will be fine ? enclosure is almost finished and PCB also etched and soldered. My enclosure in attachment.

Thanks and regards.

[Zen Mod]

Originally Posted by osscar

hello for all,
I started to worry (little) about my heatsink size for F5 when looking at these large on pictures above . Your opinion on my radiators- ? The size is 12x5x1 inches or 29x12x2,5 in cm. will be fine ? enclosure is almost finished and PCB also etched and soldered. My enclosure in attachment.

Thanks and regards.

it will be OK ;

one mistake - you must ventilate that box ;

you need some holes on bottom and top plate , or you'll cook all caps and blablah .

doesn't matter what size heatsinks are , air is a must for electronic in this case