F5 power amplifier

[Zen Mod]

remove input jfets and check them - with DMM resistance mode , or with classic matching procedure

while doing that - check again rail-out , to see have you some short on pcb

rust never sleeps

[sangram]

Each jfet is roughly 50 ohm drain to source - at least that's what I think I'm seeing...

Peter's boards don't take too kindly to desoldering. I had a really tough time removing some components which I needed to relocate

I was hoping someone could take some measurements with their amps, power off mode, and post what numbers they're seeing...

I see output to ground of 30 ohms (50+10 2xparallel) so this seems OK.

I can't figure out - there is no short as there is a finite resistance, which is basically 50 + 10 ohms, measured in either direction. The input jfets measure 24 ohms drain-to-source with the output JFETs mounted, and 40 ohms without. Is that normal? Seems awfully low to me...

[Tea-Bag]

Originally Posted by sangram

I see output to ground of 30 ohms (50+10 2xparallel) so this seems OK.

Mine measured across the output terminals when off is roughly 30 ohms. That's with output Fets installed of course.

[sangram]

OK, I'm gonna fire her up. If I'm not back to report in the next half hour, hold memorial services on 1st January

[sangram]

OK, wow. This is one clever amp.... Biasing it is a process of discovery and amazement.

Apparently, those were normal readings :whew:

[Tea-Bag]

Originally Posted by sangram

OK, wow. This is one clever amp.... Biasing it is a process of discovery and amazement.

Apparently, those were normal readings :whew:

Cool, you didn't blow up or start a fire. (yet)

[sangram]

I managed to get one channel to full bias with about 2mV of offset, it drifts a bit while the amp operates, but doesn't cross 1-2mV in either direction. The pots are *very* fiddly, even with multiturn. Heatsinks about 41 degrees, MOSFETs about 50 degrees - I need to get some thermal compound on to the other side of the Mica insulator, that should help a degree or two on the MOSFET temps (room temp is ~20 degrees, ambient 12-15 degrees at night).

I'll get the second channel biased and ready tomorrow (hopefully, if it doesn't blow up in my face) and then it's listening time

My only issue is the bias drifting. I have placed the thermistors close to (but not right on top of) the output devices. I assumed a bit of distance would help, would moving them closer help with the drift? I've seen a maximum of 5mV, and I'm trying for less than 2mV if possible. It sometimes creeps up to ward 5 to 6 mV, but then automatically starts course-correcting.

the thermistors are placed almost touching the output resistors though, I thought those would also track with the output...

[Nelson Pass]

I set the thermistors on the outputs with a drop of epoxy.

[a.wayne]

Hello Nelson ,

Can i double the outputs for better low imp operation ? Negatives ?


Regards,

[Zen Mod]

Originally Posted by a.wayne

Hello Nelson ,

Can i double the outputs for better low imp operation ? Negatives ?


Regards,

oh boy .

again ?

thread have just zillion pages , and a least 100 times that Q asked .......

yes - ya can ;

negatives - certainly smaller than any other variable you'll put with other parts in amp (PSU caps, wiring, type of resistors (strictly meaning on inductive vs. noninductive) )