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
while doing that - check again rail-out , to see have you some short on pcb
rust never sleeps
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...
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...
Mine measured across the output terminals when off is roughly 30 ohms. That's with output Fets installed of course.
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 🙁
OK, wow. This is one clever amp.... Biasing it is a process of discovery and amazement.
Apparently, those were normal readings :whew:
Apparently, those were normal readings :whew:
😀
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...
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...
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) )
I've always felt that if you add outputs some of the midrange magic may be lost since it's virtually impossible to have perfectly matched parts.
I don't have any evidence that midrange magic is a
function of matching. Perfect matching reduces 2nd
harmonic. Those SE tube designs with the coveted
midrange are supposed to have more than their share of
2nd harmonic.
😎
function of matching. Perfect matching reduces 2nd
harmonic. Those SE tube designs with the coveted
midrange are supposed to have more than their share of
2nd harmonic.
😎
I'll look at doing something similar, I have some thermal tape but the thermistors have no flat surfaces for adhesion, so maybe Arctic Alumina will have to be sourced.
I have three follow-up questions 🙂
1. We have a huge variation in weather conditions where I live. Will I need to readjust offset in summer or will the amp simply run a little (lot!) hotter and still have the offset low enough? We typically see 15 degrees all around between the hottest summers and coldest winters, and between day and night during most of the seasons - right now we're at 23/11 degrees, a typical summer day will be 38/24 (all Celsius).
2. I seem to have a pretty good thermal headroom thanks to the large sinks, can I use one heatsink for two channels/balanced F5/higher bias for a little more Class A power?
3. And the imminent question - what is the maximum (theoretical) undistorted output voltage from a single-ended F5? I have way to directly measure it as of now. 20V? 30V p-p? 50W/2 ohms (clipping point mentioned in the pdf) is ~10 Volts, so that can't be right as the 8 ohm output is significantly higher voltage...
Thanks 🙂
the whole amplifier will run 15Cdegrees hotter when your day time temperatures are @ 38degC than when it is set up for daytime temperatures of 23degC.
If the Ts=50degC at set up then it will run @ 65degC when Ta=38degC.
From there you will find that Tc and Tjmax will both be 15Cdegrees hotter as well.
The thermistors will trim back the hot running current slightly, that is why they are there, but they cannot temperature compensate for big changes in Ta.
If the Ts=50degC at set up then it will run @ 65degC when Ta=38degC.
From there you will find that Tc and Tjmax will both be 15Cdegrees hotter as well.
The thermistors will trim back the hot running current slightly, that is why they are there, but they cannot temperature compensate for big changes in Ta.
Well, I'm expecting it to run ~20-22 degrees hotter, given that heatsink efficiency would drop off with increase in ambient, and not linear either.
I wanted to know if the offset could expect to undergo some change as well, and whether it was enough to worry about.
In summer I might even be using the F5, come to think of it. My room is a furnace then.
So, Q2/3 were keeping in mind current running conditions - though I could fire it up and see how it goes. I just seem to have large enough to take a wee bit more power, I guess.
I wanted to know if the offset could expect to undergo some change as well, and whether it was enough to worry about.
In summer I might even be using the F5, come to think of it. My room is a furnace then.
So, Q2/3 were keeping in mind current running conditions - though I could fire it up and see how it goes. I just seem to have large enough to take a wee bit more power, I guess.
I want to ask an old queation before the end of the year....
Has anyone actually have a Spice Model for the Fairchild devices (FQA19N20C, FQA12P20)?
If not, what are you actually using for all these sims (IRFPs?) ?
Thx,
Patrick
Has anyone actually have a Spice Model for the Fairchild devices (FQA19N20C, FQA12P20)?
If not, what are you actually using for all these sims (IRFPs?) ?
Thx,
Patrick