Thermal unbalance of P-channel vs. N-channel
ZenMod, I ended up using a infrared thermometer for measuring body temp. I don't believe it's very accurate for this purpose but it has a range up to 60°C and spec tells, that it is +/-1K which turns out to be just OK. I did several measurements over more than 1hr of warmup.
After 1hr the N-channel sinks heated up to 52°C and the P-channels were 46°C. This is still with a bias of 1.6A/board rather than 2A which is said to be still comfortable.
As I said, the mounting situation differs a bit by the P-channels conducting some heat to the front panel which explains at least part of the Delta Temp.
I feel a bit ashamed, that I have used up your time and experiance with this monster to discuss a situation which at least is not alarming. Your comment about human skin not being an accurate test instrument was more than appropriate!!!
ZenMod, I ended up using a infrared thermometer for measuring body temp. I don't believe it's very accurate for this purpose but it has a range up to 60°C and spec tells, that it is +/-1K which turns out to be just OK. I did several measurements over more than 1hr of warmup.
After 1hr the N-channel sinks heated up to 52°C and the P-channels were 46°C. This is still with a bias of 1.6A/board rather than 2A which is said to be still comfortable.
As I said, the mounting situation differs a bit by the P-channels conducting some heat to the front panel which explains at least part of the Delta Temp.
I feel a bit ashamed, that I have used up your time and experiance with this monster to discuss a situation which at least is not alarming. Your comment about human skin not being an accurate test instrument was more than appropriate!!!
Zatopek - can you measure the tempature of the transistor itself? I am curious to see if the Delta-T is similar to the heat sink readings from one side to the other. I suspect it is.
That said, it appears your amplifier is working well, play some music and enjoy!
That said, it appears your amplifier is working well, play some music and enjoy!
Thermal unbalance of P-channel vs. N-channel
6L6 I am interested myself! As said, the thermometer is probably not really adequate to the purpose and I will buy one myself. Will probably take me a week or so to come back. A hopefully correct temp measurement will decide how much bias I can afford. The 52°C I measured is already close to what I should settle with.
6L6 I am interested myself! As said, the thermometer is probably not really adequate to the purpose and I will buy one myself. Will probably take me a week or so to come back. A hopefully correct temp measurement will decide how much bias I can afford. The 52°C I measured is already close to what I should settle with.
Unbalance of P-channel vs. N-channel
6L6, I have tried measuring temp on the surface of Q3 to Q6. Temp measured after 1hr of warmup is:
Q3/Q4: 51.6 to 52.7°C
Q5/Q6: 55.1 to 57.2°C
It confirms about the previous measurements and shows a temp difference of 4 to 5K between P- and N-channel which mostly is caused by different mounting situation of P and N with P being able to sink considerable heat to the massive front panel (450x180x10mm).
Due to test method and accessability/location of the DUT's the numbers are not very accurate and are just best effort.
Would be interested to hear about results of other DIY'ers on this thread having more bias like 1A/device or so.
6L6, I have tried measuring temp on the surface of Q3 to Q6. Temp measured after 1hr of warmup is:
Q3/Q4: 51.6 to 52.7°C
Q5/Q6: 55.1 to 57.2°C
It confirms about the previous measurements and shows a temp difference of 4 to 5K between P- and N-channel which mostly is caused by different mounting situation of P and N with P being able to sink considerable heat to the massive front panel (450x180x10mm).
Due to test method and accessability/location of the DUT's the numbers are not very accurate and are just best effort.
Would be interested to hear about results of other DIY'ers on this thread having more bias like 1A/device or so.
That's good news, and I suspect that it will not effect the operation of the amp in any way.
Set bias, adjust the DC offset, attach speakers and enjoy!
Set bias, adjust the DC offset, attach speakers and enjoy!
Strange problem with my F5 turbo...I have been trying to sort it out in a different thread but I thought some of you here might have some ideas. Here's the other thread:
F5 dc offset issue
Amplifier is 2 pair of Toshiba sj201/sk1530, 31V rails, No cascode. No diodes. 100R gate stoppers. Sounds great, no signs of trouble.
A) DC offset changes dramatically when input shorted, goes from 0mv to -180mv when input shorted. Different preamps show different DC offset even though preamps are cap coupled. Only one channel exhibits this behavior.
B) Suspected Jfet leakage. Replaced jfets on misbehaving channel with other channels jets. Issue still exists.
C) Tried small cap across feedback resistor. No change.
What should I check next? I am really baffled here. I am a bit afraid to use the amp.
F5 dc offset issue
Amplifier is 2 pair of Toshiba sj201/sk1530, 31V rails, No cascode. No diodes. 100R gate stoppers. Sounds great, no signs of trouble.
A) DC offset changes dramatically when input shorted, goes from 0mv to -180mv when input shorted. Different preamps show different DC offset even though preamps are cap coupled. Only one channel exhibits this behavior.
B) Suspected Jfet leakage. Replaced jfets on misbehaving channel with other channels jets. Issue still exists.
C) Tried small cap across feedback resistor. No change.
What should I check next? I am really baffled here. I am a bit afraid to use the amp.
dissconnect audio gnd from case , both channels
check with megaohmmeter conductivity between mosfet cases to heastsink
you can also check is there same issue with DC offset , without safety gnd ....... which is possible route from mosfets to audio gnd , in case of leakage through thermal transfer arrangement
check with megaohmmeter conductivity between mosfet cases to heastsink
you can also check is there same issue with DC offset , without safety gnd ....... which is possible route from mosfets to audio gnd , in case of leakage through thermal transfer arrangement
180mV is 0.004W into the woofer coil. Not enough to worry about.
The capacitor(s) on the tweeter and mid will block DC on those voicecoils.
What if you connect the LSK pre but not power it up. Do you still see the same 180mV offset on that channel? If so, this will suggest the cause is from this particular channel of the amp. In which case, perhaps it is the layout of the board of that channel, or it is the components (pots??) etc.
If you have a variac, you can also lower the voltage to 24VDC to see the offset stays.
If you have a variac, you can also lower the voltage to 24VDC to see the offset stays.
Will do. Right now there is more than 10R resistance between mosfets and case. But I will confirm all is good by disconnecting PSU ground to case.
Just checked mosfets resistance to chassis with psu disconnected. No continuity. Iam pretty sure the mosfets are isolated well.
Iam stumped. Any other ideas?
which value of caps you tried across feedback resistors ?
of course , you mount two caps ,not just one ?
of course , you mount two caps ,not just one ?
100pf. I only used one. I guess it would be better if both jfets had cap compensation? I will try that next. Thanks Zen Mod.
well, NFB is having two paths
either compensate none or both, to have valid conclusions
besides , 100pF is too low in value
calculate an d use something in area of 100-150KHz for that test
either compensate none or both, to have valid conclusions
besides , 100pF is too low in value
calculate an d use something in area of 100-150KHz for that test
Many to choose options from the BD139 family, do I need the power dissipation to be 1.25W or 12.5W?