It never seems to end for me.
I've got biasing down to 0.35V across the source resistors with 2mV offset.
That was last week.
As I'm finishing up the power supply side, and finalizing the slow charge etc, I hook up the amp again and for some reason its biased at 0.5V with a few hundred mV offset when I switch on!
So I turn off the power, and turn down the biasing pots and switch the amp back on again. Its at 0.08V bias with about 150mV offset.
OK, I slowly turn the bias up while lowering the offset. I get to about 0.25mV and probably 15mV offset and all of a sudden while I adjust the pots my DMM beeps and shows 0.5V offset and back up to 250mV offset!!!
I shut it down.
lower the bias pot again and after a few minutes turn on the amp and try again.
This same procedure happens again and sometimes I get up to about 0.15V bias before all of a sudden something goes haywire and it shows a bias of 0.5 - 0.6V!
Now I'm thinking there is something wrong with the bias pots. Does anyone have suggestions what else it could be?
I've got biasing down to 0.35V across the source resistors with 2mV offset.
That was last week.
As I'm finishing up the power supply side, and finalizing the slow charge etc, I hook up the amp again and for some reason its biased at 0.5V with a few hundred mV offset when I switch on!
So I turn off the power, and turn down the biasing pots and switch the amp back on again. Its at 0.08V bias with about 150mV offset.
OK, I slowly turn the bias up while lowering the offset. I get to about 0.25mV and probably 15mV offset and all of a sudden while I adjust the pots my DMM beeps and shows 0.5V offset and back up to 250mV offset!!!
I shut it down.
lower the bias pot again and after a few minutes turn on the amp and try again.
This same procedure happens again and sometimes I get up to about 0.15V bias before all of a sudden something goes haywire and it shows a bias of 0.5 - 0.6V!
Now I'm thinking there is something wrong with the bias pots. Does anyone have suggestions what else it could be?
No, it could be anywhere. My Aleph J has a similar symptom, and it wound up being a cold joint on a source resistor in the CCS. These circuits are so frugal with parts that everything interacts with everything.
Do you think the bias voltage is to the point of turning on the diodes in parallel with the source resistors. You could try biasing without the diodes in the circuit. just a thought.
John
John
I have been working for a time with F5 turbo V3 (Jims Audio PCB). In order to get more equal bias over all 8 FETs (IRPF240/9240) I have skipped them out and put in matched FETs. They are all matched with a measured Idss in the range 4.2-4.3. So far there have not been any problems to power up. Offset have been 1-2 mV, also with the unmatched FETs. I use a vario transformer and always take it step by step increasing the voltage. So also to day. I changed to matched FETs one by one and did measurement every time a new matched FET was placed on the heatsink (temp. was 43 C). So, after I have mounted the last new tested FET and with a power voltage of +/-42.5V DC, R3 and R4 started to smoke. Every FETs have been tested from shorts on the heatsinks. I have not had time to check Jfets and 2SC4793/2SA1837, but I am afraid that they also have burned.
I do not have any good theory about what have happened so suddenly. The 10 ohms resistors are ordinary 0.25 W film resistors.
Any good idea what have happened?
Eivind Stillingen
I do not have any good theory about what have happened so suddenly. The 10 ohms resistors are ordinary 0.25 W film resistors.
Any good idea what have happened?
Eivind Stillingen
Stupid me! After a short and not so good nights sleep, thinking on what went wrong with my F5 Turbo V3, I went to the BOM-list here on diyAudio.The burned 10 ohm resistors are noticed as 3W. I used only 0.25 W. Have I hit the right reason?? I have not checked Jfets and cacode transistors, and I fair they are gone as well. What about other componets? It is a hell of a job to test all power FETs and diodes. The value of R3/4, must it be 10 ohm?? I have some 50 ohm Dale (but also some "ordinary" 12 and 10 ohm wirewound)
Eivind Stillingen
Eivind Stillingen
Do you have the jfets cascoded or are you running them at full rail voltage?
I only have 0.75W caddocks at R3 and R4, it doesn't even get warm. But the jfets are cascoded, and I'm running 53VDC rails....biased at 3A. I'm not sure how you can burn R3 and R4.
I think like me, you should run just the front end, without the power MOSFETS and measure your voltages...
Haven't got there yet.... still building up the other monoblock before putting it on the speakers.
Also have buzzing issues with the transformer and working with the transformer maker to resolve the issue..... 🙁.
Thus far, Richard has been completely cooperative in this regard.
Hi Eivind,
You should have about 10 ma passing through R3 and R4 for .1 W. With 42.5v rails what are your values for R5 and R6, also R25 and R26.
Thanks,
John
You should have about 10 ma passing through R3 and R4 for .1 W. With 42.5v rails what are your values for R5 and R6, also R25 and R26.
Thanks,
John
^^The power dissipation of those resistors are determined by the supply voltage and the ratio of the feedback resistors. The output swing appears across the combination.
If only 10 mA are passing through R3/4 it is diffcult to see that this is the reason that they burned(why does the BOM-list operate with 3w). The mysterious thing is why this happen suddenly after the last matched IRPF9240 were mounted. I have done this startup prosess serveral times without any problems(se post 3270). I can of course change this last mounted IRPF9240 with another one since the problems started when this was mounted. My F5 Turbo is of course a cascode vertion.
Eivind Stillingen
Eivind Stillingen