The Polarity across the IV resistor does not change for AOR. The pot seems to have linear resistance from 0 to 1K on my meter.
Thanks,
Mark K.
Thanks,
Mark K.
Hi Mark,
When you have time again, here is another test:
Remove the TDA chip and power up and measure the DC voltage across the IV resistor, positive lead to signal and negative lead to ground. Adjust the pot all the way from maximum to minimum, or vice versa. The current through the 150R IV resistor will vary. At some point current (I) should be 2mA, which is what you need. So when V=.002A x 150R = 0.30V, current is 2mA. (Ohm's Law V=I x R).
If you can get 0.30V across the 150R iV resistor, the bias circuit on the board is working properly. If not, then the 5V supply or bias circuit is probably not working properly.
Another check - power up the board with the TDA chip in place and then check the supply voltages under load. Were your previous supply voltages measured with or without chip in place?
Ben
When you have time again, here is another test:
Remove the TDA chip and power up and measure the DC voltage across the IV resistor, positive lead to signal and negative lead to ground. Adjust the pot all the way from maximum to minimum, or vice versa. The current through the 150R IV resistor will vary. At some point current (I) should be 2mA, which is what you need. So when V=.002A x 150R = 0.30V, current is 2mA. (Ohm's Law V=I x R).
If you can get 0.30V across the 150R iV resistor, the bias circuit on the board is working properly. If not, then the 5V supply or bias circuit is probably not working properly.
Another check - power up the board with the TDA chip in place and then check the supply voltages under load. Were your previous supply voltages measured with or without chip in place?
Ben
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Ben / Ryan,
Success, good voltage: -5, +5 and -15. This was after replacing: Q1, X1, X3.
Now I'm having the same problem Mark is having. With 100R across the output, I cannot zero the offset (~160mV) and I can achieve ~.23v (.002x100r) so, its close.
But, when I remove the 100R on the output, I'm at 5v output!!! The trimmer does not change this!!
What do you guys think?
Cheers,
Greg
Success, good voltage: -5, +5 and -15. This was after replacing: Q1, X1, X3.
Now I'm having the same problem Mark is having. With 100R across the output, I cannot zero the offset (~160mV) and I can achieve ~.23v (.002x100r) so, its close.
But, when I remove the 100R on the output, I'm at 5v output!!! The trimmer does not change this!!
What do you guys think?
Cheers,
Greg
Greg,
Are both channels behaving the same?
Are the +5, -5, -15 voltages with TDA chip in place? If without TDA chip, measure with chip in place.
Have you checked your soldering? Reflow the joints for the output bias components?
Ben
Are both channels behaving the same?
Are the +5, -5, -15 voltages with TDA chip in place? If without TDA chip, measure with chip in place.
Have you checked your soldering? Reflow the joints for the output bias components?
Ben
Ben,
Yes, both channels the same and yes, voltages with chip in place.
I just wonder if I should replace all the bias components.
Cheers,
Greg
Greg,
First remove the TDA chip and measure the voltage across your 100R IV resistor. Adjust the pot. The voltage across the IV resistor should vary as you vary the resistance. At some point you should get 0.2V, which would indicate 2mA flowing through the resistor. This is the current you are trying to achieve to counteract the TDA's output current with no signal.
Ben
First remove the TDA chip and measure the voltage across your 100R IV resistor. Adjust the pot. The voltage across the IV resistor should vary as you vary the resistance. At some point you should get 0.2V, which would indicate 2mA flowing through the resistor. This is the current you are trying to achieve to counteract the TDA's output current with no signal.
Ben
Ben,
I can adjust it to .23vdc without the chip installed.
It’s close....what do you think? I cannot, zero out the offset, though.
I can adjust it to .23vdc without the chip installed.
It’s close....what do you think? I cannot, zero out the offset, though.
When you adjust the pot, what are the maximum and minimum voltages at the pot extreme positions? Same for both channels?
Without the chip you should not get zero volts.
Without the chip you should not get zero volts.
That is good. Your output bias circuits are working on the board for both channels.
The next step is to check that your TDA chip is outputting -2mA when there is no signal. Remove R32 and R33, put the TDA back in, power up and measure the voltage across the 100R IV resistor. You should measure 100R x -.002A = -.20V at each IV resistor.
The next step is to check that your TDA chip is outputting -2mA when there is no signal. Remove R32 and R33, put the TDA back in, power up and measure the voltage across the 100R IV resistor. You should measure 100R x -.002A = -.20V at each IV resistor.
Will do in the morning and report back. What’s next after that?
Really wanted to go to BA, just too much going on. How was it, spoke to 6L6, he said it was awesome.
Greg,
If you get the 0.2V at the IV resistors, then the TDA chip is outputting 2mA as it should, and everything should work when you put everything back together, and you should be able to zero the voltage across the IV resistors.
If you do not get 0.2V across the IV resistor when R32 and R33 are removed, then there is an issue where the TDA chip is not outputting the 2mA.
The capacitors are there for the bias supply and for the on-board IV resistor. If you are not using an on-board IV resistor and you are not using the circuit to null the output offset, then the capacitors, pots, and resistors are not required.
BA was great, lots of fun, and I met a lot of forum people. I met 6L6 and others for dinner on Friday, did the ACP+ build on Saturday, met Nelson Pass (and many more people) on Sunday. It was definitely awesome.
Ben
If you get the 0.2V at the IV resistors, then the TDA chip is outputting 2mA as it should, and everything should work when you put everything back together, and you should be able to zero the voltage across the IV resistors.
If you do not get 0.2V across the IV resistor when R32 and R33 are removed, then there is an issue where the TDA chip is not outputting the 2mA.
The capacitors are there for the bias supply and for the on-board IV resistor. If you are not using an on-board IV resistor and you are not using the circuit to null the output offset, then the capacitors, pots, and resistors are not required.
BA was great, lots of fun, and I met a lot of forum people. I met 6L6 and others for dinner on Friday, did the ACP+ build on Saturday, met Nelson Pass (and many more people) on Sunday. It was definitely awesome.
Ben
Ben,
Not to beat a dead horse, but do we even care that I cannot zero the offset with the boards I/V fully populated and measured at the board output, with the fact that i'm using my own I/V?
The reason I ask is that with the chip installed, and the I/V fully populated, I can only zero out to ~160mV.
Cheers,
Greg
Not to beat a dead horse, but do we even care that I cannot zero the offset with the boards I/V fully populated and measured at the board output, with the fact that i'm using my own I/V?
The reason I ask is that with the chip installed, and the I/V fully populated, I can only zero out to ~160mV.
Cheers,
Greg
Hi Greg,
If the offset nulling circuitry is working correctly and you cannot zero the offset, it is an indication that there is something wrong with the rest of the circuitry. A properly working board and TDA chip should be able to be zeroed.
Did you do the check that I mentioned in post #411?
Ben
If the offset nulling circuitry is working correctly and you cannot zero the offset, it is an indication that there is something wrong with the rest of the circuitry. A properly working board and TDA chip should be able to be zeroed.
Did you do the check that I mentioned in post #411?
Ben
Ben,
Yes, something else is wrong. I'm getting .36v with R32 and R33 removed. So, maybe the chip did get damaged when I applied 20V to it.
Cheers,
Greg
Yes, something else is wrong. I'm getting .36v with R32 and R33 removed. So, maybe the chip did get damaged when I applied 20V to it.
Cheers,
Greg
Hi Greg,
.36V across 100R is high. That is .36V/100R= .0036A. The TDA1541A current offset should be .002A.
I don't know whether the high voltage damaged it. I don't have any experience with that. Did you test the chip in a working dac prior to installing it in the D3 board? Do you have another TDA chip to test the D3?
Years ago when I first started building my own dac, I purchased a couple of cheap TDA1541 (non-A) chips for testing purposes so that I did not have to test using my more expensive chip.
Ben
.36V across 100R is high. That is .36V/100R= .0036A. The TDA1541A current offset should be .002A.
I don't know whether the high voltage damaged it. I don't have any experience with that. Did you test the chip in a working dac prior to installing it in the D3 board? Do you have another TDA chip to test the D3?
Years ago when I first started building my own dac, I purchased a couple of cheap TDA1541 (non-A) chips for testing purposes so that I did not have to test using my more expensive chip.
Ben
Ben,
Okay, I tried my R1 chip and i get exactly .2vdc and easily zeroed out, so, did damage a very expensive chip! However, i'm only getting down to 68mv on the other channel, but I think that could be the resistor being overheated, because I've installed and un-installed twice now.
This is a huge improvement. So, now I need to buy another expensive chip....
Cheers,
Greg
Okay, I tried my R1 chip and i get exactly .2vdc and easily zeroed out, so, did damage a very expensive chip! However, i'm only getting down to 68mv on the other channel, but I think that could be the resistor being overheated, because I've installed and un-installed twice now.
This is a huge improvement. So, now I need to buy another expensive chip....
Cheers,
Greg