The PSU won't cause any interaction what-so-ever. You must have a wiring fault in the original veroboard layout. The plug-board circuit should work... does it not ?
I have not tested the prototype board with signal but, as I said: Still shorting 1/2 and 6 BETWEEN OPAMPS (this time on the prototype board)
Check the photo, you can easily follow the paths. There is no other connection between the devices other than the power supply.
Sorry, I am using the term prototype board for what seems to be properly called a plug-board
Check the photo, you can easily follow the paths. There is no other connection between the devices other than the power supply.
Sorry, I am using the term prototype board for what seems to be properly called a plug-board
Last edited:
The circuit certainly looks OK as far as I can see. If it is, then there is some other explanation.
If the circuit is correct, then the test result is wrong. Either the signals are getting mixed up before the input or after the output.
If the circuit is correct, then the test result is wrong. Either the signals are getting mixed up before the input or after the output.
If the opamp you used is a fast one, then it could be oscillating. Decoupling and grounding may be inadequate.
Thanks for all your suggestions. I really apreciate them. I'm too tired today so I will check again in the coming days.
Last edited:
The tester may beep when two points are at the same potential. It does not necessarily mean that there is a short between them. In this circuit all opamp inputs and outputs will be within a few mV of ground, so a continuity tester might mislead you.
I don't know.
What I do know is that if you build this circuit correctly and test it correctly the channels will not be mixed.
I am not there performing the tests. You say that the signals cross from one channel to the other. I do not know what you have done and what results you got that caused you to come to this conclusion.
Either you have tested it and come to a correct judgement and the circuit does not work, or you have tested it and come to an incorrect judgement and the circuit does work. I can't tell. Either way, however, you still have a problem. All I am saying is that I cannot see a problem in the pictures you posted. That does not mean that the circuit is OK, but it opens up the possibility, which I did not consider when I first replied, that the circuit is actually working correctly, but that for some reason you mistakenly think it is not.
Good luck with finding the problem.
my Fluke 77 emits a tone on the diode function but that doesn't mean zero resistance it does indicate a low resistance or a forward biased diode.
if your getting continuity across the input pins of an op amp with power applied the signal is being shared across the supply line or there's an open or high impedance ground somewhere or bad decoupling caps.
have you tried diode isolating the b+ lines to the two channels? and thinking of that what is your supply can we see/get more on this part of your circuit?
if your getting continuity across the input pins of an op amp with power applied the signal is being shared across the supply line or there's an open or high impedance ground somewhere or bad decoupling caps.
have you tried diode isolating the b+ lines to the two channels? and thinking of that what is your supply can we see/get more on this part of your circuit?
Since the fault is present on 2 different implementations and the circuit is extremely simple anyway the high probability is that there is a miswiring error or an error in the test regime.
To Poldus
after rereading your post #12 not having C1 as in the original circuit could well be your problem.
after rereading your post #12 not having C1 as in the original circuit could well be your problem.
The original problem circuit board looks really untidy in the photo so I understand most answers so far leaning on the side of a wiring error.
Therefore I want to ask future contributors to this thread to please do the following:
1. Consider only the plug-in board photo and its schematic.(post #20). Opamps used here are opa1234. Any replacement opamp with the same pin configuration (post 14) may be used
2. Be aware that the 4 resistor + selector switch array has been replaced by one single resistor (6k.) but that the problem persists regardless of its value and even in the absence of such resistor altogether.
3. Input resistors are also left out for the sake of simplicity. (problem exists either way) and we are not applying an input signal this time.
4. Remember that the problem consists of pins 1/2 and/or 7 showing continuity between devices (one opamp to the other)
5. Post if you can explain how this is happening, or have put together the same circuit on your plug-in board and want to share wether the same condition exists in yours.
Note that this is a genuine concern and I don't want to waste anyone's or my time. Be sure that I appreciate all your responses so far.
Thanks
Therefore I want to ask future contributors to this thread to please do the following:
1. Consider only the plug-in board photo and its schematic.(post #20). Opamps used here are opa1234. Any replacement opamp with the same pin configuration (post 14) may be used
2. Be aware that the 4 resistor + selector switch array has been replaced by one single resistor (6k.) but that the problem persists regardless of its value and even in the absence of such resistor altogether.
3. Input resistors are also left out for the sake of simplicity. (problem exists either way) and we are not applying an input signal this time.
4. Remember that the problem consists of pins 1/2 and/or 7 showing continuity between devices (one opamp to the other)
5. Post if you can explain how this is happening, or have put together the same circuit on your plug-in board and want to share wether the same condition exists in yours.
Note that this is a genuine concern and I don't want to waste anyone's or my time. Be sure that I appreciate all your responses so far.
Thanks
Last edited:
As I have said, 'continuity' might not mean 'ohmic connection' but merely 'maintained at about the same potential'. Have you eliminated this possibility? Does 'continuity' vanish when the power supply is switched off?
That looks fine but it doesn't show the input and output connections or the power supply.
Are the input grounds, output grounds and power supply ground all connected together? If not, that could be the problem.
It is real continuity because when a signal is added at the input of either channel it is amplified and output by both devices. That is how I first detected the problem.
Yes, it is no longer there with power off. But the multimeter does show a reading of around 1400 on the screen across both devices. I have no idea what that means. Power on and, there you go: continuity.
Yes, they are. Could you elaborate on how that could be mixing the channels? Bear in mind that there is no continuity at the input pin 3, before or after the input resistor.
Last edited:
OK, that confirms that it is not continuity; just as I suspected. The claim of 'continuity' is just a red herring. We can probably ignore it, as all you have shown is that two different points in the same circuit which are supposed to be at the same quiescent DC potential (around zero) are in fact at the same quiescent DC potential. It has taken us a long time to get to this point.poldus said:
We are left with the original problem of complete crosstalk between channels. This requires something in common between the channels or coupling between them. Assuming you haven't bought a faulty batch of chips.
On 3rd March you said
Is this still the case i.e. no test with actual signal? It would be a bit silly if the rebuilt circuit works fine but you falsely think there is a fault because of the misleading 'continuity' test. Please forget about 'continuity'. Just put in a signal on one channel and see if it comes out on the other.
That's true. I have not tested the plug-in board with an audio signal. I will do it this weekend and post.
DF96: Well, you were right! Tried with a signal and there is no mixing of channels with the plug-in board.
On the plug-in board, however the tester beeps only across pins 1/2 of both devices, or pin 8. It will not beep between, say 1/2 of one device and 8 on the other as it does in the other board.
I am still clueless as to what to do with the original faulty board since I have checked so many times that the wiring is correct and nothing is shorting.
And I sure appreciate your time spent on this.
On the plug-in board, however the tester beeps only across pins 1/2 of both devices, or pin 8. It will not beep between, say 1/2 of one device and 8 on the other as it does in the other board.
I am still clueless as to what to do with the original faulty board since I have checked so many times that the wiring is correct and nothing is shorting.
Problem solved!
The fault lies in the line level filter I added after the opamps. Don't know what is wrong with it, but it should be an easy fix.
The tester's beep was indeed a powerful red herring. DF96: Had you not realized this I would have never suspected the problem was after the circuit.
Thanks everyone for your contributions. Quantum mystery solved!
;-D
The fault lies in the line level filter I added after the opamps. Don't know what is wrong with it, but it should be an easy fix.
The tester's beep was indeed a powerful red herring. DF96: Had you not realized this I would have never suspected the problem was after the circuit.
Thanks everyone for your contributions. Quantum mystery solved!
;-D
- Status
- Not open for further replies.