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Phono Pre-amp fault
Phono Pre-amp fault
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Old 9th February 2018, 05:40 PM   #81
Mooly is offline Mooly  United Kingdom
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Phono Pre-amp fault
The golden rule for an opamp used in audio (which means it will have a normal feedback network wrapped around it) is the output will go to whatever voltage is needed to maintain the difference between the two inputs to zero (zero for practical purposes here).

See if this makes sense.

The opamp has 4 volts applied to the + input. In order to see if the rule works we need to get 4 volts on the - input and the opamp output will automatically do that for us via the feedback network...

Here's how:

4 volts across 8k means a current of 4/8000 has to flow. Just ohms law. We also know the opamp inputs draw no real current to affect things. So from that we can say that this same current must also be flowing in the 10k resistor.

If we do the maths we get 0.0005A in the 8k. Multiply that current by 8000 and we get 4 volts. Now multiply the same current by 10,000 and we get 5 volts. So the total voltage across the two resistors is 9 volts.

The working simulation bears that out.

We can then go on and say that the gain is actually 1+(R2/R1). Does that work ?

(10000/8000) + 1 equals 2.25

Multiply our input voltage of 4 volts by 2.25 and we get 9 volts.

So relating that to your fault, if pin 2 (the - input) is lower that pin 3 (the + input) and the opamp output is high (yours is) then we either have a duff opamp (and tbh that would have been my less likely option... but it happens ), or something is pulling pin 2 down.
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Old 9th February 2018, 05:56 PM   #82
Mooly is offline Mooly  United Kingdom
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Phono Pre-amp fault
This shows the same basic circuit but now configured for a high AC gain. The capacitor blocks DC current.

Can you see how the same rule applies to the DC conditions ?

For the difference in voltage between the two inputs to be zero, we need 4 volts on the output.

In the second picture I have made the cap leaky (4k resistance leaky) so that it draws current. The opamp wants to try and bring the voltage on pin 2 up to 4 volts and so maintain the 'zero volt differential' between the two inputs but it can not because of the limited supply voltage and the high value of feedback resistor.

This is similar to what you are seeing. The big question is whether the opamp is duff, or..........
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File Type: jpg Opamp2.JPG (144.7 KB, 95 views)
File Type: jpg Opamp3.JPG (147.6 KB, 91 views)
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Old 9th February 2018, 06:56 PM   #83
Dunk02 is offline Dunk02  Australia
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Many thanks for taking the time to explain things. I will try to fully understand this over the next few days while waiting for the parts to arrive.

Just got the parts for the Helium micro monitor build, so might start planning that today and take off the jewellers loupe for a bit (that was giving me a headache).

Cheers
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Old 9th February 2018, 07:04 PM   #84
Mooly is offline Mooly  United Kingdom
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Phono Pre-amp fault
You're welcome I really hope the opamp fixes this one for you.
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Old 16th February 2018, 03:23 AM   #85
Dunk02 is offline Dunk02  Australia
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OK, unfortunately replacing U2 and U3 did not improve things. Got to go back over the info a
Here and start again. I suppose the next likely suspects are the resistors and caps supporting the right channel opamp?
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Old 16th February 2018, 07:39 AM   #86
Mooly is offline Mooly  United Kingdom
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Phono Pre-amp fault
Hmmm

Quote:
Originally Posted by Mooly View Post
Usually the opamp would be the last suspect and its far more likely to be a component fault around it.
At least you have eliminated it as a possibility.

Yes, you need to look very carefully at what is connected to that input of the opamp and see if there is a problem. It sounds for all the world as if there is a lower than intended resistive path to ground on that input that is pulling that pin down.
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Old 16th February 2018, 07:56 AM   #87
Dunk02 is offline Dunk02  Australia
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I shall follow that lead - cheers!
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Old 18th February 2018, 09:44 PM   #88
Hans Polak is offline Hans Polak  Netherlands
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Hi Dunk,

Sorry to hear that replacing U3 did not bring you any further, and respect for the fact that you managed to replace two SOIC's.
However, going back to your previous #42 and #44 it is obvious that the problem seems to be around U3.
DC voltage on pin2 is completely out of range, and there is also no AC voltage entering this pin.
For this reason Pin1 produces no sensible output and therefore the other half of the Op Amp, receives no input signal.
Looking at the magnitude of the signals, 30 mV on pin 2 should result in 200mV on pin 1, a gain of 7x as can be seen on U2.

In #77 I have tried to find out what components are connected to pin 2 of U3.
So try to find out with a multimeter what the values are for R36, R27 and R29 with no power supply connected and also check the resistance from these points to GND.
Then connect the power supply and measure the DC voltage on both sides of these 3 resistors.
And this time use the correct GND connection as of Pin 4, which should be the same as the outer side of the 4 Cinch connectors.
Succes,


Hans
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Old 19th February 2018, 07:05 AM   #89
Dunk02 is offline Dunk02  Australia
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Thanks Hans. I will report back with the data but work is a bit heavy this week. Keep an eye out on the weekend when I should have had some time to do what you and Molly are suggesting.
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Old 24th February 2018, 04:38 AM   #90
Dunk02 is offline Dunk02  Australia
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Molly, I think I grasp most of the info you kindly provided me. Printed out the circuits and followed along with your description - thanks again. I had to do some reading on RC networks while I was travelling for work, that was useful.

Hans, I have followed your instructions, and using U3 pin 4 as the ground I measured the following resistances and voltages. The results do not look the same as I previously posted. Also got myself some fine pointed leads for the DMM - that is making things easier,

Resis;Marked;Measured;Resistance to GND from each side;Vdc(r-gnd)
R27; 332; 3.3K; 960, 4.2k; 0.56, 0.14;
R29; 333; 33.2k; 37k, 4.2k; 4.8, 0.56;
R36; 331; 326k; 640,960k; 0.15, 0.01;

Redid the voltages referenced to pin 4 (grounded to to input) Vsupply= 9vdc
U3
1 4.8
2 0.15
3 1.6
4 0.0
5 4.8
6 2.4
7 2.4
8 2.3

U2 and U1
1 2.4
2 2.4
3 2.4
4 0.0
5 4.8
6 2.4
7 2.4
8 2.4

I also did some continuity checks and perhaps C24 and R29 are not in parallel in the feedback circuit, but in series, as determined with continuity checks either side of the capacitor.

The resistors arenít be shorted.
C23 has continuity to ground on one side only when tested with the DMM so it has not shorted.
R15 is intact 47k
R25 is marked as 10k but measures 5k only in circuit.

Does this get us any further?

Cheers
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