preamp, not passing signal.

I'm having some issues with my mic preamp. To start from the beginning :

My xlr input is setup as
Pin 1: Ground
Pin 2: Hot
Pin 3: Cold

My xlr is then connected to a electronically balanced input. I'm using a JR 12904 as the op amp. Pin 1 is the output, my hot feeds pin 3,cold to pin 2, pin 4 is grounded and pin 8 is my 18v+ supply. Pins 5, 6 and 7 are not used. My input looks like (https://www.google.com/search?q=op+amp+balanced+input&prmd=isvn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwickrev5e7PAhVKQyYKHdKABtUQ_AUICCgB&biw=320&bih=440#imgrc=Z5GfC-fJJcc7lM%3A)

The output of the balanced input then passes through a WIMA film cap, and then enters my preamp stage. (http://darrenyates.com.au/electronics/archives/16)

I used 3.3k resistors instead 2.2k, I used 100k instead of 150k. I used 47uf, 63v electro instead of 22uf. I used a 1000uf 25v electro for one 470uf and a 22uf, 50v electro for the other. My power supply is a 18v+ supply.

I then got rid of the 470uf output cap, and fed the base of a npn with the output. Fed my 18v+ supply to it and I used a 47uf, 63v cap feeding a 1k resistor (cap into resistor, resistor to ground) for my output. As Ian had instructed me for an unbalanced out. My 1/4 connector has pin 1 going to ground and pin 3 is hot.

Here is a couple pictures of my breadboard for help.

Input (https://postimg.org/image/3x5mho94t/)

Preamp stage (https://postimg.org/image/5wk7gulwd/)

Output (https://postimg.org/image/4n0y5clqb/)

https://postimg.org/image/bg0nvyrxv/dbd42ba8/

Transistor 1 is BC547, transistor 2 is BC547 and transistor 3 is BC557. The first BC547 is connected to the BC557 and the BC557 is connected to the 2nd BC547 for the output stage.

Can someone help please?

> My input looks like

That's a Google Search which is not sure to resolve to the same image for everybody or at different times.

It is in fact (I think) Fig 9 halfway down Self's page-
http://www.douglas-self.com/ampins/balanced/balanced.htm

> pin 4 is grounded and pin 8 is my 18v+ supply.

Self's very basic drawing shows no supply, assumes bi-polar, and will need added parts to work with single supply.

It would really help YOU (and others) to get a crayon and sketch the *whole* circuit.

It would really help YOU (and others) to get a voltmeter and note key DC voltages.

It would really help YOU (and others) to get a signal tracer (small guitar amp with clip-lead cord) and see where signal gets to.

I would bet your Self Fig 9 stage is standing there with its output pin smacked solidly at Ground (maybe positive rail) and no signal coming out. That makes the rest of your rig moot; get that first stage working first.

Why would it need additional circuitry to work with a single supply? The op amp I used ( a NJR12904) is a single supply dual op amp. Pin 1 is output, pin 2 is negative in, pin 3 is positive in, pin 4 is ground and pin 8 is ve+ in. I am not using pins 5,6 or 7. I don't see how an op amp, that's getting power and is also being properly grounded is not working as it should. The balanced in shows a TL071 style single op amp, which has the +/- signal feeding pins 2-3 respectively (On the replacement op amp I used anyway). Since the schematic calls for a single op amp, I figured the 2nd input/output of the dual I used needs not to be connected to anything? Both the + power supply and ground have been adequately met. My voltmeter shows 18v+ at the power supply input (Pin 8) of the op amp and (ground 0v) at the ground input (pin 4).

Datasheet for the NJM12904 states an absolute maximum supply voltage of 15V.

Not today but at some point I may start a thread with general troubleshooting advice.  These are useful skills that can be used again and again.

JR

JohnRoberts said:

Not today but at some point I may start a thread with general troubleshooting advice.  These are useful skills that can be used again and again.

JR

Click to expand...

Ok, I've started over using a NE5532. I'm starting my input build using a NE5532 in single supply operation. It has a max input voltage of 22v. The NJR does in fact only have a 15v max input, so changing it to the NE5532 should eliminate the issue with too much power being fed to the op amp. Here is my input schematic and also the build on the board.

https://postimg.org/image/gk2qdwc0f/

https://postimg.org/image/605nrih4v/


Does this balanced xlr input seem ok? Does the breadboard look correct as well? I know the two resistors at the input are in fact 12k, but they are within 10% and with my ohm meter they both read below 12k so I am using them in place of the 10k's. The two 10k's are 1%. I figured they shouldn't cause to much of an issue. 

You forgot to say what's wrong with your circuit. No output? Noise? Distortion?

Then, complete schematic is useful. Unused opamps usually are connected as followers following ground. If you have a single supply circuit as it seems you should reference everything called ground at a center point, mostly the reference of the non inverting input of the opamp. Then, to do that you could use a couple of resistors and a cap or add the extra opamp to make it more stable.

JS

joaquins said:

You forgot to say what's wrong with your circuit. No output? Noise? Distortion?

Then, complete schematic is useful. Unused opamps usually are connected as followers following ground. If you have a single supply circuit as it seems you should reference everything called ground at a center point, mostly the reference of the non inverting input of the opamp. Then, to do that you could use a couple of resistors and a cap or add the extra opamp to make it more stable.

JS

Click to expand...

The unused pins of the op amp were basically going no where. The issue I was having was there was an output, but it was only a low buzzing noise.  I have now replaced the op amp with a NE5532 since I was getting to much power at the 12904. 18v is in the perfect range for the NE5532.  Should I ground pins 5. 6 and 7 of the NE5532 or will leaving them unconnected to anything cause problems?

Leek said:

The unused pins of the op amp were basically going no where. The issue I was having was there was an output, but it was only a low buzzing noise.  I have now replaced the op amp with a NE5532 since I was getting to much power at the 12904. 18v is in the perfect range for the NE5532.

Click to expand...

  Leaving unconnected opamps is usually a no no. You don't know what they are doing, they may be sitting there doing nothing or they may be oscillating as crazy possibly causing problems with your used amp, even if you build it and you have no problems nothing prevents one day because is warmer or wetter or whatever it starts to miss behave. It's as simple as connecting the inverting input to the output and the non inverting input to whatever reference is nearby, even the rails would do the job here but the preferred option is somewhere in the middle, as the reference of the other side. Rails may be ok for some amps and cause problems in others.

JS

joaquins said:

  Leaving unconnected opamps is usually a no no. You don't know what they are doing, they may be sitting there doing nothing or they may be oscillating as crazy possibly causing problems with your used amp, even if you build it and you have no problems nothing prevents one day because is warmer or wetter or whatever it starts to miss behave. It's as simple as connecting the inverting input to the output and the non inverting input to whatever reference is nearby, even the rails would do the job here but the preferred option is somewhere in the middle, as the reference of the other side. Rails may be ok for some amps and cause problems in others.

JS

Click to expand...

So, if I read what you said correctly, I would connect pin 1 to 5, pin 6 would then connect to pin 7 (with a  100k resistor in between) I'd use a cap and another resistor to ground pin 6 and I'd ground pin 7 with another resistor. My output is then across resistor from pin 7, correct?

> Why would it need additional circuitry to work with a single supply?

To pass both-ways Audio, the output MUST be biased-up (sitting) about half way between the supply rails. For single +18V supply you should see +9V (+4V, +14V) of DC at the output.

You do this by biasing the Inputs appropriately.

Further, all opamps have an Input Range that the inputs must stay within. For the '5532 this is typically >2V above the - power pin. In single-supply, the inputs are sitting lower than this.

> The op amp I used ( a NJR12904) is a single supply dual op amp.

Then it may have an input range down to the - power pin. In fact it does.

Use op-amp theory. If you aren't putting DC at the mike, these points "float". You have a solid 0V at R4. Pin 3 "+in" is sitting at 0V. Pin 2 "-in" "wants" to be the same voltage, 0V. R1 goes to a float, so the only way pin 2 can "find 0V" is by pin 1 "out" swinging to 0V. It tries. However the output stage of the '12904 appears to only swing to 0.6V above the - power pin for real loads (to 0.05V for super-light loads). The '5532 won't even go that low.

The output is "jammed" against the negative supply. It may "come up" if an input swings more than 0.6V. A Very Loud (kickdrum) mike signal might bop part-waves of output. No normal mike signal will.

There's a paper from one of the major chip hawkers which has cookbook answers for single-supply audio.

There's a billion web pages showing single-supply opamps.

ALL opamps "can" run single supply. The ones touted for the purpose are adapted to swing very close to the negative (ground) rail. But for Audio they still have to be biased-up to halfway between the power rails.

me> one of the major chip hawkers ...has cookbook answers for single-supply...

One of several:

https://courses.cit.cornell.edu/bionb440/datasheets/SingleSupply.pdf

PRR said:

> Why would it need additional circuitry to work with a single supply?

To pass both-ways Audio, the output MUST be biased-up (sitting) about half way between the supply rails. For single +18V supply you should see +9V (+4V, +14V) of DC at the output.

You do this by biasing the Inputs appropriately.

Further, all opamps have an Input Range that the inputs must stay within. For the '5532 this is typically >2V above the - power pin. In single-supply, the inputs are sitting lower than this.

> The op amp I used ( a NJR12904) is a single supply dual op amp.

Then it may have an input range down to the - power pin. In fact it does.

Use op-amp theory. If you aren't putting DC at the mike, these points "float". You have a solid 0V at R4. Pin 3 "+in" is sitting at 0V. Pin 2 "-in" "wants" to be the same voltage, 0V. R1 goes to a float, so the only way pin 2 can "find 0V" is by pin 1 "out" swinging to 0V. It tries. However the output stage of the '12904 appears to only swing to 0.6V above the - power pin for real loads (to 0.05V for super-light loads). The '5532 won't even go that low.

The output is "jammed" against the negative supply. It may "come up" if an input swings more than 0.6V. A Very Loud (kickdrum) mike signal might bop part-waves of output. No normal mike signal will.

There's a paper from one of the major chip hawkers which has cookbook answers for single-supply audio.

There's a billion web pages showing single-supply opamps.

ALL opamps "can" run single supply. The ones touted for the purpose are adapted to swing very close to the negative (ground) rail. But for Audio they still have to be biased-up to halfway between the power rails.

Click to expand...

I'm not quite sure how to go about getting my input circuit to these standards. You're saying I should have 0v at all those locations. Does that mean I need to add a cap/resistor to each section that goes to ground? I have my input setup exactly how I posted the picture. Could I  not connect pin 1 to 5, pin 6 would then connect to pin 7 (with a  100k resistor in between) I'd use a cap and another resistor to ground pin 6 and I'd ground pin 7 with another resistor. My output is then across resistor from pin 7?
If not, how can I go about changing the input to make it acceptable so I can proceed to my preamp stage. A bunch of reading that I can't understand isn't helping. The single supply info that was provided has schematics that look like mine. So, I don't see why I can't make this one work for a dynamic mike with some modification.

PRR said:

me> one of the major chip hawkers ...has cookbook answers for single-supply...

One of several:

https://courses.cit.cornell.edu/bionb440/datasheets/SingleSupply.pdf

Click to expand...

PRR, I'm attempting to use the 2nd in/out of the NE5532 as a follower for the 1st stage. I'm going to attach pin 1 to pin 6 (output to inverting input) and then use a pair of resistors to reference half supply to pin 5, correct? I can use a 0.1uf cap from the output of the follower to ground and another 0.1 cap from ground to the non-inverting input for more stability, correct? It seems a little weird to have audio signal going into pin 2&3 and then have an actual voltage rating from a pair of resistors that split the power from 18 to 9v going into pin 6?

According those notes, I'd send pin 1 output to pin 6, I'd then use a pair of same ohm resistors to create a voltage divider, and use that as a reference for pin 5? This then gives me a half voltage reference at pin 5. Also, since I'd be using a cap at the output stage and another at the non-inverting stage it'd add additional stabilization?

Leek said:

I'm having some issues with my mic preamp. To start from the beginning :

My xlr input is setup as
Pin 1: Ground
.....
loads of confusing stuff
....
Transistor 1 is BC547, transistor 2 is BC547 and transistor 3 is BC557. The first BC547 is connected to the BC557 and the BC557 is connected to the 2nd BC547 for the output stage.

Can someone help please?

Click to expand...

STAGE 1

How about YOU DRAW your EXACT schematic ... will ALL bits connected?  With EXACT part values that YOU used.

Show the XLRs and how they are connected, 'unused' pins in the ICs, power supplies etc.

You need to do this ANYWAY if YOU want to do any work on it ... let alone have anyone help you.  Anyone who hopes to help you has to do all this legwork before they even understand what you've built.

STAGE 2

Next try to figure what the voltages should be at important points in the circuit. eg the output pins of the OPAs.

Put them on your schematic.

If something is wrong, the first sign will be wonky voltages.

You do have a voltmeter? 
_______________________________

So, if I read what you said correctly, I would connect pin 1 to 5, pin 6 would then connect to pin 7 (with a  100k resistor in between) I'd use a cap and another resistor to ground pin 6 and I'd ground pin 7 with another resistor. My output is then across resistor from pin 7, correct?

Click to expand...

How about you draw this and also show the OPAs internal to the IC?  You can get this from the datasheet.

YOU need to get used to looking at datasheets and showing connections on a schematic rather than telling people .

Otherwise YOU don't understand what you are doing .. so don't expect other people to understand either.

.. and if you post a pic, please clean it up so it is easy to read ... and make it smaller than 1024 x 768 pixels so it doesn't appear so large that we have to scroll to see anything useful ..

PRR has shown you how to do all the above.


I'll repeat my suggestion that you buy & start reading The Art of Electronics.

A bunch of reading that I can't understand isn't helping.

Click to expand...

... so its about time you made a start 

ricardo said:

STAGE 1

How about YOU DRAW your EXACT schematic ... will ALL bits connected?  With EXACT part values that YOU used.

Show the XLRs and how they are connected, 'unused' pins in the ICs, power supplies etc.

You need to do this ANYWAY if YOU want to do any work on it ... let alone have anyone help you.  Anyone who hopes to help you has to do all this legwork before they even understand what you've built.

STAGE 2

Next try to figure what the voltages should be at important points in the circuit. eg the output pins of the OPAs.

Put them on your schematic.

If something is wrong, the first sign will be wonky voltages.

You do have a voltmeter? 
_______________________________
How about you draw this and also show the OPAs internal to the IC?  You can get this from the datasheet.

YOU need to get used to looking at datasheets and showing connections on a schematic rather than telling people .

Otherwise YOU don't understand what you are doing .. so don't expect other people to understand either.

.. and if you post a pic, please clean it up so it is easy to read ... and make it smaller than 1024 x 768 pixels so it doesn't appear so large that we have to scroll to see anything useful ..

PRR has shown you how to do all the above.


I'll repeat my suggestion that you buy & start reading The Art of Electronics.... so its about time you made a start 

Click to expand...

The picture of my input stage is exactly what I have on a breadboard at the moment. I've already posted a picture of this. According to PRR, I was trying to make sure I should connect pin 1 (output of 1A of NE5532, into pin 6 of 1B NE5532's inverting input, then use a pair of resistors to create a voltage divider and connect that to pin 5 of 1B NE5532's the non-inverting input. By using a pair of caps at the input of the non-inverting input of 1B and then another at the output of 1B (pin7) it should be decently stabilized. I'll post pic. Thanks for any help ricardo.

> You're saying I should have 0v at all those locations.

You _now_ have 0V most everywhere. (Have you even put a voltmeter on it??)

You SHOULD have half-supply on most points in an audio circuit.

PRR said:

> You're saying I should have 0v at all those locations.

You _now_ have 0V most everywhere. (Have you even put a voltmeter on it??)

You SHOULD have half-supply on most points in an audio circuit.

Click to expand...

I put a meter on it before and i was getting 18v everywhere.

Here is the schematic according to notes you gave me and that I put together. Will this work PRR? If not, help me make an acceptable input stage.. I need this to work. I need something I can apply to other projects and etc if I want. And the knowledge you guys share with has been so helpful!

https://postimg.org/image/pl30wrhzb/

Can someone just hop me get a balanced input stage together using a NE5532 with a single supply? If you help with a working schematic I can breadboard it and post pic to verify it. I just need help. Please,  im here to learn.