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Simple elecret mic pre-amp: What's causing my hum?

post #1 of 14
Thread Starter 

So this is pretty much my first experience with any sort of electrical DIY aside from making cables. I wanted to make a simple pre-amp for electret capsules. I used this design: http://www.minidisc.org/mic_preamp/Simple%20Stereo%20Electret%20Mic%20Preamp.htm 

 

It picks up sound from the mic but there's a fairly loud humming in the background. Here's a photo:

 

 

I expect it's grounding issues butI really have no idea what I'm doing. The blue wire coming off the opamp is from the VCC- leg but I don't know where to wire it to so it's going nowhere. The negative wire from the mic is going to the negative wire on the battery, the ground from the 3.5mm output is wired to the ground on the pot. It's probably a total mess. I'm actually very surprised that it works at all.

 

Can someone tell me what I'm doing wrong?

 

Edit: The hum can only be heard when I set my input to full gain. 


Edited by fraseyboy - 4/21/14 at 4:07pm
post #2 of 14
Here's a good source of information on electrets and powering them:-http://sound.westhost.com/project93.htm

It's very difficult to diagnose your circuit in the absence of a diagram. The opamp Vcc would normally be connected to the positive supply, but if you don't know this, you don't know much, so be prepared to destroy a few components. Don't connect anything of value, such as expensive headphones, to the output, and look for a DC offset at the output with a meter before using it to drive anything.

w
post #3 of 14
Thread Starter 
Quote:
Originally Posted by wakibaki View Post

Here's a good source of information on electrets and powering them:-http://sound.westhost.com/project93.htm

It's very difficult to diagnose your circuit in the absence of a diagram. The opamp Vcc would normally be connected to the positive supply, but if you don't know this, you don't know much, so be prepared to destroy a few components. Don't connect anything of value, such as expensive headphones, to the output, and look for a DC offset at the output with a meter before using it to drive anything.

w

 

 

I have VCC+ connected to the positive supply, I'm just not sure what to do with VCC-. When I connect it to the negative supply the whole thing stops working, so I guess it's optional...

 

I did buy backup components expecting failure. Really surprised I haven't broken anything actually. As I said it totally works fine as a mic pre-amp. I can make recordings and they sound pretty good. There's just heaps of background noise and I figured this was grounding. Maybe shielding? 

post #4 of 14
Quote:
Originally Posted by fraseyboy View Post
 

 

 

I have VCC+ connected to the positive supply, I'm just not sure what to do with VCC-. When I connect it to the negative supply the whole thing stops working, so I guess it's optional...

 

I did buy backup components expecting failure. Really surprised I haven't broken anything actually. As I said it totally works fine as a mic pre-amp. I can make recordings and they sound pretty good. There's just heaps of background noise and I figured this was grounding. Maybe shielding? 

So why are you using a pre amp with a electret mic? They already have a FET amp built in you just have to supply power and its a vary simple circuit. Google electret mic power circuit, even without much electronic knowledge you will be able to get that working. Once the mic is properly powered you wont even need the amp. Now if  you still want to build one buy the super stereo ear kit, its only $10 and you can learn a lot from it. Oh and that circuit you made is a mess, for one thing you should solder to the solder legs not the rivets on the pot thats a good way to ruin it by getting it to hot. Start with a kit you will be a lot happier.

post #5 of 14
Thread Starter 
Quote:
Originally Posted by bootdsc View Post
 

So why are you using a pre amp with a electret mic? They already have a FET amp built in you just have to supply power and its a vary simple circuit. Google electret mic power circuit, even without much electronic knowledge you will be able to get that working. Once the mic is properly powered you wont even need the amp. Now if  you still want to build one buy the super stereo ear kit, its only $10 and you can learn a lot from it. Oh and that circuit you made is a mess, for one thing you should solder to the solder legs not the rivets on the pot thats a good way to ruin it by getting it to hot. Start with a kit you will be a lot happier.

I'm wanting to use a pre amp to achieve the goals stated on the design page: 

"A: Providing a higher electret bias voltage, 9V through a 10k resistor to give the mic's fet more headroom.  (The R91 provides 2.5V through a 6k8 resistor.)

B: Having much greater preamp headroom, and bypassing the R91's mic preamp altogether, using a hi-fi or low noise op-amp to go straight into the line input."

 

I have considered the much simpler circuits which just supply voltage but I figured this would give me lower noise and higher max SPLs. 

 

Yeah, I know it's a mess. But it does work so I don't need a kit, I just need to figure out why there's so much noise.

post #6 of 14
OK, I misunderstood about Vcc-. The opamp will not work with either of it's power pins unconnected. If you are using a single rail (one 9V battery), both Vcc- and the battery negative must be connected to ground. If connecting them causes the output to disappear, then there is a serious problem elsewhere in your circuit.

bootdsc's comments are well taken, it's not really accurate to claim that the circuit is working.

w
post #7 of 14
Quote:
Originally Posted by wakibaki View Post

OK, I misunderstood about Vcc-. The opamp will not work with either of it's power pins unconnected. If you are using a single rail (one 9V battery), both Vcc- and the battery negative must be connected to ground. If connecting them causes the output to disappear, then there is a serious problem elsewhere in your circuit.

bootdsc's comments are well taken, it's not really accurate to claim that the circuit is working.

w

Yeah this kid just doesn't know what hes doing, starting with a kit is how all of us learned how to solder and learning how to take directions is part of that. I have 17 odd years of circuit building experience but by all means don't take my advice what could i possibly know? 

post #8 of 14
Thread Starter 
Quote:
Originally Posted by wakibaki View Post

OK, I misunderstood about Vcc-. The opamp will not work with either of it's power pins unconnected. If you are using a single rail (one 9V battery), both Vcc- and the battery negative must be connected to ground. If connecting them causes the output to disappear, then there is a serious problem elsewhere in your circuit.

bootdsc's comments are well taken, it's not really accurate to claim that the circuit is working.

w

Right, fair enough. So if it works when I don't have VCC- connected it must not actually be using the opamp... What is ground in a circuit like this one? I understand ground can be like a point on the chassis but I don't have a chassis. Do I just wire all my ground things to an empty solder pad?

 

Quote:

Originally Posted by bootdsc View Post
 

Yeah this kid just doesn't know what hes doing, starting with a kit is how all of us learned how to solder and learning how to take directions is part of that. I have 17 odd years of circuit building experience but by all means don't take my advice what could i possibly know? 

You're right, I have absolutely no idea what I'm doing but I don't really see why I need to spend another $10 on a kit when I've got all the parts required to make this circuit. Building this could be just as much of a learning experience as building a kit. I just need the knowledge.

 

Sorry I disregarded your advice. I get that throwing bits together with no knowledge of what's happening is probably not good practice but I don't really mind having to take things out and try again if it's not working correctly (which it clearly isn't). 

 

Is there a way to test if my op-amp is fried? It's very possibly that I applied too much heat.

post #9 of 14
In a single-ended battery driven circuit ground is best located at the battery negative terminal. A simple strategy that is often effective is to wire each grounded point in the circuit back to the battery negative with individual wires.

w
post #10 of 14
Thread Starter 
Quote:
Originally Posted by wakibaki View Post

In a single-ended battery driven circuit ground is best located at the battery negative terminal. A simple strategy that is often effective is to wire each grounded point in the circuit back to the battery negative with individual wires.

w

 

Right. So I've wired the ground from the input socket, VCC- from the op-amp, the ground on the pot, and the negative wire from the battery to the ground lug on the output socket. So it should all be correctly grounded now. I also wired things to the legs on the pot, not the rivets. On the bright side the buzz is gone, but so has any sound from the mic. Where should I start with finding where the problem is? I've poked around with a multimeter and there seems to be voltage pretty much everywhere in the circuit. There is no voltage across the in/out legs on the pot though... I tried bypassing the pot and just wiring the output directly and now there's a voltage on the output jack but all I hear is random noise. 

post #11 of 14

Do you have a bread board you can use to rebuild the circuit? That's how i would go about testing a new circuit before trying to build on proto board, it helps to find problems and to work on the layout for the final build. There is no way for any of us to figure out what went wrong as it is but if you rebuild it on a bread board then we can help because it will be easy to see where every component connection is. Well one thing to check is the polarity of the 5 large caps and you seem to be missing the 2pf caps. Its a interesting design, i might have to give it a try since i already have all the parts for it.

post #12 of 14
Thread Starter 

I thought I could skip straight to a protoboard so I didn't buy a breadboard. Obviously I was being a bit cocky. I've checked the connections many times and everything seems to be okay. The polarity of all my caps is correct but looks like I put in 1uf caps instead of 2.2uf caps. I have no idea why... I'll buy some 2.2uf caps and a breadboard when I get round to going to the place. Would the incorrect caps cause the whole thing to not work though?

 

I left out the 2pf because the designer says it's optional: "There will probably be 2pF of capacitance just from the PCB traces, and op-amps tend to be fairly well compensated these days so it’s really not needed."

post #13 of 14
Quote:
Originally Posted by fraseyboy View Post
 

I thought I could skip straight to a protoboard so I didn't buy a breadboard. Obviously I was being a bit cocky. I've checked the connections many times and everything seems to be okay. The polarity of all my caps is correct but looks like I put in 1uf caps instead of 2.2uf caps. I have no idea why... I'll buy some 2.2uf caps and a breadboard when I get round to going to the place. Would the incorrect caps cause the whole thing to not work though?

 

I left out the 2pf because the designer says it's optional: "There will probably be 2pF of capacitance just from the PCB traces, and op-amps tend to be fairly well compensated these days so it’s really not needed."

You might consider using a 8 pin socket so you don't solder directly to the op amp. Maybe also think about why you are building this, it must not be to save money cause diy costs more by the end of a project so it must be to learn how to build. Using the right process like starting with a bread board with 2 columns and about 60 rows plus some pre made jumpers will let you focus on the circuit . Do you have a picture of the underside of the board? Would it be a fair guess to assume its a horrifying mess? taking soldering out of the list of things to trouble shoot can be a big help too. 

post #14 of 14
Thread Starter 
Quote:
Originally Posted by bootdsc View Post
 

You might consider using a 8 pin socket so you don't solder directly to the op amp. Maybe also think about why you are building this, it must not be to save money cause diy costs more by the end of a project so it must be to learn how to build. Using the right process like starting with a bread board with 2 columns and about 60 rows plus some pre made jumpers will let you focus on the circuit . Do you have a picture of the underside of the board? Would it be a fair guess to assume its a horrifying mess? taking soldering out of the list of things to trouble shoot can be a big help too. 

Ah yes, a socket is another thing I'll buy. 

 

A stereo pre-amp for electret capsules isn't really something which is available ready made as far as I know. I'm planning on using this with a simple binaural headset I made from an old pair of headphones and two elecret capsules. It already makes pretty good recordings (especially considering the mics were like $2) but it does distort at high SPLs and it's pretty noisy. 

 

The underside of the board is indeed a horrifying mess. Here's an (embarrassing) photo:

 

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