# Newbie question on dynamic mic circuit problems

1. ## Newbie question on dynamic mic circuit problems

Hey guys!

This is my first circuit I build, so please bear with me.

I made a prototype for a dynamic mic amplifier on a transistor and LM386 which contains a transformer and 4 electrolytic capacitors (and some others for bypassing and filter - 1n , 100n). Electrolytic ones are 100 uF and 220 uF 16V. The source voltage is 9V.

The prototype works, so I made a PCB for SMD components. It came out very good. But when I soldered everything I had smoke somewhere in the capacitors area :) I double checked the circuit and found a bug in my PCB. So I made it again. And I had smoke again... I figured I used wrong voltage capacitors - 6.3V instead of 16V (Vs=9V). I didn't have 16V capacitors but I had 10V tantalum ones (instead of electrolytic), so I used those. I made another PCB with them and the circuit worked for ~1 minute... and then I started hearing either clicks or noise. I desoldered the transistor and checked - it does work (used transistor tester for this). I checked capacitors - all seem to have their capacitance values. I have no idea how to check LM386, so I removed it and put a new one. Same result - still clicks or noise, or something in between.

So I'd really appreciate some ideas (and probably knowledge from experience) on how to fix or make it right.

One thing to keep in mind is that it never worked again (after those initial 2 minutes), no matter what I do.

My only idea for now is to really put 16V capacitors (ordered already).

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2. ## Re: Newbie question on dynamic mic circuit problems

Where's the schematic?

3. ## Re: Newbie question on dynamic mic circuit problems

Here it is:

The gain here is around 120 dB. That's required.

4. ## Re: Newbie question on dynamic mic circuit problems

Hi,

The gain here is around 120 dB. That's required.
I doubt this is correct.
120dB is 1:1000000

Every tiny microvolt becomes a volt....every singke mV would represent 1000V.

Klaus

5. ## Re: Newbie question on dynamic mic circuit problems

Yeah, I'm probably wrong here. But it does have 2 transformers to make the gain very high.

I'll get my condensers tomorrow, so I can make another attempt...

6. ## Re: Newbie question on dynamic mic circuit problems

The schematic is silly. It might work if you can get the LM386 to bias correctly but the noise level will be terrible.
Please refer to the LM386 data sheet, it is an ancient, low performance audio power amplifier but you seem to be using it as a high gain pre-amplifier.

Driving it differentially with no bias reference is asking for trouble but regardless, the power it can consume is far lower than would cause any significant heating or smoke. Even if the LM386 and the transistor were completely shorted it would dissipate less than 1W and barely get warm.

...and transformers produce a loss, never a gain!

Brian.

7. ## Re: Newbie question on dynamic mic circuit problems

Originally Posted by betwixt
The schematic is silly. It might work if you can get the LM386 to bias correctly
The LM386 already has both inputs biased so they work perfectly when floating.

...and transformers produce a loss, never a gain!
Of course a transformer with a low input impedance and a high output impedance produces voltage gain but the transformers and gain are not needed in this simple circuit.
A single transistor can easily have a gain of 50 times or more and the LM386 with is gain adjustment pot can have a gain of 200. Then the total gain can be 50 x 200= 10,000 (80dB) or more which is too much for a microphone.

The second transformer produces a huge signal level loss and I could not find a datasheet for the first transformer that is also not needed.

Brian.[/QUOTE]

8. ## Re: Newbie question on dynamic mic circuit problems

Silly circuit or not is not the question. I had to start with something, so there we go.

I'll tell you if it works tomorrow :) The prototype works, so there is a hope.

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Originally Posted by betwixt
the power it can consume is far lower than would cause any significant heating or smoke
Well, then I succeeded on the smoke area :) That was a capacitor of course.

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9. ## Re: Newbie question on dynamic mic circuit problems

An electrolytic capacitor connected with backwards polarity can smoke or explode.
Why did you guess that you need a voltage gain of one million times? A gain of 200 times is plenty for a hand-held microphone. The room microphone in my Sound Level Indicator project is 1820 times and it can show a whisper in the next room.
Why use antique transformers with one connected backwards?

10. ## Re: Newbie question on dynamic mic circuit problems

Because I'm using a loud speaker as a dynamic microphone. That is really interesting from my point of view. It's surprising to me to know that a speaker could be a microphone!

I doubled checked polarity - all seemed correct. But that was on the first attempt. As I said, the last one worked for 1 minute and then just noise... :/

Oh, I do have INA163 and SSM2019 but I'm not going to use it before I make this one work. Also, I have MEMS microphones with DAC built-in. But I have no idea how to use them yet.

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11. ## Re: Newbie question on dynamic mic circuit problems

A loudspeaker used as a microphone sounds really bad, like the intercoms at gas (petrol) stations, because the cone and coil of a loudspeaker are heavy so they resonate and make a "boomy" sound.
A dynamic microphone is made the same with a coil and magnet but do not resonate badly because the cone and coil are small and lightweight because they are low-power.
A loudspeaker connected to the output of a modern amplifier does not resonate with a boomy sound because the extremely low output impedance of the amplifier damps the resonance.

The intercoms at gas stations simply use a little loudspeaker as the mic and speaker and an LM386 without any transformers and without an extra transistor.

12. ## Re: Newbie question on dynamic mic circuit problems

It might. But for sake of the learning I'm going for it. I have a pair of old speakers in a closet, they're going to be half-employed :)

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BTW, I think the speaker has one advantage - it can handle any SPL sound (unlike electret mics).

13. ## Re: Newbie question on dynamic mic circuit problems

Your amplifier has a maximum allowed output level that will probably be exceeded at a SPL much less than the maximum from an electret mic.
You cannot use a high gain preamp and a high SPL into a mic.
An electret mic has a simple modification so it can be used for high SPL inside a piano or a drum called the Linkwitz Mod where the Jfet is changed from having a common source to being a source follower.

14. ## Re: Newbie question on dynamic mic circuit problems

ok, I found thr problem. I tried to change every possible component on the PCB - but that didn't help. I checked the prototype and it can work indefinitely (I tried ~30 min though).

So, I put LM386M-1 (SOIC) on adapter and used it on my prototype instead of PDIP LM386N-1. And the prototype doesn't work! Now I wonder what could be the problem with SOIC chip. I appreciate any help or ideas.
Could it be that my SOIC chips are defective?

I looked at datasheet for LM386 and the only difference is the max Vs which is 15V max anyway. And I use 9V...

15. ## Re: Newbie question on dynamic mic circuit problems

Hi,

Wrong wiring, wrong device?

Klaus

16. ## Re: Newbie question on dynamic mic circuit problems

The SOIC case has the same pin numbers as the PDIP case but maybe you have the SOIC turned around so pin 1 is really its pin 5?
You do not need the transformers and the second transformer reduces the gain instead of increasing the gain. You might not need the transistor.

17. ## Re: Newbie question on dynamic mic circuit problems

No, I doubled checked. All looks good.

I got the chips from aliexpress, both PDIP and SOIC. Could it be they screwed up with SOIC? Anyway, I'll buy a genuine one tomorrow. Hopefully, that'll be end of story.

Also, I don't think it's an issue with soldering...

18. ## Re: Newbie question on dynamic mic circuit problems

AliExpress is Chinese and maybe they sell fake or factory rejects ICs like ebay.

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19. ## Re: Newbie question on dynamic mic circuit problems

No, the genuine opamp did not help. So I give up. I have no idea what's wrong with it. It's a pity really.

The only option left is to make a drill-through PCB with PDIP package..

20. ## Re: Newbie question on dynamic mic circuit problems

A microphone, an amplifier and a speaker produce acoustical feedback howling if the microphone can hear the loudspeaker.
You said the circuit produces noise but you did not describe the noise.
Noise can be high frequency hiss, low frequency hum or acoustical feedback howling.
When a tiny amplifier makes acoustical feedback howling then it might overheat and stop or be damaged.

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