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Making a cheap microphone with a simple preamp circuit.

Fluffyboii

Member
I put the whole thing in a box (the circuit in the last photo I sent). Made a switch for disabling the bias resistor, now I am using it with a dynamic mic and works amazingly well. Only problem is powering it up. Anything that is connected to wall introduces interference to the circuit so I am stuck with these 4 AAs for now. Can I power it up from USB power without those annoying frequencies invading the circuit. For amplification values more than 100 I heard a two op amp solution is better because of bandwidth issues. How much that would effect me if I wanted to get 200 times gain instead of 100 from the this circuit. I found this diagram but when I built it on breadboard it had very bad high frequency noise. audioguru
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
Well there's a complete lack of any decoupling capacitors in that circuit, so it's not going to work very well - and batteries won't last that long either because of it.

There's even a 'sticky' at the top of this very forum:


But if you want to keep your existing circuit you need decoupling across the main supply (capacitor), on the artificial spilt supply (capacitor), and on the bias to the electret (resistor and capacitor). These are fundamental essential components in such a circuit, battery powered or not.
 

Fluffyboii

Member
Well there's a complete lack of any decoupling capacitors in that circuit, so it's not going to work very well - and batteries won't last that long either because of it.

There's even a 'sticky' at the top of this very forum:


But if you want to keep your existing circuit you need decoupling across the main supply (capacitor), on the artificial spilt supply (capacitor), and on the bias to the electret (resistor and capacitor). These are fundamental essential components in such a circuit, battery powered or not.
This is the current circuit in the box. I will definitely check the link.
 

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audioguru

Well-Known Member
Most Helpful Member
Your 5532 opamp has a minimum allowed supply of 6V but your Ni-MH batteries drop to only 4.0V and your 5V noisy power supply is also too low.
Your low resistor values are reducing the mic output level.
 

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Fluffyboii

Member
Your 5532 opamp has a minimum allowed supply of 6V but your Ni-MH batteries drop to only 4.0V and your 5V noisy power supply is also too low.
Your low resistor values are reducing the mic output level.
Oh, so missing RC filter is the problem that causes everything other than a battery introducing a hissing sound. I actually have a 47uF capacitor across positive and negative therminals. So I need a RC filter and a capacitor across R4 (R6 in your example) as Nigel said. What values are suitable for the RC filter. By the way my NE5532P variant claims to have an low minimum input voltage of 3.5V, I tried 4 alcaline batteries to get 6V but the difference in noise was unnoticable. I coudnt use a boost converter because I could hear the operating frequency of the boost converter invading the circuit. It consumes 2 ma at 6 volts. Why do I need a capacitor at the pin 1, output pin. Some examples does not include that.
 
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audioguru

Well-Known Member
Most Helpful Member
An old style of power supply makes a low frequency hum sound when decoupling capacitors are missing in a preamp circuit. But now you describe the sound and say it is a high frequency hiss. Then your power supply is not an old style that makes hum, instead it is a new small style (like a boost converter) that makes hiss. Decoupling capacitors in your circuit should reduce the hiss.

The low value capacitor parallel with the negative feedback resistor reduces high frequency gain which reduces hiss, high frequency consonants in voices and reduces high frequencies in music.

All forms of the old NE5532 dual opamp have a minimum supply of 6V. A lower voltage does not cause hiss, instead it causes distortion.
 

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Fluffyboii

Member
An old style of power supply makes a low frequency hum sound when decoupling capacitors are missing in a preamp circuit. But now you describe the sound and say it is a high frequency hiss. Then your power supply is not an old style that makes hum, instead it is a new small style (like a boost converter) that makes hiss. Decoupling capacitors in your circuit should reduce the hiss.

The low value capacitor parallel with the negative feedback resistor reduces high frequency gain which reduces hiss, high frequency consonants in voices and reduces high frequencies in music.

All forms of the old NE5532 dual opamp have a minimum supply of 6V. A lower voltage does not cause hiss, instead it causes distortion.
Maybe I saw a wrong schematic, I coudn't find the 3.5V mentioned later. Do you have a schematic suitable for dynamic microphones with gain up to 200, I guess I will build another circuit from scratch since this one is hot glued to the plastic and adding stuff would be too difficult anyway. I actually found 10 brand new NE5532 chips laying around in my house because some aliexpress seller sent them instead of NE555 chips like 2 year ago :D While building one from scratch I want to add bass and tremble control but it seems to complicate thing too much so I may not. Can I change the TL071 in the pinned "excellent electret mic preamp" circuit Nigel Goodwin posted with NE5532 it looks like I only need to change 7 pin to 8 and 6 to 1 but I don't know the different characteristics of both op amps and I don't have the knowledge to fully understand a schematic and modify a circuit. I guess they will not teach us in collage until the 3th year. I know there are multiple schematics on the internet but most of them lack the decoupling capacitors or has other problems as you mentioned.
 

Fluffyboii

Member
Sorry I asked everything from you which is extremely lazy, instead I will try to learn it myself, the corrected circtuit you sent only contains first amplifier so I am assuming I don't need two of them the get 200 times gain. I will change the 10K resistor with 5K and 100K with 1M potansiometer connected series with 100K resistor. So from my current understanding it will have 1.1M/5K+600ohm = ~196 times gain adjustable down to ~18 times since my dynamic microphone has a 600 ohm impedance. I did put a switch between bias resistor and microphone to make it work with both dynamic and electret microphones in my first circuit. I will research more about it to find the other values of maybe change both of the resistor value to something smaller. If you ask why I switched to a dynamic from electret, it is because I got a good deal on one and I like the single direction characteristic of it over the omi directional electret microphone.
 

audioguru

Well-Known Member
Most Helpful Member
Your preamp uses inverting opamps with low value resistors then its NE5532 dual opamp IC is used. Nigel posted the electret mic preamp that I designed and it uses R1 to power the Jfet in an electret mic. R1 should be removed if a dynamic mic (coil and magnet) is used.

The TL071 is a single opamp but the NE5532 is a dual with 2 opamps in it so look at the datasheets to get the proper pins.
 

Fluffyboii

Member
Your preamp uses inverting opamps with low value resistors then its NE5532 dual opamp IC is used. Nigel posted the electret mic preamp that I designed and it uses R1 to power the Jfet in an electret mic. R1 should be removed if a dynamic mic (coil and magnet) is used.

The TL071 is a single opamp but the NE5532 is a dual with 2 opamps in it so look at the datasheets to get the proper pins.
I just made the same circuit after getting a TL071. Only difference is 500k pot and used 2X 1K resistors in series instead of the 2.2K so it should have 250 times gain. Much cleaner audio when I compare it with the first one. I will put it in a metal case with a battery soon.
 

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Fluffyboii

Member
Still got some fluctuating noise from the boost converter, tried multiple things but nothing except adding a 100ohm resistor series to supply power helped (that resistor fixed the startup noise issue due to the capacitor charging too fast). I tried the inductive divider in this site https://www.eetimes.com/how-to-control-input-ripple-and-noise-in-buck-converters/ but nothing really changed. I also added a 100uF capacitor in the input terminals of the converter and 10p capacitor between 6. and 2. pins to reduce high frequency gain. It is much better than the first circuit I built. First one wasn't even usable with boost converter due to the noise. Can we modify the RC filter in your circuit to get better performance with the boost converter,
 

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audioguru

Well-Known Member
Most Helpful Member
My electret mic preamp circuit does not have a high frequency filter, instead it lets the TL071 opamp produce a flat frequency response to 30kHz then and when its gain is 200 then it has a natural rolloff of high frequencies that drops at 6dB per octave.

Why charge a battery then use a boost converter to power the preamp with noise? Instead use the boost converter to charge a "9V" Ni-MH battery that can be used for a few long days for each overnight charge.
 

Fluffyboii

Member
My electret mic preamp circuit does not have a high frequency filter, instead it lets the TL071 opamp produce a flat frequency response to 30kHz then and when its gain is 200 then it has a natural rolloff of high frequencies that drops at 6dB per octave.

Why charge a battery then use a boost converter to power the preamp with noise? Instead use the boost converter to charge a "9V" Ni-MH battery that can be used for a few long days for each overnight charge.
I see, I really need a 9V rechargeable battery in this thing, I just don't have one. It would make thing so much easier. Now I feel like a idiot for wasting so many hours on it.
 

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