Making a cheap microphone with a simple preamp circuit.

Hi, my usual headphones died recently and I had to send it to warranty. So I was looking for a microphone. First I tried using my earphones built in microphone and got a cable to split the audio cable to mic and speaker which worked but the volume was unacceptably low. So I got a cheap condenser desk microphone for 5 bucks and It sounded ok, still low volume.

My interest in building one spiked for some reason I got a MAX9814 board with AGC from overseas but It will take over a month to arrive. So I put together the simple single transistor pre amp with the parts I had laying around. And It worked. I was so sure that it will not work but it works surprisingly well. Anyway I made some changes on it, tried some different values. It consumes around 5ma with 12 volts and less than 3ma with 9 volts so It should last forever with a single 9V battery. I am currently using 4 AAs because I hate 9V batteries. The original schematic is below.

I purchased some tantalum capacitors for better audio quality and 4 electret microphones for experimenting with. I also ordered necessary parts for other circuit I found from GreatScotts channel. It has probably better audio with less noise since it got high and low pass filters and an amp IC. I will also try to build that one to see how it performs.

I got multiple questions about amplifiers though:

Since electret mics are condenser ones it should be ok to connect them in parallel configuration but what about series. They have transistors built in so I assume series connection is pointless, but it would be good if I could get a 2 parallel 2 series connection for better quality. İf I connect 4 of them in parallel would I need to adjust the 10Kohm resistor to something with quarter value to get the same amount current to all of the individual mics.

Can I use more than one transistor to amplify the output even more. I dont remember the name of that configuration but I know connecting multiple transitors in a spesific manner can make a amplifier. Although I am sure that it would increase the noise even more.

Can these preamp circuits applied to dynamic or ribbon microphones. I guess I need to remove the voltage connection to the mic itself for that to work.

Can I get a big condenser capsule and make it work without a phantom power with a simple power supply.

When I try to feed the single transistor preamp with any electricity source other than batteries it gets intense interference, using power adapter was impossible because of that and also USB power from pc was quite noisy. I tried using a linear voltage regulator with lots of filtering but it didn't effect the outcome.

Also I don't understand how should I connect jack wires. I will sent a picture of the current setup which is connected to only at the right channel but I guess it is ok since it is mono.

I want to learn more about microphones and preamplifier circuits. I will solder all of these on perfboard when I settle with a good circuit and got the jack connectors with other stuff.

ak

AnalogKid said:

Newbie questions about Sound amplifier

Hello i'm a beginner and i have a question about the following schematic. I can't understand the use of first transistor at the input; if the audio input is sinusoidal the amplifier exit from active state every time the signal become negative and the signal will be clipped. I'm missing something?

www.electro-tech-online.com

ak

Click to expand...

I already saw that forum treat but It doesn't really answer my questions. And I dont know what "ak" means.

"ak" = signed Analog Kid

Why do you want to parallel microphones? One electret mic picks up sounds from a wide angle, called omnidirectional.
A microphone has a very low signal level so a lot of amplification is needed. That is why a shielded audio cable is always used, but your breadboard circuit uses ordinary wires that are not shielded and are antennas for interference, especially electrical hum. The cable feeding a recorder or amplifier also must be a shielded audio cable.

Your NJM5532 opamp uses a 5V supply but its minimum specified supply is 6V.
The 1k resistor R1 powering the electret mic is only 1k so it reduces the output level from the mic. Use 4.7k instead. Its 5V or more must not have any noise on it.
The 47k resistor R2 produces a gain of only 21 times but a gain of 100 might be needed so change it to 10k.
With R2 at 10k then only one of the two 470nF capacitors C1 and C3 is needed for a good low frequency response.

audioguru said:

Why do you want to parallel microphones? One electret mic picks up sounds from a wide angle, called omnidirectional.
A microphone has a very low signal level so a lot of amplification is needed. That is why a shielded audio cable is always used, but your breadboard circuit uses ordinary wires that are not shielded and are antennas for interference, especially electrical hum. The cable feeding a recorder or amplifier also must be a shielded audio cable.

Your NJM5532 opamp uses a 5V supply but its minimum specified supply is 6V.
The 1k resistor R1 powering the electret mic is only 1k so it reduces the output level from the mic. Use 4.7k instead. Its 5V or more must not have any noise on it.
The 47k resistor R2 produces a gain of only 21 times but a gain of 100 might be needed so change it to 10k.
With R2 at 10k then only one of the two 470nF capacitors C1 and C3 is needed for a good low frequency response.

Click to expand...

I see, I will make the changes. I am planning to use a buck converter to get the ideal voltage from 2 AA batteries in series. Would it cause any problems.

A buck converter reduces the voltage. A boost converter boosts the voltage but might not work from 2V to 3V (two battery cells).
The converter works by oscillating which might be heard in the sound.

audioguru said:

A buck converter reduces the voltage. A boost converter boosts the voltage but might not work from 2V to 3V (two battery cells).
The converter works by oscillating which might be heard in the sound.

Click to expand...

My bad, the one I ordered is indeed a boost converter and it can boost up from 2v to 24v. I will try and see how it works, I can use a 9V battery if I need to.

audioguru said:

Why do you want to parallel microphones? One electret mic picks up sounds from a wide angle, called omnidirectional.
A microphone has a very low signal level so a lot of amplification is needed. That is why a shielded audio cable is always used, but your breadboard circuit uses ordinary wires that are not shielded and are antennas for interference, especially electrical hum. The cable feeding a recorder or amplifier also must be a shielded audio cable.

Your NJM5532 opamp uses a 5V supply but its minimum specified supply is 6V.
The 1k resistor R1 powering the electret mic is only 1k so it reduces the output level from the mic. Use 4.7k instead. Its 5V or more must not have any noise on it.
The 47k resistor R2 produces a gain of only 21 times but a gain of 100 might be needed so change it to 10k.
With R2 at 10k then only one of the two 470nF capacitors C1 and C3 is needed for a good low frequency response.

Click to expand...

I built the circuit first on bread board. Amplification was low but it was there. So I got it soldered. Amplification is not there It is same volume as the directly connected mic.

Ok forgot about what I said, it started working. But it got too much gain I will add a adjustable resistor series to 10K. BTW you said me to use a 4.7K instead of 1K for better sound. Woudnt using a higher volume limit the sound more. Should I change it to 4.7K.

It started sounding like hot garbage out of nowhere. I didn't done aynthing I just wanted to test it twice to se if it works and now it sounds terrible. But definitely working I need to find the reason

Inquisitive said:

"ak" = signed Analog Kid

Click to expand...

I though he just forget "47"


hah...

My aplifier stopped amplifying for some reason. It works when power is applied but the amplification is too low or non existent. I checked the solder joints and saw no issues. So apparently it worked for a while then decided to not amplify.

I do not know if your supply voltage is 6V or higher.
I do not know if your perfboard is stripboard.
I cannot see connections on pin 1 and on pin 3 of the NE5532. I can see connections only on pin 2, pin 4 and pin 8.
I cannot see if the unused opamp is properly disabled.
I cannot see if the microphone is actually an electret type.
I cannot see if the mic cable and output cable are shielded audio cables.

Is the NE5532 a fake one from ebay?
What is the preamp feeding? If it is an amplifier with a speaker then is there acoustical feedback howling?

Fluffyboii said:

So I was looking for a microphone.

Click to expand...

What are you connecting it to?

If it's a PC or phone, the mic connection provides a bias voltage for an electret capsule, something like 5V via a 4K7 resistor for a PC input.

And note that the output level from an electret mic capsule can vary phenolimally between types; cheap ebay ones are often extremely poor, while a relatively cheap one from a major supplier can give a really good output.

rjenkinsgb said:

What are you connecting it to?

If it's a PC or phone, the mic connection provides a bias voltage for an electret capsule, something like 5V via a 4K7 resistor for a PC input.

And note that the output level from an electret mic capsule can vary phenolimally between types; cheap ebay ones are often extremely poor, while a relatively cheap one from a major supplier can give a really good output.

Click to expand...

Sorry my board is a bit messy. IC is original, mic is electret. Actually it started working again. I tried with different electret mics and the gain was a bit too much so it was clipping a little. It doesnt work with 11 volts but works with 5. Also my pc front mic jack was problematic. Since I dont know if it will stop working in the future I cant say anything but I will get you more detailed pics.

I got it running again with some slight modifications. First I got the jacks further away from each other and glued them better. After that I added a 10K potentiometer series to the 10K resistor. And I listened to audiogru's advice and changed the 1K resistor to 4.7K. It has some weird bass to it with 2 of my electret mics but one of my electret mics works very nice. I Coudnt get it to work with 11 volts as I said for some reason but 5V works fine for now. I will use a boost converter when it arrives. I will upload some voice demos from 2 different mics and pictures of the last setup. BTW my soldering iron was dying so it looks super ghetto.

There are some badly taken photos of mine

It died after turning it on off and on, so much inconsistency. It was only working while I pushed the jack, I spotted a crack on solder on the output jack fixed it and now there is nothing. Also the other working one stopped working too, I feel like it is cursed.

I made a new one that works and doesnt look terrible