Building an ULTRA sensitive PC MIC

[Peter_wadley]

Basically the manager at a local radio shack asked if I could try and build him the following circuit...

He would like me to build him an ULTRA sensitive microphone circuit.

The circuit is for his wife.. for her job she uses a low power microphone

* THIS IS FOR A COMPUTER BY THE WAY *

She wants to be able to get up and walk around a small room while still being about to talk to her customers..

So the manager buys a small electret mic and gives it to me.. from what I knew at the time I told him that I will need an audio amp IC to amplify the sound going into the mic..

So I tell him the 386N amplifier should do..

I wire it up with a gain of 20 (The default gain) and the bass booster..

See Bottom Left Schematic on PAGE 5.

and hook it up to an 8hm: speaker.. and used a 9V battery

Ok, what I now need to do is interface it with the PC microphone port.. I notice my standard PC mic has a stero plug..

Question 1: What is each part of the plug connected to in the computer? Why would a mic with one input have a stero plug

I know I am going to need an attenuator or I will blow the PC input.. need help with that one..

Should I use LINE IN or MIC IN??

Basically I just want to get this thing hooked up the computer so that I can start working on making it as sensitive as possible..

I hope I can get this project made *WELL* so that I can bring it in along with my resume for a summer job

Thanks
Peter Wadley

 

[Nigel Goodwin]

I hope I can get this project made *WELL* so that I can bring it in along with my resume for a summer job

I hate to be the bearer of bad news

But you've blown any chance of a summer job with anyone who reads these forums!.

You DON'T use a power amplifier like an LM386 for a microphone preamp, you use a preamplifier - either an opamp or discrete transistors.

 

[Peter_wadley]

Isn't the LM386 a..

Low Voltage Audio Power Amplifier... oh I see now.. POWER

Right now I have the following.. could I use any of these?

- LM741.. dont want to use since it is bipolar
- LM358.. I dont think this one will do..
- and 2 TL081's.. this should do correct??

Yeah, he already paid me some money to make it so I am sort of put on the spot with this.. still I want to at least make an attempt..

 

[mneary]

**broken link removed** might help. One thing omitted is that you should bias your electret with a 4k7 from the ring terminal.

 

[audioguru]

How are you going to control the mic volume? Up close it will be too loud. Far away it will not be loud enough.

How are you going to control the terrible sound picked up by the mic when the lady is speaking in a direction away from the mic?

What about echoes?

What about noise from the traffic outside and the air conditioner inside?

Many performers use a wireless headset microphone for its good clarity when the performer is moving around. The lady needs one.

 

[Blueteeth]

You'll need an AGC circuit at the very least. Amplitude controlled gain. That way, for low amplitude sounds, it'll have high gain, and high amplitude sounds would give low gain. I had a small kit a few years ago that did this (veleman kit?) and it was pretty impressive, maintained a fairly constant output volume regardless of where I was in the room. But of course, with the higher gain (when you're firther away from the mic) it'll also pick up ambient sounds better.

Also note, the 'mic' inputs on even the high-end soundcards, in my experience, tend to be crap. Sure its only for voice comms and not ultra high quality recording but still, when changing the sample rate from 16khz to 48khz makes no audiable difference, one has to question the analogue side of soundcards.

Failing that, as audioguru suggested, some sort of wearable wireless mic. The 'decent' ones are expensive, cheaper ones will work just fine for conversations but may be prone to interference from almost anything. What about those bluetooth headsets ('universal soldiers' as I call em) for mobile/cell phones that seem to be on everyones nut these days? range of 10m, probably 20 if the PC side has a half decent bluetooth dongle and its pretty immune to interference, provides telephone quality audio, and apparently is very confortable to wear (no headband). I hate them with an ever growing passion, but sounds like its perfect for this app. After all, it is what its designed for.

BT

 

[Leftyretro]

https://pdf1.alldatasheet.com/datasheet-pdf/view/105863/PHILIPS/NE572.html

 

[audioguru]

Philips had a Compandor Cookbook that explained it pretty well.

 

[Peter_wadley]

His wife also hates head sets.. the manager was telling me about her sensitive ears.. wasnt really listening though.

How are you going to control the mic volume? Up close it will be too loud. Far away it will not be loud enough.

How are you going to control the terrible sound picked up by the mic when the lady is speaking in a direction away from the mic?

What about echoes?

What about noise from the traffic outside and the air conditioner inside?

Many performers use a wireless headset microphone for its good clarity when the performer is moving around. The lady needs one.

I could add 10K logarithmic pot to output.. for volume changes

She is not going to be running around everywhere though..

From what I have gathered.. she wants to be able to talk to her customers while computing.

BUT

She cant stand her headset.. or any for that matter.. thats what makes this so fun :|

As for echoes, air conditioners and traffic ... that is her problem.. not mine

Bottom line.. I was paid, although I told him to wait until it was made , so I shall produce..

As for buying a pre-made product... why is this even a suggestion? This is an electronic DIY site..

I dont think a sales women will be belting out any vibrato notes inorder to close a sale; so an expensive headset is not necessary.

I guess I will just use the TL081 and adjust the gain until I come to a good sound.

All I am really frightened of is frying my computers Mic input.. what are the chances of this happening?

Should I test the output Wattage of the preamp firstly?

I will also go through the links provided - thank you - I dont really want to have to order special IC's and parts for this .. I have school work and would rather get this over with sooner then later.

Thanks for helping out

Edit: I dont want to come off as a jack*** about the 'go out and buy it' comments, its just that the particular comment is fustrating when you want to be creative :{}

 

[Peter_wadley]

I found and built this TL081 circuit.. is there anyway to make it more sensitive?

Circuit

 

[Peter_wadley]

Wow! I just put a 100K POT in place of the resistor which is on the cathode side of the 10uf cap.. it is VERY VERY sensitive.. i think this might just work.

 

[TheVictim]

I'm not as experienced as everyone else here, but if you're going to use an OpAmp I really don't see what is so bad about the 741. You can split off the power supply voltage with a couple of 100k resistors and that can bias the non-inverting input with Vcc/2, then power the op amp off a single supply voltage. Capacitively couple your audio to the inverting input. A 100K pot feeding back to the inverted input from the output will allow you to adjust gain.

You'll still have all the other problems to deal with (a 100uF cap across the power supply will help with noise), but you WILL have a LOT of gain.

 

[mneary]

The 741 isn't a bad part. The TL071 is just better. It uses less current, has a lower input bias current, and 10X better large signal slew rate. IIRC, it also drives closer to the rails without distortion, and has half the noise.

 

[Nigel Goodwin]

The 741 isn't a bad part. The TL071 is just better. It uses less current, has a lower input bias current, and 10X better large signal slew rate. IIRC, it also drives closer to the rails without distortion, and has half the noise.

Exactly, the 741 was fine in it's day, just that it's day was so very long ago - the TL0xx series chips are just so much better in every respect.

 

[audioguru]

Before you said the lady was going to talk to customers while walking around the room. Then the voice level from the mic will change all over the place and it s high sensitivity will pickup all kinds of noises.

I changed a few parts in your mic preamp to make it work properly.

EDIT: I corrected the formula to calculate the voltage gain.

 

[Peter_wadley]

OK,

I tried the setup with out your mods - thanks guru - and the results were astonishing to say the least.

I stood back about 20 feet and spoke with a normal voice.. the sound recorder picked it up as if i was standing right next to it..

I then went on an online game and asked people how the mic sounded and they all said about 8/10 for clearness

I am going to try your modifications after school then bring it into the store for him to test it.

Thanks alot!

Also It was faintly picking up a bit of radio noise, would enclosing in a conduct project box fix this?

 

[audioguru]

I tried the setup with out your mods - thanks guru - and the results were astonishing to say the least.

Good.

Also It was faintly picking up a bit of radio noise, would enclosing in a conduct project box fix this?

The mic must have a shielded audio cable or very short wires.
The preamp circuit should have a pcb or be made on stripboard with short wiring.
Don't use a breadboard which picks up all kinds of interference by its long wires.
I have many preamp circuits in plastic boxes and they are fine. Use a metal box connected with a wire to the circuit's ground to stop strong local stations from interfering.

 

[Peter_wadley]

Guru,

Your name holds true!

With your modifications the circuit is more then I had expected! Unbelievable, I think Ill make an extra mic for myself!

Ill be bringing the prototype to the manager tommorow.

By the way, if it isnt too much to ask, what is the gain set at with the 100K resistor?

I hope someday I will have the knowledge and ability to optimize circuits like this! Makes life so much easier!

Thanks again

 

[audioguru]

Hi Peter,
My schematic said wrongly how to calculate the gain of the opamp. It is (R3/R4) + 1. So the voltage gain is 101. I corrected my schematic.
With that much gain the frequency response of a TL071 or TL081 is flat up to 32kHz. If the voltage gain is higher at 1000 then the frequencies above 3.2kHz will be reduced.

 

[Peter_wadley]

Alright,

Great