looking for MIC circuit

[ash1412]

Hi All,

I am looking for lifesize MIC schematic.

**broken link removed**

can any one help?



Regards
Ashwani

 

[Audioguru]

The mic preamp for the Lifesize mic might be built inside it since it has power through its cable.
But there are no details about it.

 

[ash1412]

Thanks for ur reply.

Basically they are using LM2903 & LM833.

Any recommended preamp for Electret MIC.
I want to design MIC same as lifesize.

Any reference design would be very appreciated.

Thanks in Advance
Ashwani

 

[Audioguru]

I have an electret mic preamp circuit that I have used many times. It uses an inexpensive TL071 low noise opamp but better ones are available. The one you found uses an LM833 dual opamp for stereo?
My preamp works perfectly until the 9V battery drops to about 6V.
Use the 470k trimpot until you know how much gain you need then replace it with a fixed resistor.
I have two pcb designs for it if you want.

 

[ash1412]

Thanks for ur reply.

can you share your preamp circuit?

my mail id is: ash1412@gmail.com




Thanks in Advance
Ashwani Kumar

 

[Audioguru]

Do not post your e-mail address on the internet. Thousands of SPAMMERS will find it and sent you millions of SPAM.
Here are pcb designs for my electret mic preamp circuit:
EDIT: They do not open and I don't know what is wrong with this website.

 

[Audioguru]

Instead of GO ADVANCED and uploading my files, i will try to ADD AN IMAGE:

 

[ash1412]

Thanks.

Regards
Ashwani

 

[ash1412]

Hi,

I have assembled all components on prototype pcb. When I am testing this circuit, I am able to listen my voice(in-front of MIC) on speaker with high noise & continuous beep sound.
I am always getting continuous beep on speaker.

please suggest, how to reduce this beep sound & noise.

Thanks
Ashwani

 

[Audioguru]

When the mic can hear the speaker then there is "acoustical feedback howling" because the sound goes around and around.
To stop the howling or beep:
1) Turn down the gain control or volume control.
2) Separate the mic and speaker and point them away from each other.
3) Use a good speaker with a flat frequency response instead of a cheap speaker that has a peak in its frequency response.

 

[ash1412]

Thanks for your reply.

I have followed the steps, To stop the howling or beep:
1) Turn down the gain control or volume control
=> by varying the 470k(potentiometer) resitance continuous beep sound down, but
MIC sensitivity also decrease(MIC is not able to sense 1M away voice).

2) Separate the mic and speaker and point them away from each other:
=> both are approx. 5M away from each other.

3) Use a good speaker with a flat frequency response instead of a cheap speaker that
has a peak in its frequency response.
=> I am using Creative speaker.

I want to increase MIC sensitivity(sense voice signal 1-2M away from MIC) keeping low noise and continuous beep .


Any suggestion/changes in design would be very appreciated.


Thanks in Advance.
Ashwani

 

[Audioguru]

I did professional PA systems and boardroom teleconference systems for many years. I tried "feedback reducers" that helped slightly and I tried "frequency shifters" that made weird sounds and helped slightly.

Look at performers. They "eat" the mic (it is very close to their mouth) and scream very loudly. The speakers are in front of them pointing away from the mic and the mic is pointing back and away from the speakers.

Frequently the performer does not have a live mic. A recording is playing and they pretend to be talking or singing to avoid feedback.

 

[ash1412]

Thanks for your reply.

I am new in this field. I want to use this mic in teleconference system, how is this possible without noise?



Regards
Ashwnai

 

[Audioguru]

A good teleconference system uses a digital echo canceller circuit to prevent sounds in the room picked up by the mic or mics from coming out the local speakers. It can transmit and receive at the same time which is called "full duplex". It is expensive and the circuit is very complicated.

A cheap old speakerphone circuit uses "voice switching" which attenuates the local speakers when somebody in the room talks and it attenuates the local mics when the distant end is playing on the speakers. It is called "half duplex". It frequently cuts off words and background sounds mess it up.

 

[ash1412]

Thanks.

do you know any other good preamp for Electret MICs(half duplex)?
I am using ready-made DM368 processor based board, which have audio input & output port. I want to use audio in (MIC) for conference.

Any suggestion would be appreciated.

 

[Audioguru]

do you know any other good preamp for Electret MICs(half duplex)?

Most electret mic preamp circuits are similar. Maybe a more expensive opamp can be used for less hiss.

I am using ready-made DM368 processor based board, which have audio input & output port. I want to use audio in (MIC) for conference.

A digital teleconference system does not have the 4-wire-to 2-wire converter used for a telephone line so the only feedback will be long distance feedback. A half-duplex circuit is separate from a mic preamp. Motorola (Freescale) make some half duplex speakerphone ICs that work better than most cheap speakerphones. I made a pretty good voice-switching speakerphone with a Motorola IC about 29 years ago.

 

[ash1412]

can we use multistage preamp ckt?

 

[Audioguru]

The preamp circuit I showed uses an opamp that is multi-staged. It has 14 transistors, 3 Jfets, 1 diode, 11 resistors and 1 capacitor inside.
You can make your own preamp circuit with separate transistor like they did 50 years ago.

 

[ash1412]

No
I was asking, if I use same two preamp ckt(based on TL071) i.e. output of one preamp connect input to another preamp. I think noise more boost up as compare to signal.

one more ques:
When I am using this preamp ckt with PC (giving external 9V) for audio recording, Its able to record high quality audio. Why different behavior with speaker?

 

[Audioguru]

I was asking, if I use same two preamp ckt(based on TL071) i.e. output of one preamp connect input to another preamp. I think noise more boost up as compare to signal.

My preamp circuit has a gain of 215 times when the 470k trimpot is turned to maximum. That is plenty of gain. If you want more gain then increase the 470k to 2.2M and/or reduce the 2.2k to 470 ohms. With more gain then you will have more background noise and more electronic noise. Two preamps each with a gain of 215 times makes a total gain of 46,225 times which is stupid.

When I am using this preamp ckt with PC (giving external 9V) for audio recording, Its able to record high quality audio. Why different behavior with speaker?

The preamp cannot drive an impedance less than 10000 ohms. But a speaker is only 8 ohms so it must be driven by a power amplifier. The preamp drives the power amplifier.