Electret mics come with a j-fet amplifier that reduces their impedance quite a bit. You might not need a buffer amp. If you do, start with the schematic in post #2 and simplify for a fixed gain. But try it without a pre-amp first and see if you need one. Be sure to use twisted pair or twisted/shielded pair cable to minimize noise pick-up.

 

Electret mics come with a j-fet amplifier that reduces their impedance quite a bit. You might not need a buffer amp. If you do, start with the schematic in post #2 and simplify for a fixed gain. But try it without a pre-amp first and see if you need one. Be sure to use twisted pair or twisted/shielded pair cable to minimize noise pick-up.

Thank you for your help today, you taught me a few things on audio, that's great help, appreciate it.

 

What is the environment for this intercom - factory floor, outdoor football stadium, kitchen, bedroom - ?

ak

 

What is the environment for this intercom - factory floor, outdoor football stadium, kitchen, bedroom - ?

ak

It's residential property, usually in a quite area (not CBD). In bedroom, living room, kitchen, shower etc..

 

I did a pre-amp designed for 20KHz with a 1500 ohm termination and it was good for 50 feet of coax. I don' t know why you think you have a noise problem.

 

I did a pre-amp designed for 20KHz with a 1500 ohm termination and it was good for 50 feet of coax. I don' t know why you think you have a noise problem.

We want to pick up conversation about 1 meter away from the intercom. At 1 meter away from the intercom, it's too quite at the far end. If I increase the gain digitally to get better volume, I have too much background noise. That's why I am guessing I have noise problem.

But, as you already know, I don't know much about audio. So my guess could be totally wrong

 

If I increase the gain digitally to get better volume, I have too much background noise.

Increase the gain analogally. We've been doing it that way for 100 years without getting any digitizing noise in the audio.

 

Increase the gain analogally. We've been doing it that way for 100 years without getting any digitizing noise in the audio.

building one now.

edited:
Gain of 10 a good number??

 

There is a difference between acoustic noise from the environment and electrical noise from the wiring and circuit (and a different form of the environment). As a quickie test, listen to the signal with your best headphones. acoustic background noise will have more rumble and bass; electrical noise will be more hissy.

ak

 

There is a difference between acoustic noise from the environment and electrical noise from the wiring and circuit (and a different form of the environment). As a quickie test, listen to the signal with your best headphones. acoustic background noise will have more rumble and bass; electrical noise will be more hissy.

ak

My ears are not very good with audio/music stuff, I will let someone else have a listen to see if they can tell the different.

 

Like this:

Forgot to ask, in this configuration, do you use directional mic, or un-directional mic?

 

in this configuration, do you use directional mic, or un-directional mic?

See post #2

Most of what makes a microphone directional or not is the enclosure. Just build the darn thing!
Radio Shack sold an intercom in 1975 with 50 feet of wire (not even twisted) that used 2 transistors. You have a world of better stuff to use and you're analyzing it to death.
If you can't beat a 2 transistor intercom with j-fet op-amps, present your schematic because you must have made an error assembling it.

 

See post #2

Most of what makes a microphone directional or not is the enclosure. Just build the darn thing!
Radio Shack sold an intercom in 1975 with 50 feet of wire (not even twisted) that used 2 transistors. You have a world of better stuff to use and you're analyzing it to death.
If you can't beat a 2 transistor intercom with j-fet op-amps, present your schematic because you must have made an error assembling it.

Just to update:

I built a preamp with TLC2272 (not sure it's a good one for this, but that's the one I have now), inverting amp config, gain = 40, one stage amp only, background noise is reduced significantly.

Next, I will build one with two mic and diff amp, but I don't have a different amp right now, so I will have to buy one first. Will report back once I have it tested.

Thanks again #12

edit:

I can still hear a tiny little hum, but you need to pay special attention to hear it, I think it's good enough for my application.

 

I remind you that a TL-071 is a great op-amp for low noise audio.
I remind you that you should restrict your bandwidth to about 300Hz to 3000Hz for voice only. That will kill a lot of power frequency hum.

 

I remind you that a TL-071 is a great op-amp for low noise audio.
I remind you that you should restrict your bandwidth to about 300Hz to 3000Hz for voice only. That will kill a lot of power frequency hum.

Thanks again #12, I will try these out. I have to go home now, it's almost 6pm here local time. Bad thing will happen if I don't go home now...

Will do an update sometime next week after I get the parts and test it out.

 

The TLC2272 looks like an excellent, low noise amplifier, too. I don't know why you want to change. Both of them are fast enough for 3KHz, wide voltage range, and the 2272 has lower noise.

 

The TLC2272 looks like an excellent, low noise amplifier, too. I don't know why you want to change. Both of them are fast enough for 3KHz, wide voltage range, and the 2272 has lower noise.

I am thinking this, what do you think? it has -3db from 250Hz too 3.5KHz. The following is my LTspice sim and bolt pot

 

Gain of 50? Put it in a metal box so it doesn't pick up power frequency hum.
You still have room to negotiate with the R1/R4 pair. If your wires pick up some noise, you can use less resistance in that position.

 

Gain of 50? Put it in a metal box so it doesn't pick up power frequency hum.
You still have room to negotiate with the R1/R4 pair. If your wires pick up some noise, you can use less resistance in that position.

Don't worry much about my gain, I will properly adjust it again when I test it and physically put it in a enclosure. And thanks for pointing out R1/R4 pares, I was not comfortable with them, they were origionally 22K and 220K. But I don't have 220K with me, only 360K.

If R1/R4 proves no good, I will change it to lower value.

Thanks again #12. Will update this post once I done.

 

Hi guys

This is my update, I have adjust the circuit a little bit. But no major change. It works well for my need. These are the summary:

• Work very well in a bigger room (at least according to my ears)
• I can pick up the noise of a PC fan if a PC is less than 2 meters away, but the noise level is very low, not a concerns for me

Let me know if anything else I should test.

Thanks guys