Hello

My friend asked me to make him a cheap and simple solution for recording 3 lavalier microphones into one Pc soundcard without use of a mixer or any other
equipment.


Basically what I have made is a simple OP amp summing amplifier with 3 inputs for the microphones.
Questions :
What am I really looking for on the input of the OP amp?
Do I want to match the impedance?
Or do I want to have a higher input impedance by increasing Rin and RF ?
If I do that then should I use a buffer stage on the output?

******

The microphones are electret lavalier microphones (ones you put on your shirt)
and they are meant for plugging directly into an integrated motherboard soundcard (the pink jack on the front/back of the PC )

The microphones have an output impedance of 2.2k according to the specifications . By practical methods I have determined that they have no internal resistor.

After some searching I found that the input circuit of the soundcard on the motherboard looks something like this :

http://www.hobby-hour.com/electronics/computer_microphone.php

After some practical testing I have confirmed that the input circuit is indeed of a similar nature.
Also after probing around , I have found that the input circuit gives a biasing voltage of about 3,6 volts and has an inbuild resistor (like in the circuit in the link above)

Basically my circuit looks like this :
View attachment mic_preamp.pdf

Note that There is no need for gain , just a simple summing of the 3 microphone signals so It can all be recorded by one input on the soundcard.

As an input I denoted electret mics , but there will actually be pads which will connect to jacks into which the actual mics will be plugged in.

The link to the mics:
http://www.alza.cz/genius-mic-01c-d157575.htm

******
I have built and tested the circuit on the breadboard with the actual mics and recorded a few samples into my integrated sound card on my laptop and it works.
The sound quality isn't supreme , just slightly worse than if I plug in the microphone directly but that is okay.

Is there anything in my circuit that I can add subtract or any way that I can improve the design ?
I'm particularly looking at advice on the Rin and RF resistors on the input of the op amp.

The quality of the recording does not have to be excellent , it just has to be clear enough for the listener to understand what the 3 people are saying without trying hard to make out every word.

Thanks in advance.

Tim

 

I would add a cap between the R7/R8 junction and ground, to attenuate any power supply noise.
You could try higher values (4k7?) for R1-R3.

 

I would also insert a unity gain buffer on each of the mic inputs before the passive mixer resistors.

I'd do it something like this: http://i.imgur.com/28bc2uC.png

 

The data sheet says that with -15,0,+15 supply the opamp can swing 26v ptp. Meaning 2v form each rail. And draws 10ma. With only 4.5v supply who knows how close it can swing but it hardly has any wiggle room. And I guess the high impedance bias supply from the souncard is just not good enough. I suggest you try running the opamp from a separate battery. Try 9v and see what happens
edit...I just noticed that you mention 3.6v. Is that whenit's loaded by your preamp ? If so, that's a non starter. I'm surprised you get anything out ! Ther are some opamps that will swing rail to rail and designed for 5v operation. Someone else may know a type number..

 

I would add a cap between the R7/R8 junction and ground, to attenuate any power supply noise.
You could try higher values (4k7?) for R1-R3.

I will be running this unit with 3 AA batteries . Hence my 4.5 volts , also there iwll be no power supply noise but I guess it still makes sence to add a ceramic there to improve transient response ( not sure if i'm saying the right thing here ).
Yeah I might try . I stuck with 2.2k as it is always recommended with electrets .

I would also insert a unity gain buffer on each of the mic inputs before the passive mixer resistors.

I'd do it something like this: http://i.imgur.com/28bc2uC.png

Thanks a bunch! I though I might need buffers at all stages . Also for R7-R8 I chose higher value resistors because I'll be powering the unit through batterys and it would give me a smaller idle current.

What effect does the 10K resistor have at the output?
I've seen it many times on the inputs of amplifier Ic's but never found out what that resistor does. Dont the cap and resistor make a highpass filter ?

The data sheet says that with -15,0,+15 supply the opamp can swing 26v ptp. Meaning 2v form each rail. And draws 10ma. With only 4.5v supply who knows how close it can swing but it hardly has any wiggle room. And I guess the high impedance bias supply from the souncard is just not good enough. I suggest you try running the opamp from a separate battery. Try 9v and see what happens
edit...I just noticed that you mention 3.6v. Is that whenit's loaded by your preamp ? If so, that's a non starter. I'm surprised you get anything out ! Ther are some opamps that will swing rail to rail and designed for 5v operation. Someone else may know a type number..

Thanks for your reply.
this might actually be an issue if it does indeed only swing to within 2 volts of the power rails .. I When I tried the circuit out it was with 5 volts which gave me about 1 volt of room to work in which was fine . As I mentioned before , this isn't an amplifier but just a signal summer.
The swing will not be greater than 100mV .

I might try a 9v though.
I am actually powering the whole circuit with a serapare batter and not from the bias voltage from the sound card.
I measured 3.6 volts on the sound card bias without the microphone connected and about 3.3 volts when it was connected. That's why i decided to power my circuit from an external source.

 

I'm afraid I come from the valve (tube) era so op amps still mystify me a bit ! But with all 3 mics plugged in I make the circuit have a gain of about 4.

But I agree that should still not mean the output swing exceeds 100Mv.

I would like to know why it's being suggested that three buffers are needed on the input. the mics certainly don't need it as their impedance is 2.2K and they have an output FET.

You could make it a true follower by feeding the input network to the junction of R7,R8 and strapping the op amp -ve input to the output pin.

However my queries don't explain distortion and I have a feeling that the very low operating voltage may be the cause. Perhaps the linearity of the op amp is compromised at that sort of voltage. It would normally expect 15-0-15.

And also, if it was me struggling with that little circuit I would use at least 4 cells and connect the microphone "GND" network to the cenre point of the battery supply ie 3v.

In that case, IF there is no dc component coming from the mics you could dispense with the input capacitors and R7,R8 and connect the three 10k resistors direct to the +ve input.

Please feel free to correct me !

 

In that case, IF there is no dc component coming from the mics you could dispense with the input capacitors and R7,R8 and connect the three 10k resistors direct to the +ve input.

I was hoping that, at least, the OP would have come back my now - I hate having the last word !

Anyway, I have since noticed that according to the OP's cct diagram there will be DC on the output. I thought commercialy made electrets included a capacitor in the output. Confirmation or otherwise of that would be useful info.

 

Hello sorry Cornishlad I was pretty busy the past few days .
Unfortunately my deadline was last night so I had to finish it up and etch the circuit .
My final design is the same as the PDF that I posted plus a buffer stage on the ouput (since i'm using a dual op amp then why not).
Coupling caps all 4.7uF ceramics .
Works a charm , the quality of the sound was better than I expected.
In fact , the sound was better and stronger than if I had just plugged in the microphone into the soundcard (as intended to be).
Also I listened to above mentioned advices and decided to use a 9v battery to power the circuits . Consequently , the bias resistors of the electrets were changed from 2.2 to 4.7k .
Rin = 10k
Rf = 15k
Thanks for everyone's input

note:

Yes there is a cap on the input of the soundcard . I linked the input circuitry of the soundcard in my first post. I also coupled the output by a cap just in case so it doesn't interfere with the biasing voltage and the other way around.

 

You could make it a true follower by feeding the input network to the junction of R7,R8 and strapping the op amp -ve input to the output pin.

etc...

Please feel free to correct me !

Glad to hear you had success with the mixer. I'm sure going to 9v was the secret. I would like to correct one thing I said in #6. As no one else has !

The main priciple of your mixer circuit is the virtual earth point created by neg feedback on the - ve op amp input. Having realised that I have to say that if the inputs were put to the +ve input pin this would not apply and therefore I don't know how good a mixer it would be.

As a bonus. think that you would also have got more signal from the electrets at 9v

 

I didn't correct you because I wasn't sure by what you meant.
Yeah probably the 9volt did the trick , that and the ceramic caps imo.
Unfortunately due to the deadline and pile of schoolwork that I had to do I did not have time to probe it and make a more detailed analysis.