MIC Input Circuit

[Suraj143]

I want build a MIC pre amp actually wants two MICs & two seperate outputs & going to build two of these below circuits.I have below problems.

*Can I replace the 741 opamp IC & replace with a TL072?
*How Can I add a MIC gain (sensitivity) control?
*Can I plug dynamic MICs, Instrumental or condensor MICs as well?

 

[canadaelk]

Almost any op-amp will work in this circuit You may need more gain for some microphones, for this change R1 to a higher value (hint: volume control). The value of C2 is to high, maybe 100pF? Add an output capacitor (1 MF to 10 MF).
Dynamic microphones are OK as is, condensers need a bias to work. Adjust R1 to suit the output voltage of the microphone. E

 

[audioguru]

The design of a 741 opamp is 52 years old and produces too much hissss to be a mic preamp. Also its poor slew rate cuts high audio frequencies above 9kHz.
A TL072 is a dual audio opamp has less noise but it is also fairly old. Use a new audio opamp instead.

I agree that the 100nF capacitor value is much too high and will cut all audio frequencies above only 29Hz.

The gain is only 57 times (increase the value of R2 to increase the gain) which is probably too low. Usually a mic preamp has a gain of 200 times that can be reduced with a volume control.

The circuit will work with a dynamic mic but not a condenser mic that must be powered. Today an electret mic is usually used but your circuit will not power it.

 

[Nigel Goodwin]

There's a 'sticky' at the top of this very forum, that gives AG's simple electret preamp.

Basic opamp circuits

This question is asked repeatedly, and Audioguru always posts a really helpful graphic showing inverting and non-inverting opamp circuits for both single and dual supplies. So to avoid him doing this every time, I'm placing his graphic here, and making this thread 'sticky'. I've also uploaded...

www.electro-tech-online.com

 

[rjenkinsgb]

Another key point is, what will the preamp be feeding?
If that is an existing piece of equipment, the output type & impedance need to be configured appropriately.

 

[Nigel Goodwin]

Another key point is, what will the preamp be feeding?
If that is an existing piece of equipment, the output type & impedance need to be configured appropriately.

That would be a VERY vague and rare requirement, the low output impedance of an opamp can feed most other circuits perfectly well.

 

[rjenkinsgb]

As in, just another electronics stage - in which case it's not critical, as Nigel says.

Or, is it to feed the mic input or line in on a PC, or a balanced line input on a mixer etc..
Those have rather different requirements.

Re. other inputs:
A dynamic mic would work connected directly to the input and ground.

An electret mic capsule would work with an appropriate bias resistor between the mic capsule + and +12V.
(Preferably two resistors in series & a cap to 0V, rather like the upper half of the opamp bias circuit)

Instruments - "active" things such as a keyboard or guitar pedal output could connect direct.

A guitar itself needs a much higher input impedance to avoid overloading the pickups.

You could try adding a 1M or 470K resistor from the bias divider (R4 - R5 junction) to the opamp input and input cap junction; also then make the input cap 0.1uF.
That should give a suitably high input impedance.

A condenser mic is a very different thing; they need a high voltage bias (and a load in the region of a gigohm, for a bare condenser capsule).

Complete mics have internal preamps and normally connect using a balanced line via an XLR plug, with a 42V "phantom power" feed to the mic outputs via 6k8 resistors, in whatever they connect to.

(However there are a vast numbers of fakes on sale, mics with electret capsules sold as "Studio condenser" or similar).

This is an example circuit (rom Gearslutz.com) for a real condenser condenser mic capsule; note the charge pump in the lower half, which produces around 50V, and the 1G bias resistors.

https://www.gearslutz.com/board/attachments/geekslutz-forum/301810d1342987154-mxl-990-resistor-replacement-how-far-off-schema-sp-1-mxl990.gif

 

[audioguru]

People "over there" call an electret mic a condenser mic.
I have two very high quality and very expensive Audio-Technica "studio" mics. On the box it says "phantom powered" but in the owners manual its says "electret condenser".

 

[Nigel Goodwin]

As in, just another electronics stage - in which case it's not critical, as Nigel says.

Or, is it to feed the mic input or line in on a PC, or a balanced line input on a mixer etc..
Those have rather different requirements.

Both are perfectly fine fed from the output of an opamp, the line input of a PC is probably specifically designed for it - the mic input obviously isn't intended for line inputs, but a simple series resistor (or a two resistor attenuator) makes it so. Mixer inputs are balanced or unbalanced, and again are perfect for feeding from an opamp, and again are specifically designed for that input.

There's really no issues at all - the output of an opamp is as universal as it gets.

 

[Nigel Goodwin]

People "over there" call an electret mic a condenser mic.
I have two very high quality and very expensive Audio-Technica "studio" mics. On the box it says "phantom powered" but in the owners manual its says "electret condenser".

As I think we all know, a condensor mike and an electret one are quite different things - something the advertisers don't seem capable of understanding.

I went to a guys house once (delivering a TV or something), and he had a really nice grand piano - which was gold plated internally - and he was a pretty impressive pianist (he actually runs a chip shop!).

Anyway, he wanted to record himself playing - so he went to his local music shop and asked for what he needed. So they sold him a nice condensor mike, a mike stand, an HDD based recorder/mixer, plus a plug-in CD-Writer, so he could copy from master to a CD.

All he really wanted was a cassette recorder , or better still one of thoe portable Zoom etc. handheld audio recorders - but it might have been slightly before that?. Anyway, the music shop obviously decided to scam him for as much money as they could.

Basically he hadn't got a clue what to do with it - and of course he originally couldn't record anything with it because of the condensor mike - although he did eventually manage to turn the phantom power ON, it was down about four menu levels!. He's recorded himself playing, and his daughter singing (she was AMAZING!) - and he wanted to know how to adjust the relevent levels - I pointed out he couldn't, as it was recorded in one take on a single track with a single mike.

He asked me if I could show him how to work it? - but I didn't have time - so I offered him the services of my daughter (who was about 13, a pianist, bassist, singer etc, a qualified recording engineer, and all round smart arse!). So I dropped her off (pimped her out!!) and went shopping at the Supermarket - she made herself some cash from teaching him how to work it, and had a play on his golden piano.

The point of this ramble, is that the differences between condensor and electret can be VERY critical.

 

[Suraj143]

OK thank you for the info.Now I got a clear understand a Electret MIC and a Condensor MIC.

I'm going to build audiogurus electret MIC pre amp.I have some small doubts.See the attachment.

1) If I remove the R1 in the attached circuit can I plug a dynamic MIC?
2)If I make only "one electret MIC circuit" how can I connect it to a stereo AMP in?
3)For dynamic MICs also do I need a gain of 200? The 470K Preset is hard to find, & its better I can use some lower value like 100K if gain is not that much needed.
3)9V is harder to find & going to use 12V power is it ok?

 

[audioguru]

I'm going to build audiogurus electret MIC pre amp.I have some small doubts. See the attachment.

1) If I remove the R1 in the attached circuit can I plug a dynamic MIC?

Yes.

2)If I make only "one electret MIC circuit" how can I connect it to a stereo AMP in?

Simply connect the OUT of the preamp circuit with very short wires or with a series 100 ohm resistor feeding a shielded audio cable to both amplifier line inputs.

3)For dynamic MICs also do I need a gain of 200? The 470K Preset is hard to find, & its better I can use some lower value like 100K if gain is not that much needed.

The 470k trimmer resistor is very common over here. You need to adjust it to match the distance and loudness of the person speaking at the mic.
You can use a 100k trimmer resistor if you replace R4 with 470 ohms and replace C2 with 100uF.

3)9V is harder to find & going to use 12V power is it ok?

12VDC is fine.
Is a TL071 audio opamp available over there?

 

[rjenkinsgb]

A couple of tips re. other sources:
If you want it to be compatible with a guitar, make R2 & R3 1M each instead of 100K. The TL071 has exceptionally low input bias current and will be fine with 1M or even higher.

For "automatic" electret bias, use a stereo / TRS socket with the amp input connected to tip and the 10K bias resistor connected to ring.
That is the convention used in dedicated PC mic inputs; for an electret mic use a stereo plug with tip and ring both connected to the mic +, for anything else use a mono plug.

A mono plug will have no bias and simply short the bias resistor to ground.

You could use two sockets in parallel for 3.5 & 1/4" inputs, or just a 1/4" one and a stereo headphone adapter down to 3.5mm
If you use two sockets, you can ignore the bias resistor and just connect the input on the 1/4" one.

 

[Suraj143]

OK one last question.

I want to make a PCB for a pre amplifier (with a tone controller).But it has to couple this mic pre amp.

My question is how can I connect this micr pre amp to my preamp? How can I feed audio source?

 

[rjenkinsgb]

Just connect the output of the preamp to the input of the tone control.

It does not need both coupling caps in series, so leave out (bpass) the input cap on the tone section. Both parts can run on the same supply voltage.

 

[Suraj143]

Just connect the output of the preamp to the input of the tone control.

It does not need both coupling caps in series, so leave out (bpass) the input cap on the tone section. Both parts can run on the same supply voltage.

Thanks.So how can I input the Line IN signals when I want to listen to music?

 

[audioguru]

In the tone control circuit, the 1uF value of C1 is way too high to feed the 1M of R11, passing earthquake frequencies down to 0.16Hz. 0.1uF (104 or 100nF) will pass earthquake frequencies down to 1.6Hz. Use 0.022uF (223 or 22nF) for all audio frequencies.
C2 in the tone controls circuit should be 1uF.

Use a switch to disconnect the output of the mic preamp then switch the line signal or a guitar pickup signal into C1 of the tone control circuit.

 

[Suraj143]

In the tone control circuit, the 1uF value of C1 is way too high to feed the 1M of R11, passing earthquake frequencies down to 0.16Hz. 0.1uF (104 or 100nF) will pass earthquake frequencies down to 1.6Hz. Use 0.022uF (223 or 22nF) for all audio frequencies.
C2 in the tone controls circuit should be 1uF.

Use a switch to disconnect the output of the mic preamp then switch the line signal or a guitar pickup signal into C1 of the tone control circuit.

Very Clear answer.Thanks