need help to build a microphone preamplifier

[Azhar]

Hi all
I need help and advice on building a microphone preamplifier using 741 op amp!
I googled and found these ones <<please see attached photos>> but I am not sure how accurate they are, especially that I read someone commenting as fellow on the schematic on photos#2:

"Diagram error
Pins 2 and 3 to the Op amp are shown reversed
That is:- R4 should be wired from pin 6 to pin 2 (junction of R1 input circuitry). Voltage divider R2, R3,C2 should be wired to pin3 of LM 741.
Overall gain of the LM 741 is best controlled by replacing R4 with a 220K Ohm pot. This replaces VR1."

Any advice which is better to use? or maybe suggest a better circuit?
thank you in advance!

[bgsteiner]

Quick question what type of microphone are you planning on using. High end microphones and most xlr microphones excluding those like the SM58 require +48V phantom power in order to operate.

[Azhar]

Quick question what type of microphone are you planning on using. High end microphones and most xlr microphones excluding those like the SM58 require +48V phantom power in order to operate.

I was thinking of An electret (condenser) microphone, but I'm happy to use other types of mics if I need to

[bgsteiner]

Most condenser microphones will need phantom power to operate correctly I would recommend looking into the circuitry for a good stable phantom DC supply.  They don't really take that much amperage. It would be best to have a selector switch to turn on and off the phantom power incase you ever use the preamp with other types of microphones. I would do some research intot he exact model you intend to use to make sure you get the power it needs or doesn't need.

I bring this up because the power is sent along the data lines so you would need to isolate this voltage from the main preamp circuit.

[TMM]

Phantom power supply:
http://sound.westhost.com/project96.htm
I have built one of these and it is complete overkill for 1 mic but it is very low noise and parts are cheap. Just use a lower power transformer (6VA is plenty) if only using it with 1 mic.

[ElectroIrradiator]

Don't use a 741 for a mic pre, it is sh!te.

Look for schematics, which uses the NE5534 or NE5532. This classic opamp is really inexpensive yet great for an application like this, and has been used in many a commercial preamplifier.

The first schematic you link is useless, you will be listening to the noise on your 12V rail. Horrible design fault.

I have attached a schematic of a commercial preamplifier. It uses a transformer for isolating the +48V phantom power, though there may be other solutions to that problem. Note that many DIY schematics employing phantom power runs a high risk of blowing up the opamp due to poor design.

Schematic was intentionally not scaled down due to the poor quality of the original. Making it smaller might have compromised whatever readability remains.

[Flump]

hi Azhar
you might want to consider using LM386 instead
very few components needed for these.


also these might help
https://www.eevblog.com/forum/beginners/microphone-pre-amp-circuit/
https://www.eevblog.com/forum/beginners/microphone-tone-control/

[ElectroIrradiator]

hi Azhar
you might want to consider using LM386 instead
very few components needed for these.

No, he really doesn't want to consider using a noisy speaker output power amplifier for a condenser microphone preamplifier...

[Flump]

I built 2 last week and used them inline with the mic on my 2m radio
everyone commented that the audio was good quality and no one
said anything about the noise level.

maybe on paper they dont look good but in real world as a mic pre amp
they work fine

[ElectroIrradiator]

I built 2 last week and used them inline with the mic on my 2m radio
everyone commented that the audio was good quality and no one
said anything about the noise level.

maybe on paper they dont look good but in real world as a mic pre amp
they work fine

Ham radio is not exactly a critical test, particularly since it is a highly BW limited system. You won't even be able to hear any noise in the treble region, since the transmitted BW is around 300 Hz to 3 KHz. Depending on the application, then a single BC547 preamp may 'sound good' if you are not too fussy.

Azhar,

What do you intend to use the mic for?

[Azhar]

Ham radio is not exactly a critical test, particularly since it is a highly BW limited system. You won't even be able to hear any noise in the treble region, since the transmitted BW is around 300 Hz to 3 KHz. Depending on the application, then a single BC547 preamp may 'sound good' if you are not too fussy.

Azhar,

What do you intend to use the mic for?

I'm studying engineering (first year) and I need to build A low noise microphone preamplifier using op amp or transistors for my 'Troubleshoot Basic Amplifier Circuits' class
Therefore it doesn't have to be ideal I guess, as far as it does the job

[ElectroIrradiator]

In that case I'd definitely use the NE5534 (single) / NE5532 (double), unless you are under strict orders to use a 741. The NE5532 is pretty much the de facto industry standard for low noise, high quality audio work. Additionally it is inexpensive and readily available, and is a known performer. Better opamps for audio exist these days, but I doubt you could be faulted for choosing the '32/'34 for a mic preamplifier.

Your schematic number 2 is a starting point, though not the best one, if you ignore the opamp pinouts of the 741. Note though, that it contains two errors, plus there are three points I would tweak/test/consider perhaps changing.

[Azhar]

How about this circuit?

[grenert]

I've built the simple electret mic preamp circuit on this page, and it works well:
http://www.minidisc.org/mic_preamp/Simple%20Stereo%20Electret%20Mic%20Preamp.htm
Note that it is a stereo preamp, so uses a dual opamp.  You'd only build one channel if you needed a monophonic preamp.  The opamp pinouts are different for single and dual opamps, so be sure to check a pinout diagram for the one you use.

As others have said, the 741 is a horrific opamp, replace with something else.  TL071 or NE5534 (single) are both cheap and good in this application.

[ElectroIrradiator]

How about this circuit?

Much better, though some of the component values may require tweaking for optimum performance.

[mhzghz]

These videos may also help: