need help to build a microphone preamplifier - Page 1
Forum section: electronics / beginners

author: Azhar
author_rank: Regular Contributor
timestamp: November 02, 2013, 02:10:28 am
content: Hi allI 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 errorPins 2 and 3 to the Op amp are shown reversedThat 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!

author: bgsteiner
author_rank: Contributor
timestamp: November 02, 2013, 04:15:50 am
content: 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.

author: Azhar
author_rank: Regular Contributor
timestamp: November 02, 2013, 04:59:38 am
content: Quote from: bgsteiner on November 02, 2013, 04:15:50 amI was thinking of An electret (condenser) microphone, but I'm happy to use other types of mics if I need to

author: bgsteiner
author_rank: Contributor
timestamp: November 02, 2013, 05:17:49 am
content: 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.

author: TMM
author_rank: Frequent Contributor
timestamp: November 02, 2013, 05:32:37 am
content: Phantom power supply:http://sound.westhost.com/project96.htmI 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.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 02, 2013, 07:19:16 am
content: 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.

author: Flump
author_rank: Frequent Contributor
timestamp: November 02, 2013, 10:52:20 am
content: hi Azharyou might want to consider using LM386 insteadvery few components needed for these.also these might helphttps://www.eevblog.com/forum/beginners/microphone-pre-amp-circuit/https://www.eevblog.com/forum/beginners/microphone-tone-control/

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 02, 2013, 11:05:00 am
content: Quote from: Flump on November 02, 2013, 10:52:20 amNo, he really doesn't want to consider using a noisy speaker output power amplifier for a condenser microphone preamplifier...

author: Flump
author_rank: Frequent Contributor
timestamp: November 02, 2013, 11:19:10 am
content: I built 2 last week and used them inline with the mic on my 2m radioeveryone commented that the audio was good quality and no onesaid anything about the noise level.maybe on paper they dont look good but in real world as a mic pre ampthey work fine

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 02, 2013, 11:27:49 am
content: Quote from: Flump on November 02, 2013, 11:19:10 amHam 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?

author: Azhar
author_rank: Regular Contributor
timestamp: November 02, 2013, 12:33:47 pm
content: Quote from: ElectroIrradiator on November 02, 2013, 11:27:49 amI'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' classTherefore it doesn't have to be ideal I guess, as far as it does the job

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 02, 2013, 01:21:02 pm
content: 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.

author: Azhar
author_rank: Regular Contributor
timestamp: November 02, 2013, 01:45:36 pm
content: How about this circuit?

author: grenert
author_rank: Frequent Contributor
timestamp: November 02, 2013, 03:08:53 pm
content: 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.htmNote 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.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: November 03, 2013, 07:05:35 am
content: Quote from: Azhar on November 02, 2013, 01:45:36 pmMuch better, though some of the component values may require tweaking for optimum performance.

author: mhzghz
author_rank: Contributor
timestamp: November 05, 2013, 07:16:32 am
content: These videos may also help: