Making a Mini Mic preamp module?

#1

I need to build a little preamp to attach a mic to a camera, anyone got any knowledge about this? Would have mic level XLR in, would go through a gain knob and then out to a line level stereo mini-jack output for the camera. Sound quality needs to be as good as possible.
Commercial ones are passive but i could power it off rechargeable batteries if it needs to be active.

This would really be ideal to record the sound for the induction videos.

Is this easy to do using available components? Would something like this cut it?

https://www.maplin.co.uk/p/mindsets-low-noise-stereo-pre-amplified-kit-n47fl?cmpid=ppc:components:pla:google&gclid=Cj0KCQjw8vnMBRDgARIsACm_BhK97Ts_al3futX_85ZqklhViZiPWlsNIyUm6DejTH6_v5UsEjHJ51UaAhyTEALw_wcB&gclsrc=aw.ds

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#2

You’ll need to build an active amplifier to go from Mic level to Line level. Passive preamps do some clever things to match output and input impedance, but won’t actually make anything any louder (which is exactly what you’re trying to do). Difference between line and mic level here.

What you’ll need to do it build an amplifier which can amplify a signal between 100x and 1000x. Thankfully you shouldn’t need to provide much current, just more voltage (which makes the electronics easier).

You might want to check this out: http://www.instructables.com/id/Build-The-5-Microphone-Preamp/

What kind of microphone are you using? I’m assuming that it doesn’t need phantom power - which would mean a more complicated circuit.

That kit you posted isn’t really aimed at this kind of thing, but it might just work. It says it provides a gain of 100x, so could be at the low end. Worth a try and if nothing else it could be a good source of components for a second attempt. The thing you’ll need a keep an eye out for is that the microphone is providing a balanced signal (two wire both containing the signal but at opposite voltages), and the line output is going to be unbalanced (just one output wire). The lazy approach to this is to ignore the balanced signal and just use one of the signal wires - but you could get a noisier signal.

At the core of this, you’ll want to check out this Wikipedia page about op-amps. They’ll be your friend in this quest. https://en.wikipedia.org/wiki/Operational_amplifier

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#3

Thanks! Beachtek do dslr preamplifiers that are passive, not sure how they work without batteries but they seem quite popular. Other systems use 9v batteries which seems to give a higher output. I basically need a nice high output signal to makeup for the poor in-camera pre-amps.

I found this that looks interesting, not sure I could power it from 9v though? I’ll be using a boom condenser mic like the rode ntg-2

http://www.fivefishstudios.com/diy/doa12/

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#4

Do you need phantom power?

If not any low noise opamp will do it from
tl 071
op 27

Ne5534
Ect

I doubt that you will need anything better than the op 27 or ne5534 . Tl071, tl072, tl074 are ok but very cheap

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#5

the ntg-2 needs power but can take an AA battery instead no real need yet
although something upgradeable would be nice.

What do I need besides the op amp and jack/XLR sockets to make things work off a small battery? Could use 9V but something rechargeable would be nice.

#6

Two pp3 rechargeable batteries

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#7

Great. i’ll have a go at building something based on the op 27. I’m also getting an ntg2 since I need it for a job next week. Should give us really good sound for the induction vids!

#8

You’ll need some other small discrete components as well as the op-amp chip itself. If you get the op amp as a kit, those should come with it, otherwise you’ll need to do some soldering and circuit design yourself. If you’ve not done that kind of thing before the kit might be easier as a first attempt.

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#9

https://www.eeweb.com/blog/extreme_circuits/balanced-microphone-preamplifier

http://ethanwiner.com/spectrum.html

http://www.circuitdiagramworld.com/audio_circuit_diagram/Microphone_preamp_schematic_circuit_21296.html

Op 37 is the de comp version of op 27 prity much the same opamp

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#10

First circuit seems most like what I need but it’s designed for dynamic mics up to 200 ohms. The mic I’ll be using is static and 250 ohms. Can I modify the circuit to take this into account?

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#11

Yes should not be a problem

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#12

You need 48v phantom power.

See link 2

And a few diodes for protection

I would drop the gain and put a second stage in with a pot to set the levels

You could up the input Z but 1 k should be fine.

That is about right for the first stage but maybe drop the two100k resistors to 22 k and put a bit more gain in a second stage or just use the microphone input on the camera as second stage

This should do 48v with a few mods

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#13


lEN

1d

Hi. I am in Toronto, Canada and this is the first time on this web site. For a low output/low impedance mic I am trying to build a preamp that is powered by 48 volts phantom power from the mixer the mic signal is going to. Of course no DC voltage from the phantom voltage must get to the microphone because I will be connecting a balanced dynamic mic or a ribbon mic to the preamp. I figure that maybe a fet or two fet transistors would be used in this circuit or a single dual IC such as a 5534. Thank you for any help that you can give to me. I keep searching the web but I keep coming up with circuits that show preamps but not how the amplified audio output goes into the mixer that provides the 48 volts. Thanks to you in advance for helping me out with this problem.

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