build a condenserr microphone

[überfuzz]

Hi! I have two small microphones lying around that I thought I try and use in my acoustic guitar. Have anyone seen or have a circuit to drive one with 9V? I have something simple in mind, something like this:



Cheers!

[soundguruman]

It really causes too much feedback for an acoustic. You need to be able to tune the resonant frequencies out of the audio.
That's why many are using magnetic PUs.
Microphone works for recording, but not very well for live sound.

[überfuzz]

Yeah... People told me so, but I'd like to have a go anyway. I have an EQ build into the guitar I'm going to tinker with. The EQ has a feature allowing for sweeping the, or moving the, attenuated frequencies.

[soundguruman]

Yes it has to be a parametric EQ, and it has to "cut" at least 30 db to work effectively.

[nickb]

Be warned, it quite common for these to contain a built it FET http://en.wikipedia.org/wiki/Electret_microphone

Anyway, here's a suggestion (if you got some 250Meg ohm resistors and a bit more than 9V).Neumann-u64.pdf

[überfuzz]

Be warned, it quite common for these to contain a built it FET hhttp://en.wikipedia.org/wiki/Electret_microphone

Anyway, here's a suggestion (if you got some 250Meg ohm resistors and a bit more that 9V).[ATTACH]24163[/ATTACH]

It do contain an "in-house" FET. I don't see how that will be problematic in an acoustic guitar though... My bet is that it will take much more punch than I'm able to get. Anyhow, if it turnes out to be problematic I should be able to modify the microphone in to a source follower. (It seems to be a common source follower, judging by a circuit diagram I found.)

[nickb]

I was just thinking that you don't want to use too small a drain load resistor else you vaporize your internal FET.

I'm not sure what you are getting at with the source follower. Are you thinking about driving the cable? The drain resistor will probably end up being around 10k - in the absence of any data for the microphone choosing the resistor to get 4-5V on the output will be close enough. You then have to drive the capacitance of the cable. Ten feet of coax could easily be 1000pF which will roll off at 15Khz. IMHO I wouldn't expect that to be a problem.

[überfuzz]

I don't quite follow you... Anyhow, The plan is to mount the microphone in a small housing, or some nifty way, with a 9 volt battery. I actually planed to start testing without the resistor. The microphone is rated 1.5 to 10 V. 9V drop over the microphone should be fine right?

[nickb]

I I actually planed to start testing without the resistor. The microphone is rated 1.5 to 10 V. 9V drop over the microphone should be fine right?

You will need the battery and a resistor. You won't get any signal otherwise. Try 10k ( not less than 1k) and measure the voltage at the resistor - the closer to 4.5V the better.

My point about the cable was you might loose a bit of the highs with a long cable.

[J M Fahey]

Start with 4K7 in series with the microphone and the 9V battery and tweak if needed, but it should start working quite well.

Couple it with a 1uF electrolytic to a 10K pot; Audio/Log if available.

Make some type of clamp to mount it floating in the middle of the soundhole.

Sounds good but boomy (normal because the guitar cavity *is* a low frequency resonator, exact same as a bass reflex cabinet).

I was *very* successful making a dual input preamp, using a bridge piezo for punch and loudness, plus the mouth electret , starting from "0" and *slowly* raising it until overall sound started getting fuller.

Very good sound with best of what both worlds could offer.

The combination had much of the character of a good voice condenser microphone placed very close to the guitar, at a small fraction of the cost.

[Enzo]

Regardless of what some think, there are good sounding commercial acoustic guitar pickups that are in fact small microphones mounted inside the guitar.

Fishman Rare Earth Blend Acoustic Pickup | Sweetwater.com

I used to get a pretty darn good sound out of my Gibson J-50 with an old EV lav mic, From back when lavaliers were even larger than an EL84. I made a small holder from duct tape and a coathanger, and hung the mic from the lower edge of the sound hole, pointing up across the hole. Worked good.

[überfuzz]

The microphone is done. It sounds surprisingly good, although the signal is somewhat weak. It definitely puts the pieso that was mounted in the guitar to shame.

Now it seems to be a mater of adjusting the tone by finding the optimal position. The way Enzo suggested is ok, I might end up with something similar.

[Enzo]

SOme of the ones I have seen have like goosenecks for the mic element to allow positioning. yes indeed, it matters where inside the mic sits. All kinds of resonances and things inside there. Mics can be inside or outside for different approaches.

[überfuzz]

Off topic
This microphone made me tinker and fiddle with this particular guitar. The guitar has a slight resonance around 440Hz (A). At first I started to suppress this range sightly with the build in parametric EQ. But hey, I like the sound of my guitar why mess about with the EQ now when I have a microphone with a much more linear frequency response. Argh! The big questions in life...

[Enzo]

You could pretend...


Decades ago, I played my acoustic guitar. I got a couple knobs - not controls, just knobs - and put them on suction cups. I then stuck the knobs on the face of my acoustic with the suction cups. Made it look electric anyway. Now and then while playing, I'd reach down and "adjust" a knob. it really confused some people. Other people got the joke.

But then it doesn't take a lot to amuse me.