Olympus Microphone/s powering

DRONE7

Member
2008-08-21 11:12 am
I have bought cheaply a pair of well specd Olympus mics designed to work with their Pro dictation/audio recorders.
ME30W Microphone Kit | Olympus Pro Dictation

The manual indicates that they are powered from the Olympus devices...
So I am considering powering them like this...
YouTube
Recording and Measurement Microphones
audio - Wiring an electret condenser microphone? - Electrical Engineering Stack Exchange

2,2k resistor and 100mF cap 3volt supply

Any comments appreciated....I'm a microphone newbie..:)

Ideally I want to use them as room eq and speaker test kit.
 
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DRONE7

Member
2008-08-21 11:12 am
@rayma
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Thank you for your concern... Fortunately we are quite some distance (and a vast sea) away from the fires in Australia.
How Far is it Between Sydney, Australia and Auckland, New Zealand

Currently we are experiencing some odd colours to our skies and sunrise/sets due to the ash clouds blowing our way with a local sampling station reporting particulate matter above WHO guidelines.

However, they are all minor effects in comparison to what our poor Australian neighbours are going through...The bushfires there are horrendous with several hundred homes lost, multiple human fatalities and uncountable loss of native wildlife (estimate to date 480 million creatures !)...with no end in sight.
 
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DRONE7

Member
2008-08-21 11:12 am
Yep, what Ray said. :up: Most mic power on 1/8" connectors is 2.5V with the resistor and cap built into the recording device. The circuit show simply supplies all that stuff externally.

Interesting....so possibly if I can measure 2.5V or thereabouts from a mic input on something like a computer sound card would that indicate the electret could be connected directly ?

If making an external 9v circuit for better noise performance what precautions, if any, would be needed to connect to such an already powered input..?
 

rayma

Member
2011-04-29 8:37 pm
If I can measure 2.5V or thereabouts from a mic input on something like a computer sound card
would that indicate the electret could be connected directly ?
If making the 9v circuit what precautions, if any, would be needed to connect to such an already powered input..?

The circuit needs a direct power supply connection to the 2k load resistor to work,
so it can't use another circuit's DC voltage hat might be present unless a third wire is connected.

Just use the output coupling capacitor to block the electret's DC from being applied to the input.
 
If the input provides mic bias already, adding external biasing circuitry in front will only ever make things worse, given that both bias resistors are in parallel small-signal wise. Less signal, more distortion.

Mind you, the mic bias voltages generated by onboard audio in particular often leave much to be desired in terms of cleanliness, generally dominating input amplifier stages by quite a bit even though these can be rather more noisy than you'd really want as well. Occasionally you'll also find performance marred by ground noise from internal ground loops.
Since those same inputs tend to accept some rather healthy input levels at the same time (assuming a lower step of preamplification is used), using an external preamp seems like an attractive proposition. Usually about 20 dB with low output impedance (to dominate any noise coming in via bias resistors) will do the trick.

While microphones based on small electret capsules are never going to be the last word in noise level (it doesn't get an awful lot better than about 30 dB SPL, A-wtd), the poor linearity at high levels can in fact be addressed:
Siegfried Linkwitz proposed hacking the capsule itself to rearrange the FET amplifier circuit from common source to source follower - which basically works but is fiddly and may take a number of attempts before producing a capsule that is actually working and not just ruined.
Johne Conover suggested making use of a second external cascode JFET and letting semiconductor physics sort things out (see around Fig. VIII and onwards). For good results, said second JFET should have a somewhat larger Idss and a substantially higher Vp than the one inside the capsule. I imagine that going with a higher supply voltage would enable using even higher Vp parts than the 2N5484 he had; also some gain trimming is likely to be required for good stereo channel matching since output level would depend directly on transistor parameters known to be potentially quite variable.

Arguably it's all a bit of a moot exercise these days, as you can get a decent enough pair of phantom-powered pencil condenser mics around the 100€ mark on this side of the globe, with capsules that are like 3 time the size in spite of being called "small diaphragm", giving more attractive self noise and level handling right out of the box.