Miniature dynamic mic pickups out of 140-160 Ohm earplug capsules

[Mikheil]

Hello,

I got recently interested in building new pickup-system for my new OM acoustics. In other instruments I do have installed little ECM Oktava microphones, however due to a significant costs and cutting the new guitar's body for some on-board preamp/EQ, I would like to build a new dynamic mic-based pickup this time .....

Being not sure of those little dynamic microphones available locally, I tried and tested few high-quality earplug capsules by Sennheiser, Nokia and others ....... the results were really impressive but only with higher coil-impedance 120-160 Ohm capsules by Samsung; Sennheisers and Nokias were only of 24-32 Ohm .....

So this is how the story begun! And now, I am searching for some PRE-preamp circuit to get the <1mV sensitivity capsules boosted up to 20mV ........

Does anyone experimented with such little dynamic capsules ? Any work-horse PRE-preamp circuits available ?

Thank you for your feedback!

[bbsailor]

Hello,

I got recently interested in building new pickup-system for my new OM acoustics. In other instruments I do have installed little ECM Oktava microphones, however due to a significant costs and cutting the new guitar's body for some on-board preamp/EQ, I would like to build a new dynamic mic-based pickup this time .....

Being not sure of those little dynamic microphones available locally, I tried and tested few high-quality earplug capsules by Sennheiser, Nokia and others ....... the results were really impressive but only with higher coil-impedance 120-160 Ohm capsules by Samsung; Sennheisers and Nokias were only of 24-32 Ohm .....

So this is how the story begun! And now, I am searching for some PRE-preamp circuit to get the <1mV sensitivity capsules boosted up to 20mV ........

Does anyone experimented with such little dynamic capsules ? Any work-horse PRE-preamp circuits available ?

Thank you for your feedback!

[ATTACH=CONFIG]49307[/ATTACH]

Consider using a Shure A95U series of microphone matching transformers. Wire your dynamic capsule with two conductor shielded low impedance microphone cable with an XLR connector on the end of the cable to go into the XLR transformer input but with the transformer located at the amp high impedance input. This will minimize cable capacitance effects on high frequency response alterations and noise pickup. If noise is a problem wrap the dynamic capsule in a metal screen material that is grounded to the microphone wire ground.

Joseph J. Rogowski

[Mikheil]

Hi Joseph,

Thanks for your comment and thoughts!
Well, in-brief on what I am looking for: this should be low-voltage <9V PRE-preamp circuit with 1-2 transistors, or alternatively, one Op-amp (more expensive option!) of LT, INA, OPA, THAT, MAX series etc....... further, these two capsules should be blended and then going out by a stereo End-pin jack which might carry a second pickup's out as well .......

There are many circuits found in the internet but I really would prefer to having some tested versions of the work-horse circuits as I do not wish to experiment with components and values etc .......

I have built this way combo-amp copying Fender Acoustasonic preamp last year, it was just a couple of hours to wiring the circuit which is ready-to-go requiring no calibration nor variations; I can not afford to wasting time for just potentially relevant circuits, looking only for tested ones.

Thanks!

[J M Fahey]

I can not afford to wasting time for just potentially relevant circuits, looking only for tested ones.

Youīll have to search around for some commercial dynamic capsule preamp which works with 9V, and copy it, not sure somebody will custom design and, more important, develop ("waste time") such a preamp for you, just for the asking.

Personally Iīd look for a couple such commercial preamp schematics for "inspiration" , nothing wrong with that, fire up the old Protoboard and start experimenting.

As of:

Op-amp (more expensive option!)

Iīd think quite the contrary, even best Op Amps cost peanuts.
Only caveat would be, since it seems your project will be powered by a 9V battery, that among similar possibilities you choose the less power hungry one , for best battery life (so no NE553x and similar current hungry ones).

The beauty of Op Amps is that, at least in theory, "they are all the same" by definition, so you may get a preamp which does what you need, and replace the original 50 cent Op Amp with a high quality very low noise NASA spec one which will cost 2 bucks.

[Mike Sulzer]

For a low impedance, low output voltage mic you would generally avoid using an op amp as an input stage because of high noise voltage. A transformer seems so easy, but if you do not want that, then you might consider a circuit with a bipolar junction transistor front end since this device can give very low noise voltage and the noise current is not an issue with low source impedance. (Some JFETS also give low noise voltage.) http://jap.hu/electronic/micamp.html is a typical circuit. You do not need more than a gain of about 10, and so you might consider a simple two transistor circuit, although a low current op amp that can operate on a low voltage would not be too hard on the battery.

[Mikheil]

Thanks Juan,
The real trouble is that all those super-quiet Op-amps (LT, INA, THAT series etc) which cost $5-10 are not available locally, ordering overseas costs far more, so no sense to look for them.....
LM4562 and OPA2134 op-amps are the best available locally, not really expensive ($5-3) and certainly might be an option.....
But, I am still looking the tested schematic with all the required specs......

[Mikheil]

For a low impedance, low output voltage mic you would generally avoid using an op amp as an input stage because of high noise voltage. A transformer seems so easy, but if you do not want that, then you might consider a circuit with a bipolar junction transistor front end since this device can give very low noise voltage and the noise current is not an issue with low source impedance. (Some JFETS also give low noise voltage.) http://jap.hu/electronic/micamp.html is a typical circuit. You do not need more than a gain of about 10, and so you might consider a simple two transistor circuit, although a low current op amp that can operate on a low voltage would not be too hard on the battery.

Hi Mike,

Thanks for sharing your thoughts!
Yes, I do hear such a comment on using bi-polar transistors repeatedly, many people suggest such inputs over using op-amps ...... I was also told to use 'common base preamp' design which potentially might has even lower noise-specs and higher gain etc......

I also looked into old Hi-Fi amps (high-class Soviet audio) and their schematics and made several interesting cuttings from the circuits, however they all use dual-supply of 18-22 V and I have no idea on how to modify it to 3-4 v .......

Attached are few examples of those recommended circuits, what would be better to look for ?

[J M Fahey]

Try the single transistor one.

[Mikheil]

Try the single transistor one.

Common-base vs Common-Emitter mic preamp circuits: real and visible values for <1mV sensitivity dynamic mic inputs ??

[Mike Sulzer]

Common-base vs Common-Emitter mic preamp circuits: real and visible values for <1mV sensitivity dynamic mic inputs ??

I see no reason to use common base. Dynamic microphones are expected to give better frequency response when they look into an impedance a few times larger than their output impedance rather than loading them down with a lower impedance. Signal to noise should be somewhat better also. A simple single transistor circuit might have more gain than you want with a bypassed emitter resistor, but you can always unbypass part or all of it to give some feedback.

[Mikheil]

I see no reason to use common base. Dynamic microphones are expected to give better frequency response when they look into an impedance a few times larger than their output impedance rather than loading them down with a lower impedance. Signal to noise should be somewhat better also. A simple single transistor circuit might have more gain than you want with a bypassed emitter resistor, but you can always unbypass part or all of it to give some feedback.

Thanks, Mike!
Sure, I know that low-impedance dynamic input should have around 10 times higher impedance, so it should not be less that 2.5-3 kOhm ...... however, 200 Ohm dynamic mics may perform well plugged into high-impedance inputs, as an example, in my actual guitar I have ECM Oktava mic with 250 Ohm impedance (with miniature audio-transformer) which is wired to B-Band on-board preamp/EQ with 10 mOhm input impedance and all works just superb........

Well, I have some 1, 2, and 3 bipolar NPN transistor base circuits too and I am preparing to build and test few circuits at once .....

[Mikheil]

because of high noise voltage. A transformer seems so easy,

Mike,

I would like to ask on Audio transformers ......

I have one little mic transformer of Oktava-5 mic which is very small around 1x2 cm, it has 6 pins (see on to attached wiring image); I do not remember precisely but it has around 600 Ohm coil-impedance and pin-outs in the centers of coils ......
This tiny transformer has two contact input from the side of microphone and balanced 3-contact out which I use as 2-contact out for guitar's preamp......

Would such ECM mic transformer work with dynamic capsule ?

[Mike Sulzer]

Would such ECM mic transformer work with dynamic capsule ?

I think it would work. Possibly, the voltage step up might not be quite as high as you would like, but try it and see.

[Mikheil]

Dear All,

Just finished with circuit and testing it with capsules, the result is phenomenal! I was thinking it might only potentially be possible...... what I mean: I was hoping that these dynamic capsules might function not only as mics capturing sound waves but also capturing vibration by their metallic housing, and, Yes, they do both superb ......

More details:

1. Noise - far low than Zero
2. Gain - it really is too much, 1V or even more
3. THD - far low than Zero

I only need to cut some LF and raise Highs little bit ..... input and output caps should be replaced to <1 mF to cut Bass, but I do not know how to add more highs...

Any thoughts ?
Thank you!

Schematic here:

[J M Fahey]

Those are earphone capsules, not real microphones.

You have what you have and be grateful for that, you canīt boost highs which were never there to begin with.

I guess your guitar also use Piezos under the bridge, which tend to have way too much Highs, a judicious mix of both pickup outputs will give you a very balanced end result.