### Author Topic: Microphone circuit basics  (Read 844 times)

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#### aurmer

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##### Microphone circuit basics
« on: March 14, 2018, 05:37:39 am »
I have a media comms system which we are using for live communication between director and camera operators. The complaint is that turning up the comms to 100% volume is still too quiet. I believe we have an impedance matching problem.

So I contacted the manufacturer to ask what the comm device was designed for (we have 200ohm mics right now with 64ohm headphone speakers). They responded saying the following.

Quote
-The aviation headset headphones are dynamic with an impedance of 300ohm. The comm system supplies a bias voltage of 10Vdc. Microphone impedance is 50 to 600ohm with an operating voltage of 8-28Vdc.
-Itâ€™s also OK to use aviation headsets which provide their own microphone bias voltage.

So I am a bit confused, and I seek to learn a bit more about these circuits. How can a range of 50 to 600ohm be right? Wouldn't I want to just match the impedance of the jack itself? With that in mind, I did the following test... I plugged a naked plug into the mic jack and I measured the voltage given the loads seen in the attached image (This was tested on two different communications devices within the system. Results were identical).

My math tells me that, given my measurements, the source impedance is 2.7kohm. And it seems that I was able to confirm the "10Vdc bias voltage" claim.

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Now, here are my techincal questions.
- Given my situation, what is my best bet to getting more signal from a microphone? Should I expect a lower impedance microphone to be louder or a higher impedance to be louder?
- We want to use a dynamic mic. Should I be using an isolation technique to keep the Vdc bias voltage from reaching the mic? Shure educational material suggested this to me.
- Should I just skip this whole headache and get some inline mic preamps?
- And is it strange (or completely normal?) to have such an impedance from the bias voltage source? I mean, it seems that any typical microphone wouldn't see the full 10Vdc of bias voltage with so much of that voltage dropping before it gets out of the plug. Perhaps these mics only need 1-4V of bias voltage?

If anyone with experience in this could point me in the right direction, I would greatly appreciate it. TIA.
« Last Edit: March 14, 2018, 05:39:25 am by zetharx »
If I just asked the wrong question, shame on me for asking before I was ready for help. Please be kind and direct me to a resource which will teach me the question I SHOULD be asking. Thank you.

#### capt bullshot

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##### Re: Microphone circuit basics
« Reply #1 on: March 14, 2018, 09:53:47 am »
So, this is what I understood:

The manufacturers headsets use microphones with bias supply. These usually are electret type.
You want to use dynamic microphones. These do neither require nor like a bias supply.

Dynamic microphones in general have lower output level than electret ones, no matter of impedance or bias supply. I'm not familiar with levels of different impedance dynamic mics, but I'd expect higher levels from the higher impedance ones.

So what you shoud do: decouple the mic from the bias supply (the preamp should take care of this)
So what you need to do for more volume level: add a preamp (that can be supplied from the bias supply)

Anyway, take this as a more general advice, I don't know any details about aviation or other kind of headsets.
« Last Edit: March 14, 2018, 09:55:42 am by capt bullshot »
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#### aurmer

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##### Re: Microphone circuit basics
« Reply #2 on: March 14, 2018, 03:05:55 pm »
Thanks, I have reading to do about electret microphones, and I will head down to the local electronics parts store and get an electret mic for some testing.
If I just asked the wrong question, shame on me for asking before I was ready for help. Please be kind and direct me to a resource which will teach me the question I SHOULD be asking. Thank you.

#### Audioguru

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##### Re: Microphone circuit basics
« Reply #3 on: March 14, 2018, 06:29:31 pm »
An electret mic is a condenser mic with a Jfet inside to reduce the extremely high impedance. The electret material is permanently charged with about 48V so the mic diaphragm becomes a moving voltage divider. The Jfet needs to be powered with about 0.5mA and a few volts and have a series load resistor of about 10k ohms in series with the 9V power supply.

The output impedance of an electret mic is about 3k ohms. If the resistor powering it parallel with the input resistance of the preamp are also 3k ohms then its output level is cut in half.

#### whizgeek

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##### Re: Microphone circuit basics
« Reply #4 on: March 15, 2018, 12:38:51 am »
I think you may be hung up on Microphones. Maybe the current ones are full of dirt, and you can just replace them, but you have no mention at all of the other parts of the circuit.
Is there an audio IC powering the headphones ?

#### Cerebus

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##### Re: Microphone circuit basics
« Reply #5 on: March 15, 2018, 01:05:28 am »
An Electret microphone is just a capacitor microphone that relies on a permanently charged Electret to provide the polarizing charge for the microphone capsule.

Conventional capacitor microphones use an externally supplied high voltage supply to charge the capacitive capsule via a high value resistor (typically 1G or more). The charge voltage on both conventional and Electret designs can range from a few 10s of volts to a few 100 volts depending on the particular microphone capsule design. Most modern Electret microphones have small to very small capsules and use relatively low voltages.

The Electret in an Electret microphone can either form the back capsule or the diaphragm, higher quality ones usually use an Electret back capsule as the Electret film is typical heavier than standard capacitor microphone diaphragm film.

The operating principal is the same for all capacitor microphones. The charge on the capacitor is essentially fixed, so as the diaphragm moves backwards and forwards it alters the capacitance of the capacitor. This causes the voltage across the capacitor to change according to V = Q/C (where V = voltage, Q = charge and C = capacitance). (I don't know where audioguru gets the idea that any kind of voltage divider effect is involved, this is a pure basic capacitance effect.)

All capacitor microphones need a preamplifier. Because of the (essentially) fixed charge of the capsule it has a very high impedance and the preamplifier has to have a correspondingly high input impedance and is almost always nowadays realized with a junction FET as the input device. Back in the day vacuum tubes were used and are still used in some audiophool designs. The differentiating factor between Electret and conventional capacitor microphones is not the presence of the JFET preamplifier but the permanent polarizing voltage on the former and the externally applied polarizing voltage on the latter.
Anybody got a syringe I can use to squeeze the magic smoke back into this?

#### Bassman59

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##### Re: Microphone circuit basics
« Reply #6 on: March 15, 2018, 03:33:37 am »
I have a media comms system which we are using for live communication between director and camera operators. The complaint is that turning up the comms to 100% volume is still too quiet. I believe we have an impedance matching problem.

Is this a Clear Com system?

#### aurmer

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##### Re: Microphone circuit basics
« Reply #7 on: March 15, 2018, 03:38:07 pm »
I have one Sennheiser HMD 280 Pro and one Sennheiser HMD 281 Pro. Both headsets have the same tech. Both sound the same when used.

The system I am using comes from Blackmagic Design. They have two products with built-in comms which we use. The ATEM Studio Converter (published info on the comms feature can be found on page 7 of this manual) and the Blackmagic Studio Camera. These devices have no mic input gain adjustment. Correct me if I am wrong, but I am focusing on the microphone because (as long as we are committed to using this system) the mic is the only thing I can change.

Since I don't know enough about mic circuits, I think we will try simply buying other headsets from amazon with the plan to return what doesn't work. Placing a powered preamp between the headset and the input jack should work, but it would be only our last resort because there is no outlet near the camera tripod.
If I just asked the wrong question, shame on me for asking before I was ready for help. Please be kind and direct me to a resource which will teach me the question I SHOULD be asking. Thank you.

#### Zero999

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##### Re: Microphone circuit basics
« Reply #8 on: March 15, 2018, 10:58:25 pm »
The impedance doesn't need to be matched. Ideally the input impedance of the amplifier should be much higher, than that of the microphone.

Yes, it does sound like the communication system is designed for electrect microphones. Another possibility, if it's based on a very old design is it could be originally made for carbon microphones, which have and even higher output than electrect mics. The latter may seem crazy in a modern system, but it's often the case, even through the carbon mics are no longer used and replaced with electrect or dynamic mics, with a built-in amplifier, because the high level signal helps in high electrical noise environments.

In any case, a transistor can be used to amplify the signal from a dynamic mic, so it works with a system designed for electrect or carbon mics.

The attached schematic will enable a dynamic mic to be used with a system designed for an electrect mic. If it's not loud enough, it's trivial to increase the gain, but cross that bridge when you get to it.

« Last Edit: March 15, 2018, 11:01:18 pm by Hero999 »

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