# How to achieve 35 millivolts peak to peak with normal speech 1 inch from electret?

#### rabbiporkchop

Joined Apr 7, 2017
16
I plan on using this electret

I plan on using a schematic similar to this except I was planning on using a 1.5 volt Type N battery.

The input impedance of the amplifier circuit is roughly 600 ohms.
I read somewhere that changing the pull up resistor value changes the gain of the circuit. Is this true?
If the gain is a fixed value I have plans to build an appropriate attenuator pad to reduce output to 35 millivolts but I would prefer not to do this if possible.
I was wondering if anyone would be able to help me calculate values of resistors to achieve the desired output level of 35 millivolts into a 600 ohm load powered by a 1.5 volt battery...

#### MrChips

Joined Oct 2, 2009
23,290
An electret microphone has a built-in FET in its circuitry. The external resistor is the pull-up or load resistor.
There is an optimum value for this resistor. You can try increasing the value of this resistor until you get the desired amplitude. Or you can set it for the optimum setting and use an attenuator after the AC coupling capacitor.

#### ronv

Joined Nov 12, 2008
3,770
I plan on using this electret

I plan on using a schematic similar to this except I was planning on using a 1.5 volt Type N battery.

The input impedance of the amplifier circuit is roughly 600 ohms.
I read somewhere that changing the pull up resistor value changes the gain of the circuit. Is this true?
If the gain is a fixed value I have plans to build an appropriate attenuator pad to reduce output to 35 millivolts but I would prefer not to do this if possible.
I was wondering if anyone would be able to help me calculate values of resistors to achieve the desired output level of 35 millivolts into a 600 ohm load powered by a 1.5 volt battery...

You will need a gain of about 100 to amplify the output of the MIC to 35 mv.
It would be easier to use 9 volts instead of 1.5 volts to power the amplifier.

#### rabbiporkchop

Joined Apr 7, 2017
16
You will need a gain of about 100 to amplify the output of the MIC to 35 mv.
It would be easier to use 9 volts instead of 1.5 volts to power the amplifier.
Do you think another electret would be a better choice for 1.5 volts in for 35 millivolts out?

#### rabbiporkchop

Joined Apr 7, 2017
16
Go for an Hearing aid amplifier, or use an Op amp for bigger gain...

http://www.talkingelectronics.com/projects/HearingAid-2/HearingAid-2.html
I haven't had an opportunity to measure the output level but I'm pretty sure that it is higher than I would like. I'll get around to measuring it tomorrow. I definitely have some feedback issues that need to be looked at. As you can see in the picture I have very limited space inside to work with,
Hence my battery choice.

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#### rabbiporkchop

Joined Apr 7, 2017
16

#### joeyd999

Joined Jun 6, 2011
4,477
I found an interesting article that seems to contradict everything I believed about impedance matching.
There is no contradiction. Impedance matching makes sense for two applications: max power transfer and transmission lines.

If you're signaling over a distance considerably shorter than the wavelength, impedance matching just wastes power.

On the other hand, low OI and high II is asking for trouble on long lines.

#### rabbiporkchop

Joined Apr 7, 2017
16
There is no contradiction. Impedance matching makes sense for two applications: max power transfer and transmission lines.

If you're signaling over a distance considerably shorter than the wavelength, impedance matching just wastes power.

On the other hand, low OI and high II is asking for trouble on long lines.
Do you think if I did the same thing the other guy did and place a 22k resistor in line with my audio wire that any distortion would happen or would that alter the frequency response? If it does simply reduce the amount of power with no negative effects I think that's probably the route I would choose.

#### joeyd999

Joined Jun 6, 2011
4,477
Do you think if I did the same thing the other guy did and place a 22k resistor in line with my audio wire that any distortion would happen or would that alter the frequency response?
How much could it possibly cost to construct it and find out? Call it a cheap education.

#### rabbiporkchop

Joined Apr 7, 2017
16
I finally found the circuit I was looking for.

#### GopherT

Joined Nov 23, 2012
8,012
Here is everything you want to know about an op amp controlled electret microphone.

Just adjust the gain resistor to get your desired output voltage swing. All of the math is shown in detail to get you exactly to your goal.

http://www.ti.com/lit/ug/tidu765/tidu765.pdf

#### rabbiporkchop

Joined Apr 7, 2017
16
You mic already has the jfet built in.
You will need to amplify it, not attenuate it to get 35 mv.
What are you trying to do?
Give this a read:
http://www.sengpielaudio.com/calculator-transferfactor.htm
I may have been wrong on desired output level. It is over driving my transceiver as is.
It definitely needs attenuation. I have no oscilloscope for precise measurements. I'll be experimenting in the next week when my 50k audio pots show up in the mail.

#### rabbiporkchop

Joined Apr 7, 2017
16
You will need a gain of about 100 to amplify the output of the MIC to 35 mv.
It would be easier to use 9 volts instead of 1.5 volts to power the amplifier.
I suspect my millivolt number is probably incorrect and maybe closer to 3.5 millivolts

#### ronv

Joined Nov 12, 2008
3,770
Yes, Makes sense.
I think I would try this:

#### rabbiporkchop

Joined Apr 7, 2017
16