Good morning,

I'm trying to build a battery powered electret preamp for use with aviation radios and I've tried a couple of circuits I've found on the net. Unfortunately, while I get enough amplification to use through a mixer in my studio, there isn't nearly enough output for use in an aircraft. I'm getting about 50mV peak from these circuits but I need about .75V at 1kOhm.

Could anyone point me to a higher output circuit please?

I can post the circuit I'm using now if needed. Perhaps I just have to tweak the resistor values but I'm not very good with the math.

Thanks,

Tom.

 

I don't know what circuits you've tried, but the one on this page might work.

http://www.rc-cam.com/rc-cam3a.htm

Scroll a little over half way down the page to: "(3) Microphone Preamplifier"

The output level can be adjusted bu changing the value of R2x. Increasing the value increases the output. You could replace it with a 10K resistor in series with a 1M pot, and adjust the pot to find a value that will meet your needs.

Ken

__________________
"To invent, you need a good imagination and a pile of junk."
Thomas A. Edison (1847 - 1931)

 

Post the mic preamp circuit you tried and we will see why its gain was too low.
Does your electret mic have 2 wires or 3 wires?

The camera mic preamp circuit is not good.

Attached Images

poor mic preamp.PNG (61.0 KB, 63 views)

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Uncle $crooge

 

Here's the circuit although I think I'm using a 2N3568 instead of a 2N3904. It's actually a commercial headset made by Nady, the HM-10 so I'm dealing with a tip-ring plug (I guess that means a two-lead mic, doesn't it?)

Tom.

Attached Images

preamp.JPG (22.5 KB, 69 views)

 

I simulated your horrible mic preamp circuit. An electret mic that is powered from a 10k resistor has a 3.3k impedance (R1 in my circuit).

With a 47k load then the gain is 24 which is not too bad.
With a 1k load then the gain is only about 2 which is not nearly enough unless you scream.

The transistor is not biased correctly because at high levels it clips the bottom. So even if you scream it sounds bad.

Attached Images

sim mic preamp.PNG (39.0 KB, 46 views)

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Uncle $crooge

 

So now that we both agree that it won't do the trick...

Could you point me towards something that will?

What I'm trying to build is a box that will plug into an aircraft radio's mic & headphone jacks, and allow the pilot to use modern in-ear-monitor earbuds and super light headset microphones. Sorry, I know very little about electronics. I've created the headphone path complete with mono/stereo switching and all I need now is a circuit that will lift the signal level from the electret mic output to a "carbon mic" level (aparently pilots used to use mics from early telephone systems and that standard stuck) which is higher than typical audio line inputs.

I could just rip a preamp out of an old headset, it's quite small, and put it in the box but I would like to learn something along the way.

BTW, I'm using a 9-volt battery as the power supply and because this is not a noise-cancelling mic it will be positioned very close to the mouth and will have to handle high SPL's. So figure the useful output will normally be very high in the mics range.

Thanks,

Tom.

 

Here is a high quality mic preamp for an electret mic.
It has a pot to adjust the gain from 1 to 25. Reduce the value of the 3.9k resistor for more gain if needed.
It has a high max output level with very low distortion.

Attached Images

electret mic preamp2.PNG (7.6 KB, 91 views)

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Uncle $crooge

 

Thanks!

There looks to be a resistor just prior to the output with just "47" under it. What is this?

Also, you seem to be using two different symbols for capacitors, is there a difference (and do I have to worry about direction with capacitors?)?

Tom.

 

The resistor labelled "47" is 47 ohms. It keeps the opamp from oscillating if a shielded cable connects to the output (shielded cable has capacitance to ground).
The 0.1uF input and output capacitors are non-polarized film type. The ones with a rectangle marked "+" are polarized electrolytic type.

Polarized capacitors must be connected with their "+" end toward a positive voltage as shown. New ones actually have their "-" end marked but the "+" end was marked instead many years ago and schematics always show the "+" end.

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Uncle $crooge

 

Last question (I hope)

Do you have a preferred supplier? I seem to be running around all over the place for these parts. Somebody preferably online?

Thanks,

Tom.

 

I buy parts online from Digikey and Newark (international Farnell) in North America. They have everything and if I order before 8:00PM it is delivered the next morning.

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Uncle $crooge

 

Nice amplifier, but it's too good. I'm gonna get killed for this, but here goes.

For aircraft communications (high noise environment), you need to cut off below about 250 Hz and roll off the highs. I would change C1 to 0.01uF, C2 to 0.1uF, and place 1000pF (1nF) across VR1. I would put 1000pF (1nF) across the microphone. I would also clip the output with 1N4148 diodes in series and antiparallel (4 diodes total) allowing more average voice output while keeping the radio from over modulating.

Yes, it will sound awful, as "bad" or worse than an old telephone.

 

Usually a noise-cancelling "communications" microphone already sounds bad like an old telephone. It cuts the rumble and cuts the hiss. Try the preamp like it is first, before you make it also sound bad.

What do two bad things connected together sound like? An awful throat microphone?

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Uncle $crooge

 

Preferred supplier? If you use ebay, then look at nfceramics. Look through their stuff and find something that contains all the parts you need (like item 7570417878?). Kits are in the range of US$10 and they ship to the US for $3.50.

 

I'm assuming he's using a common computer microphone...... he mentioned that it's not noise-canceling. In 100db-plus at 25 to 200 Hz, you really cannot afford quality.

Sure, try it first without the mods, but leave room for the few extra parts.