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Need help for electret mic preamp

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Hyperion1

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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.

https://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
 
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.
 

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Here's the circuit

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.
 

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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.
 

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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.
 
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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.
 

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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.
 
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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.
 
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.
 
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.
 
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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?
 
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.
 
Sorry, Mneary.
I thought he was going to use a noise-cancelling communications mic.
If the airplane has a pistons engine and propeller then the ordinary mic will be extremely noisy.
It will sound better if he eats the mic and screams. Then the gain can be turned down.
 
I have a similar problem as above...

Please can someone help me out...

I'm in the South African Mountain Club Search & Rescue Team. We do allot of work around helicopters (the Oryx, a beast of a bird) from what we have measured its an environment in the region of 114dB. We are jumping in and out; going up and down a hoist cable and working with ropes etc etc. We also need to talk to the pilots (AM radio) so ideally a throat microphone suits us best. We also need to talk to each other (VHF radio). As you may well know they both need different microphones and after loads of pre-amps and rolls of solder I'm still none the wiser! It doesn’t help when you know diddly SQUAT about electronics... I'm a carpenter for piets sake!!

We already have one working "Suicide Bomber Kit" that connects both Ground radio and Ground to Air radio to one Mic and one moulded in ear speaker through a "black box" we have also connected a voice recorder to the black box to capture everything being transmitted.

So my problem is this: I cant get the circuit to work with my IC-A6!
Is someone able to work out what is required so I can make it up?
If it could be put in dummy form that would be even better :) and if you’re able to give me a Vero board layout, now that would mean I owe you a case of beer and a pizza!!!:D
 
Sorry for the delay

Apologies for not coming back to my own thread...

The amp is working great. The mic that I am using is a Nady HM-10 non-noise cancelling headworn mic. Its not a Countryman but its not an el cheapo pc mic either.

When David Clark says their M7a mic is noise cancelling, they are at least mostly referring to the fact that the capsule is an open design allowing two sound paths to the front and back of the mic diaphram. Ambiant noise flows down both channels and cancels where voice only comes through the "front" passage of the mic assembly. The amp is located in the mic assembly and might have some filtering circuitry; I didn't want to go tearing one appart. Wouldn't know what the components were anyway. But you're right, it sounds like crap.

The aircraft that I fly in have noise gates built into the intercoms so all I had to do was to tweak down the pot until the background signal fell below the threshold during normal operations. BTW the cabin of a flying C172 is about 90-95dB at low altitude (drops as you go up) which is a little quieter than a live stage environment which is what this mic is intended for. If I really wanted to drop the noise I'd need a notch filter and its really not that bad.

I knew that I was going to have to place the mic very close to my mouth anyway so that's how I set it up. It could probably use a band pass filter but more for efficient transmission power usage rather than sound quality. There is no point in transmitting 5khz as the receiving station will filter it out anyway, just wasting power. Although, when I get on the radio, ATC knows who I am...:D

The only other problem I ran into is that on a TRS mic plug in the studio we use the tip as first signal and ring as second (stereo mics) with grounding on the sleeve but in aviation the mic signal is sent on the ring not the tip. Don't know why but it is.

As far as an open helicopter...? Geez, all I know is that helicopters typically use much lower Ohm mics than GA aircraft, same with military. It should be possible to build a splitter that sets the resistance and output for each type but basically you'd be wearing a little mixer on your belt.

Thanks to all,

Tom.
 
My car has a good muffler. I can barely hear the engine running and my neighbours can't hear it.
But everybody is wakened by a little airplane at night when it is thousands of feet above us.

Why don't private airplane engines have mufflers? They don't cruise at full power so the loss of a little amount of power by the muffler won't be noticed.
They are not in a Formula 1 race. A modern muffler is not very heavy.
Then everybody who is near or in the airplane won't be deafened.
 
Indeed quieter helicopters would be far more pleasant... but until then I need to deal with the flaws in other people’s designs :)

I've tried to attach the circuit I've been using (hope it comes up).
If not I'll try again... Do you guys have any comments? What should I change? Hyperion1: Did you use Adioguru's circuit? Did add or change anything? If possible I'd like to stay away from an extra battery supply but if you gurus recommend it I give it a bash...

All I need to build a circuit that my IC-A6 understands. I would imagine this would just the same as found inside any normal aviation headset... It doesn't need to be military spec or the same as the ones found inside a chopper?? Or does it?? I would imagine if I wanted to plug into the intercom system then it would; correct?
 

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