Audio amplifier

[zachtheterrible]

Hi all,
I just bought a 900MHZ RX and TX that can transmit an analog signal. I want to modulate the TX with the signal from an electret mic.

The transmitter requires a 1V peak to peak signal. I tried one of the audio amps from a "bug" FM transmitter without realizing this requirement, and I could only hear something on the other end if I blew really hard into the mic. I'm looking for an amplifier that will give me that 1V peak to peak. Would an LM386 do it? Unless I got my calculations wrong (most likely I did) it would only give me a voltage of .2V. The signal from an electret mic is tiny. The operating voltage is 3v.

Any ideas? I know it's painfully obvious :lol:

 

[audioguru]

Hi Zach,
You need a mic preamp with a voltage gain of from 33 to 100. The minimum supply voltage for an LM386 is 4V. A few "ordinary" opamps have a min supp v of 3V but your battery will probably drop to 2V over its life. There are millions of new surface-mount opamps made for low-voltage cell phones and MP3 players but I haven't tried them.

An MC34119 bridged little power amp has a min supply v of 2V and would do the trick. It is an 8-pins through-holes IC and its supply current is from 2.5mA to 4mA. :lol:

 

[mstechca]

you could make a voltage divider from two resistors in series from VCC to ground. The resistor connected to VCC should be about 9 times higher than the resistor connected to ground. Then you can use the voltage from the center of the two resistors.

Here's an example:

If you have 10V, connect 1K to ground, and 9K to VCC.

1K / (1K + 9K) = 1 / 10 = 0.1

0.1 * 10V = 1 volt!

I don't know if this will completely work, but it is a start.

The resistor values should be high enough to prevent excessive current from entering the circuit.

 

[audioguru]

MStechca,
I don't understand how your 2 resistors are going to amplify the 30mV p-p signal from the microphone to 1V p-p for the transmitter.
The supply is 2V to 3V not 10V.

 

[mstechca]

I don't understand how your 2 resistors are going to amplify...

You don't need to, because I was explaining how to get a lower voltage from the supply.

The supply is 2V to 3V not 10V.

Then the resistors need to be adjusted accordingly.

Just use: R1 / (R1 + R2) * supply

and the result is the voltage to use.
R1 is the resistor connected to ground, and R2 is connected to VCC or +ve.

For a 3V supply, use 1K for R1 and 2K for R2
For a 2V supply, use 1K for both R1 and R2

Increase both resistances by the same number (pick the number) until the current provided doesn't blow up the rest of your circuit.

and PLEASE do NOT use ONE OHM for R1 and ONE or TWO OHMS for R2.
because then you will be delivering AT LEAST 1 1/2 AMPS from the measly 3V supply (because I=V/R). worse yet, if you are running 3V off a battery, that battery will most likely die in the next 5 to 10 minutes (or even less).

 

[zachtheterrible]

Audio, I think that'll do the trick just fine. How much gain do those mic preamplifiers in the FM transmitter circuits generally provide? A 2n3904 transistor can have a gain of 200 at the most if I recall correctly, and you're saying I only need from 33 to 100, and its very safe to make an amplifier using about 1/4 of that transistor's maximum gain. Could I make an amplifier with a transistor, seeing as I don't have a MC34119 on hand?

Mstecha, as battery voltage drops, the voltage in the voltage divider is going to drop as well, making it necessary for me to change resistances. I'd just use a voltage regulator, which does that automatically. My voltage supply isn't 10v, it is two AA batteries: 3V and 2V as they die.

 

[zachtheterrible]

Well I just found out that the reason that I had such a low output voltage from that microphone amplifier of mine is because I was using a piece of crap electret microphone. Now that I used a decent one I can get 1v peak to peak signal without the need for any more amplification. Thanks for the help though :lol:

 

[audioguru]

Hi Zach,
I dunno what MStechca is doing making a mic preamp from 2 resistors.

I simm'd the mic preamp from this popular FM transmitter. The results are awful as shown in my next post.

 

[audioguru]

Hi again Zach,
The Sensitive FM Transmitter's mic preamp probably works OK in its circuit if you don't speak loud enough to have more than about 30mVp-p of output. With 30mV input then the transistor saturates like mad because it is incorrectly biased and its gain is too high. So I added an emitter resistor to reduce its gain and made it closer to correct bias.
It has more than 1Vp-p output with a 2V and 3V supply, with about 10% distortion.

This sim is with a "typical" transistor and changes a lot if the transistor has a higher or lower current gain.

 

[mstechca]

I dunno what MStechca is doing making a mic preamp from 2 resistors.

NO! :lol:

The two resistors make a voltage divider, not a preamp. Don't you already know that resistors DO NOT provide amplification by themselves?

:shock: :shock: :shock: :shock: :shock: :shock: :shock:

 

[Dr.EM]

I know what you were trying to do, lower the 10v supply to 3v, but i'm not sure where you got the idea he was using a 10v supply?

Zach, did you buy the new mic, or did you just find a better one in your "junkbox"? The Panasonic wm-61a are very good, I want to get some without having to order from america and pay £22 extra :cry:

 

[audioguru]

The Panasonic wm-61a are very good, I want to get some without having to order from america and pay £22 extra :cry:

An electronic surplus parts store on the web in America sells these Panasonic mics very cheaply, about $1.00US. They are all rejects AND DON'T WORK! (but they look pretty). I bet Zach got one.

The electronic manufacturers I worked for DESTROYED any defective items to prevent them from getting on the street. They were neatly put in the garbage in the evening and next morning stuff was all over the place. I don't think the mess was caused by racoons looking for old parts to sell!

 

[Dr.EM]

They are only about $1 each on digikey too, but they have £10 handling charge on "small" orders (I don't need 75+ mics :shock: ) and delivery to here is £12, then theres tax. Its quite absurd really, pity theres none on ebay.

 

[audioguru]

Hi Dr. EM,
You're being ripped-off by Digikey! :shock: :shock:
1) You pay them a handling charge so they can run down the street and buy a bunch of rejects from the surplus joint.
2) You pay some more so they can test them to find a few "good" ones.
3) 12 pounds is rediculous for shipping unless you want delivery on the same day as your order. How? The Concorde doesn't fly anymore.
Is your Queen's Post Office so busy with radio and TV that they don't receive mail anymore?
4) Your government charges tax for something that isn't even bought there? Over here, all shipments over the border are marked in big letters, "GIFT" because they are tax-free.
Actually, I think Digikey sneaks stuff across the border here since I order it from digikey.ca. Don't you have a mirror site called digikey.uk? :lol:

 

[Dr.EM]

There is some kind of UK digikey thing, not too sure about the tax, but still definately a rip off. Haven't actually bought any but yet but it still seems the only way. There are some decent looking Panasonics at CPC Farnell though, they might have to do, but they are £3 each too anyhow.

 

[mstechca]

1) You pay them a handling charge so they can run down the street and buy a bunch of rejects from the surplus joint.

omg :shock:
good thing I don't BUY my "toy"(according to audioguru) transmitters and receivers.

2) You pay some more so they can test them to find a few "good" ones.

Are you assuming this?

3) 12 pounds is rediculous for shipping unless you want delivery on the same day as your order. How? The Concorde doesn't fly anymore.
Is your Queen's Post Office so busy with radio and TV that they don't receive mail anymore?

What's pounds in dollars, and how far away is the destination?

Actually, I think Digikey sneaks stuff across the border here since I order it from digikey.ca. Don't you have a mirror site called digikey.uk? :lol:

Don't you guys know that maybe there is a "LOCAL" electronics store?

Audioguru, you should know MUCH better :shock:

There is SAYAL electronics for you. There are 5 locations. Check at sayal.com for a location near you. Yes, there is one in missisauga! The only extra thing you pay is the TAX. There are NO shipping charges when you go into the store and buy the parts.

and ordering online is a NO-NO! :shock: :shock: :shock:
especially when I hear that one guy ordered something from ebay, and the order was not processed correctly. In fact, he shared his story on here about a week or two ago.

 

[audioguru]

MStechca,
I order most of my parts online at Digikey and Newarkinone without fear.
They are big companies with excellent encryption of your credit card info. They have a huge stock of Name-Brand parts at good prices and a discount for quantities.
They don't have a handling charge if your order is $25.00 or more.
They deliver cheaply by courier the next day of even cheaper by mail in a couple of days.

I have Sayal, Active, 2 Chinese stores and an Arabian store locally. I buy "emergency" stuff like switches and pots that I can look at, plastic enclosures, strip-board and surplus switching wall-warts there. Some LEDs I bought at Sayal didn't work but were replaced by them, the Chinese ones had the colours mixed-up and log pots were linear and vice-versa.

 

[Dr.EM]

Yeah, I have a Maplin electronics fairly locally now, but like Audio, its for emergency parts only really. My large or more obscure orders come from Rapid electronics. Great service from them, just they don't have those mics. The Digikey order would have to be over £75 for me to have the handling charge removed :shock: must be a UK thing again...