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Electret microphone amp circuit ?

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audioguru

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Cheap shielded audio cables use two or three thin strands of wire for a ground. They make a terrible shield.
Good shielded audio cables use many strands of wire that are braided together to make a good shield.
 

tron87

New Member
Ive copied the edited(fixed) schematic circuit audioguru posted on bread board but now with a tL072 dual op amp the only problem is im not getting any sound though the earphones at all even with the 500ohm output resistor removed .
 
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audioguru

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What is the impedance of your earphones? Maybe they have an impedance less than 1k ohms. An opamp can drive nothing less than 1k ohms.
Maybe your earphones need a power amp IC to drive them.

Post your schematic showing which pin numbers connect to what and show the DC voltage that you measure at the output pin of the opamp. It should be half the supply voltage.

Is your electret mic with 3 pins or with 2 pins? My schematic is for an electret mic with 2 pins and the pin connected to the metal case must be the one connected to 0V.
 

tron87

New Member
Resistance of the earphones are <100ohms a Lm324 drove the earphones quite well but had alot of noise so changed to a TL072.

[pinout of the TL072]
pin1 connected to one side of 100k feedback resistor and to earphones via series 500ohm resistor.
pin2 connected to other side of 100k feedback resistor and to 1k resistor ->+3.3uf->0v.
pin3 connected to electret v+ via 100n cap and two 100k resistor in voltage divider configuration.
pin4 ->0v
pin5 nc
pin6 nc
pin7 nc
pin8 ->9v dc

The electret microhphone is a two terminal one 0v,other v+.Im reading 2.3vdc on the output with earphone connected.
 
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audioguru

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Your earphones might be 8 ohms each which is only 4 ohms in parallel and need a power amp, not an opamp to drive them.
If your preamp circuit has a 1k or 2k ohms load then it will work perfectly.

A lousy old LM324 is spec'd to drive a load as low as 2k ohms. A TL072 is also rated to drive a load as low as 2k ohms, not 16 ohms (two 32 ohms earphones in parallel) and not 4 ohms (two 8 ohm earphones in parallel).
 

tron87

New Member
Would using a TL072 as a preamp and then a say Lm386 as the main amp be better than just using a single TL072/LM386 opamp ?
Is gain and volume two completely different things when its comes to audio op amps ?
Im using some earphones from a ipod how do i know what the max voltage/current is i can use without burning them out ?
 

audioguru

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A TL072 is a dual audio opamp that can drive a load that is 2k ohms or more.
An LM386 is an inexpensive audio power amplifier that can drive a load that is 8 ohms or more.

There is no such thing as a single TL072/LM386 opamp.

Gain is how much amplification a circuit has. Volume is usually adjusted with a volume control that attenuates the input signal to an amplifier.

Most ipod earphones are 32 ohms per ear but some are 16 ohms per ear. If you connect both ears in parallel then the total is only 16 ohms or 8 ohms that cannot be driven from an opamp.
Most earphones have a max allowed power of 100mW. They produce a sound pressure of 100dB with only 1mW and 100mW produces 120dB which is extremely loud and might destroy your hearing.
 

tron87

New Member
Im using a regulated 9v dc wall wart to power the amp circuit but am getting a lot of undesirable noise in the earphone when compared with powering it with a 9v pp3 battery which is crystal clear.I have a 1000uf cap across the voltage rails for decoupling theres still alot of undesirable noise what more can be done to filter this out ?
 
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audioguru

Well-Known Member
Most Helpful Member
You forgot to describe the noise.
Is it low frequency hum? Is it high frequency hiss? Is it a high frequency squeal from a switching power supply?
Is it acoustical feedback howling becayuse the mic can hear the earphone?

I suspect you got a over-priced switching power supply from RadioShack that has a lot of high frequency ripple.

The opamp output should have a series coupling capacitor to the earphone to block DC but pass AC.
 

mvs sarma

Well-Known Member
Best is, to use a battery for the pre amp. If the wall wart is to be used, perhaps we have to open and add capacitors of 4700nF or so across each diode and then the supply would be quieter.
 
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