mic preamp opamp working only on a single supply

[piconano]

i m trying to build a mic pramp with opamp, but from some reason it only works when the opamp is a single supply.

simple noninverting opamp dual supply, amplify fine a signal from the radio, but when i connect an electret mic(with 10k resistor bias to the +vcc)
it dont work

but when the opamp is a single supply it works

why?

 

[MikeMl]

Look here (post #1)

 

[cowboybob]

i m trying to build a mic pramp with opamp, but from some reason it only works when the opamp is a single supply.

simple noninverting opamp dual supply, amplify fine a signal from the radio, but when i connect an electret mic(with 10k resistor bias to the +vcc)
it dont work

but when the opamp is a single supply it works

why?

Welcome, piconano!

Could you provide us with a schematic(s) of what you've built so far?

 

[piconano]

Welcome, piconano!

Could you provide us with a schematic(s) of what you've built so far?

sure
i made a pdf with schematics

 

[MikeMl]

The problem with the bottom circuit in your attachment is that there is no DC path to provide the input bias current to the non-inverting input of the opamp. Ideally, there will be a resistor from Ground to the input which has a value of R1*R2/(R1+R2).

This circuit can not operate on a single supply. It would require -Vcc to be at least -1.5V with respect to the ground symbol.

 

[piconano]

The problem with the bottom circuit in your attachment is that there is no DC path to provide the input bias current to the non-inverting input of the opamp. Ideally, there will be a resistor from Ground to the input which has a value of R1*R2/(R1+R2).

This circuit can not operate on a single supply. It would require -Vcc to be at least -1.5V with respect to the ground symbol.

but there is a resistor from the ground to the input( i don't care about the gain in this stage i just want it work first) ,and it operates on dual supply with -vcc.

the circuit works with a radio signal but not with a mic signal

 

[Nigel Goodwin]

but there is a resistor from the ground to the input( i don't care about the gain in this stage i just want it work first) ,and it operates on dual supply with -vcc.

The diagrams you've posted both show no resistor connected to the +ve (non-inverting) input, which is why it doesn't work - connect a resistor (47K or so, it's not critical) from the +ve input to chassis.

 

[MikeMl]

Here is a single supply version that meets the dc path to both opamp inputs requirement. I tailored the gain and frequency response to be reasonable for a mic circuit.

 

[cowboybob]

Mike, what did you use for U1?

 

[Nigel Goodwin]

Mike, what did you use for U1?

An opamp

It's not in any way critical, even a 741 would work quite happily, but the TL0xx series would be preferred

 

[MikeMl]

Mike, what did you use for U1?

This one:

It has the following attributes:

 

[cowboybob]

Thanks, guys.

 

[audioguru]

The 220pF capacitor chops away all the important high frequencies in English and most other speech (consonants and sibilants).

 

[MikeMl]

The 220pF capacitor chops away all the important high frequencies in English and most other speech (consonants and sibilants).

Not for voice radio communications, as in Ham radio. The bandwidth of audio stages is usually constrained to 300 to 3000Hz. As posted, the preamp has a -3db drop off of gain at 20Hz and 7kHz, more than enough for voice.

Hell, I can't hear anything above 2.4kHz in one ear, and 4kHz in the other. I can understand voice comms on the phone, ham and aircraft radio just fine.... I even have to prove it to the FAA every year...

 

[audioguru]

I am 68 years old and protected my hearing so I can still hear high audio frequencies. No guns, no very loud rock music(?) and no engines without good mufflers.
Compared to live speech, telephones and AM radios without producing consonants and sibilants sound very muffled to me and to many other people.

When I worked with telephone conferencing my customers complained about muffled sound. So I measured the round trip for a telephone call and found 3kHz at -12dB. I complained to Bell who said it was normal because their limit is -15dB at 3kHz for a round trip (-7.5dB to the central office and another -7.5dB from the central office). Then I made and sold transmit equalizers that boosted 3kHz 10dB and everybody was impressed by the improvement.

 

[piconano]

The diagrams you've posted both show no resistor connected to the +ve (non-inverting) input, which is why it doesn't work - connect a resistor (47K or so, it's not critical) from the +ve input to chassis.

my bad of understanding
i did that ,put the resistor, and it worked. kinda.
the problem- it doesnt get the max gain.
gain resistors are 100k and a variable resistor(vr)
when the vr exceeds the 500 ohm the signal disappear and- will comes back distorted if i raise my voice

in the single supply op amp that didnt happened

 

[Nigel Goodwin]

my bad of understanding
i did that ,put the resistor, and it worked. kinda.
the problem- it doesnt get the max gain.
gain resistors are 100k and a variable resistor(vr)
when the vr exceeds the 500 ohm the signal disappear and- will comes back distorted if i raise my voice

in the single supply op amp that didnt happened

POST YOUR CIRCUIT!!!!!!

You're doing something badly wrong, and without knowing what you're doing how can we help?.

 

[audioguru]

Piconano,
You did not provide an updated schematic so I did:

 

[piconano]

POST YOUR CIRCUIT!!!!!!

You're doing something badly wrong, and without knowing what you're doing how can we help?.

sorry
i think i m stupid at reading comprehension which is not far a way from me understanding electronics

 

[MikeMl]

Your problem:

At R1 <1K, you are getting a gain > 101. With 100K source impedance for the input bias current to the non-inverting input of the opamp, and <1K source impedance for the input bias current to the inverting input of the opamp, the bias current causes a differential DC voltage between the inputs, which is amplified by the gain (>101), which causes the output level of the opamp to shift so far that the audio signal is clipped.

Set R1 to get a gain of about 200. With no mic input, measure the DC voltage at the output pin of the opamp. I'll bet that it sits very close to the positive supply voltage.

If you really think you need gains exceeding 100, then use two opamp stages, one with a gain of 10 followed by a second one with a gain of 10 to 50.

What Opamp are you using?