rc4580 as audio amplifier

[frank_ckt]

Hi All,
This is my first post.
I was trying to create an audio amplifier using rc4580. I have or want to use an electret microphone or a dynamic microphone. I am not getting any output from the amp when I attached the electret mic. I do see that the gain increases when having a signal generator as an input.
My elecret mic was purchased at Radioshack. Their diagram show + to Vcc, white wire is output and needs a capacitor in series with it. I used a .1uf. The ground (bare wire) goes to negative of battery.

Later on I will attached a speaker at the end of the circuit.
thanks,
f

 

[cowboybob]

Welcome, frank_ckt!

Could you provide a schematic of your circuit for us?

Just from your post, you might have failed to put a 2.2k (might take a 10k) bias resistor between the Vcc and the electret + wire.

We can better advise if we can see what you have done thus far.

 

[frank_ckt]

Welcome, frank_ckt!

Could you provide a schematic of your circuit for us?

Just from your post, you might have failed to put a 2.2k (might take a 10k) bias resistor between the Vcc and the electret + wire.

We can better advise if we can see what you have done thus far.

Hi cowboybob,
Thanks for the reply. I have copied the circuits in the following youtube video using the RC4580. I will use a 9V battery to power the circuit.

In the above video, I didin't use the filters after the op amp. I just wanted to see if I get any output.

I have also tried using the circuit in the above video with the RC4580 and can't get output.

I hope the circuit in the video is okay. Otherwise I can draw it and post it. Let me know.

thanks,
frank

 

[cowboybob]

Which schematic (they are different) did you use? The top circuit is dual supply powered (therefore no input bias needed) and the bottom one has single power supply and therefore has an input bias.

It would be best if you could post the exact schematic of the circuit as you built it, especially how you hooked up the electret mike (include wire color codes).

Hand drawn is fine.

 

[audioguru]

We have no idea what circuit Frank made or what power supply voltage since he did not post its schematic.
I suspect his RadioCrap electret mic has 3 wires, not the normal 2 wires. RadioCrap does not list what its 3 wires are for because they do not know. They spec its size instead.
I suspect that Frank does not have the datasheet for the RC4580 dual opamp and is using the pins for a 741 single opamp.

Maybe he is trying to drive a speaker with it???

 

[cowboybob]

Thanks, AG.

frank_ckt: From the pic, my bet is on:

Silver (also shield) - ground
Red - Vcc
White - sig

As you can see, the White lead is buried inside the ground (shield) lead.

 

[frank_ckt]

Which schematic (they are different) did you use? The top circuit is dual supply powered (therefore no input bias needed) and the bottom one has single power supply and therefore has an input bias.

It would be best if you could post the exact schematic of the circuit as you built it, especially how you hooked up the electret mike (include wire color codes).

Hand drawn is fine.

Hi Cowboybob,
Attached are schematics I cut and paste from the videos which I've edited values for. Also attached is the diagram wiring for the electret mic from radioshack. Yes it has 3 wires. The datasheet for RC4580 is also attached.
Please note that I have tried different combination of the white wire (ground, and connected to were mic+ should be connected) and still no output.

Audioguru, thanks for the reply. I am driving earphones with it. At the moment, I have a friend's oscope and attaching it to the output. I am also using a 9V battery.

 

[frank_ckt]

Also at the moment, I am using one op-amp of the RC4580.

 

[audioguru]

Your first amplifier is inverting with an input impedance of only 1k ohms so it is shorting most of the output from the mic. Its voltage gain is only 20 which is reduced to less than 7 by it shorting the mic.
Use a non-inverting design instead with an input impedance of at least 30k ohms. Increase the gain to about 100.

Your second circuit is non-inverting but the 2.2k bias resistor values are so low that they are killing the battery and killing the output level from the mic. the gain is 28 but is reduced to about 9 by the low value bias resistors. Use 100k bias resistors instead of 2.2k and increase the gain to 100.

The opamp can drive 100 ohms or more. 32 ohms is common for headphones which it cannot drive well. Forget about it driving 8 ohm headphones.
It can drive 600 ohm headphones if its supply voltage is high enough.

Your schematics show a 2-wires electret mic but your mic has 3-wires so it won't work in your circuits.

 

[audioguru]

I forgot to say that both schematics are missing a very important 100uF capacitor parallel with the battery mounted close to the power supply pins of the opamp.
The output capacitor value of only 104 (0.1uF) on the first schematic will pass only high frequency squeaks. Its value must be calculated for the impedance of your headphones.

 

[frank_ckt]

Hi Audio guru,
I've redraw the second circuit. See attached. I didn't get to update some values since I drew it prior to your response. But R1=100k ohm, R3= 10k ohm, R2=100 ohm. I've draw the mic the way I think it should be wired?

Your first amplifier is inverting with an input impedance of only 1k ohms so it is shorting most of the output from the mic. Its voltage gain is only 20 which is reduced to less than 7 by it shorting the mic.
Use a non-inverting design instead with an input impedance of at least 30k ohms. Increase the gain to about 100.

Your second circuit is non-inverting but the 2.2k bias resistor values are so low that they are killing the battery and killing the output level from the mic. the gain is 28 but is reduced to about 9 by the low value bias resistors. Use 100k bias resistors instead of 2.2k and increase the gain to 100.

The opamp can drive 100 ohms or more. 32 ohms is common for headphones which it cannot drive well. Forget about it driving 8 ohm headphones.
It can drive 600 ohm headphones if its supply voltage is high enough.

Your schematics show a 2-wires electret mic but your mic has 3-wires so it won't work in your circuits.

 

[audioguru]

Maybe I should give up because you do not understand this simple opamp circuit.
Now you are VERY BADLY shorting the signal from the mic and are cutting most sound frequencies.

 

[frank_ckt]

Hi Audioguru,
Please don't give up as you can see I don't have much experience. Thanks.

 

[cowboybob]

Made an error. Will replace in a moment.

 

[cowboybob]

OK.
frank, try this circuit (in SIM it works fine. There is some minor phase shifting).

With a 200mv P-P input sig and no load, output is about 1.25v P-P.

With a 20k load, output is about 640mv P-P and with a 10k load, output is about 380mv P-P. See the trend?

Since you have access to a scope, you might want to check the Electret for an output.

Keep trying. A little bit of success will carry you much further than you might guess.

 

[audioguru]

OK.
frank, try this circuit (in SIM it works fine. There is some minor phase shifting).
With a 200mV p-p input sig and no load, output is about 1.25V P-P.

You will never get 200mV p-p from an electret mic. The gain for low frequencies is 1000. Your quote says the gain is 1.25V/0.2V= 6.25 times. Who should we believe?

The 0.01uF input capacitor value is so low that it cuts frequencies below 800Hz. No lower mids and no bass.

 

[Experimentonomen]

Maybe this could work:

 

[cowboybob]

You will never get 200mV p-p from an electret mic. ...

Never said you could.

The gain for low frequencies is 1000. Your quote says the gain is 1.25V/0.2V= 6.25 times. Who should we believe?

OP1 is a 741. Used it intentionally since you always get riled up at its mere mention. Just gave him the numbers from the SIM, AG.

I don't think, at this point, that he gives a rat's butt about the particulars of the circuit. My guess is that at this point, he just wants something that will work, however poorly.

As he said:

Hi Audioguru,
Please don't give up as you can see I don't have much experience. Thanks.

He's new to this. A little slack is warranted...

 

[audioguru]

Maybe this could work:

Your schematic is getting better but it gain is only 23 times.
Two capacitors have values too high. Don't you know the simple formula?
You show a mic with only 2-wires.