giusepped

May you suggest to me a simple power amplifier?
I have been using one based on TDA052 which is difficult to find here...
Regards

audioguru

Your mic preamp with two inputs won't work. You cannot feed a signal into the extremely low output impedance of an opamp.

You need to have separate preamps for each mic then an inverting opamp as a mixer.

Use an LM386 small power amplifier to feed the headphones. Its output power is less than the TDA7052. Its voltage gain is preset to 20.

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preamp.PNG (18.4 KB, 10 views)

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giusepped

Two questions:
1) I wanna use one input at time
2) May I put a condenser between the output of the voltage follower and input 1 or
3) may I put a switch on the supply line of the opamp, in such a way that when the input 1 is inserted, the opamp will not affect
Regards

audioguru

The voltage follower doesn't do anything except short the second input. A capacitor in series with its output would allow some low frequencies in but still short high frequencies.

You need a separate preamp for each mic then an inverting opamp as a mixer.

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TheVictim

Isn't there a way to override the preset gain of the LM386 using a couple of capacitors?

giusepped

Sorry, if I insist.
But if I don't put anything on the Pin Number 1 (my mic), the ooutput of the opamp will feed my preamp, dont'it?
If I do not put anything on the second input, and instead I use my mic and also turn off the OPAMP, why the mic would be shortned?

audioguru

The datasheet for the LM386 says that if additional negative feedback is added to reduce the gain then it will become unstable and will oscillate if the new gain is less than 10.

If the gain of 20 is too high then simply use an attenuator made with two resistors to reduce the input level.

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Uncle $crooge

audioguru

Yes.

You don't want to feed a mic signal to the output of an opamp if it has power or if it doesn't have power. Use two preamps and a mixer circuit instead.

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Uncle $crooge

Hero999

Yes, it's on the datasheet.

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TheVictim

The output impedance (resistance to AC) of the opamp is so low that the signal coming from your mic is going to be shorted out to almost nothing by the very low resistance. Remember that electric current always takes the path of least resistance in proportion to all the resistances at that point in the circuit.

giusepped

You are right, but now I have no much money, no much time and no much tools.
So, I thought to put a double switch in order to completely disconnect the OPAMP.
It is not a professional solution but I think it does work.
G

TheVictim

Your two preamps and a mixer can be part of the same IC package, so you really wouldn't be looking at that much of a time/cost increase.

giusepped

Yes ,right, but I have not tool to make the pcb layout, I have only general purpose pcb boards. By the way, have you some mixer to suggest?
Regards
G

giusepped

So, sorry of boring everyone, I thought to do this solution, instead of using a mixer.
If I use a pre-powered mic, I will switch-off the R1 resistor. In this way, the OPAMP should
separate the two preamp. If I use a non-powered mic, I will switch on the S2 switch, and in this way the R1 will power it.
Don't you agree?
G

Attached Images

kw6.png (17.7 KB, 7 views)

audioguru

I have no idea what you are doing with the opamp as a follower and switching R1.
Where is the second microphone going to connect to?

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