Multi earphone amp system-help

[giusepped]

The "horrible" original preamp circuit probably used a horrible LM358 dual opamp that is noisy and has crossover distortion.

No. it is a BA15218, maybe not very famous.
I attach the datasheet
G

 

[audioguru]

I would like to know why the invering configuraiton is a problem, because in an audio amp the phase of the signal does not care. I think the designer put the inverting configuration to prevent oscillations
G

A non-inverting opamp circuit doesn't oscillate. Its input impedance is extremely high. In my circuit, the output of the microphone feeds two 100k resistors in parallel which is 50k ohms so the microphone's high output impedance of about 3k ohms is not loaded down.

The inverting opamp circuit has an input impedance that is too low at 1k ohms. So the output of the microphone is loaded down and therefore its output level is reduced.

With such a low input impedance the inverting opamp circuit needs a huge input capacitor which is polarized. It is probably polarized backwards. My non-inverting opamp circuit uses a little non-polarized metalized-poly input capacitor.

 

[giusepped]

Ok suppose I use the circuit of audio guru.
THe problem is that the input in my system come from a microphone which is
already connected to another pre-amp. May I connect that microphone to the input of your circuit? Or it will modify the input impedence?
G

 

[audioguru]

Ok suppose I use the circuit of audio guru.
THe problem is that the input in my system come from a microphone which is
already connected to another pre-amp. May I connect that microphone to the input of your circuit? Or it will modify the input impedence?
G

Why not use the output from the other preamp instead of adding another preamp?

 

[giusepped]

Becasue I need two outputs: one for the power amp and one for the line output (e.g. for the tape recorder)
G

 

[giusepped]

...and because the other amp feeds an amp which feeds a chain of 20 earphones!
So, I would like to to a parallele chain for one more 20s...

 

[audioguru]

The output of the existing preamp can probably drive many power amps and line outputs.

If the other preamp is an opamp then it can easily drive a load as low as 1k ohms. The input of a power amp is probably 10k and the line level load is also probably 10k. Add another power amplifier and the total load becomes 3.3k ohms which the preamp can drive easily.

 

[giusepped]

I cannot connect to the output of the existing preamp because it is a closed box. I see only
the mic input. I would pick this input and jump it to my preamp.

 

[audioguru]

I cannot connect to the output of the existing preamp because it is a closed box. I see only
the mic input. I would pick this input and jump it to my preamp.

Then remove R1 from your mic preamp and let the other preamp power the electret mic.

 

[giusepped]

Ok, thanks audioguru.
But to make a very flexible solution (the client is very fuzzy!) I propose this solution, with
two input on my box. One for pick up the signal from a powered mic, and the other for a non powered mic. (See the schematic)
What do you think?
G

 

[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.

 

[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.

 

[TheVictim]

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

 

[giusepped]

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.

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]

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

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.

 

[audioguru]

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?

Yes.

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?

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.

 

[Hero999]

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

Yes, it's on the datasheet.

 

[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.