audioguru

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

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Uncle $crooge

mneary

Since we don't know much about the load, probably should put 100 ohms in series with the output to prevent oscillation.

giusepped

Tahnk you all for your replies.
I would insist on the fact that the desing it's not mine, I bought a kit in a local store,
because I have no time to do the PCB layout.

Anyway, the PREAMP is working pretty well. Sorry, for the quick design,
I used gschem for the first time. The OPAMPS are the two you find in the 5218.
I did not check the cutoff frequencies, I will check.
However, the mic is really a condenser with two terminal. It has a FET preamplifier built in.
At least, the local vendors said to me that it is a condenser with two terminals.
Now, while I will try to implement the circuit of audioguru, may you suggest I to share the
input of the first mic with second stage? I need two outputne for the power AMP and one for the LINE....
G

giusepped

The load is the input of a power AMPLIFIER TDA7052 (Philips), connected by means of an esternal shielded cable
G

giusepped

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

giusepped

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

Attached Files

ba15218.pdf (65.3 KB, 3 views)

audioguru

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.

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Uncle $crooge

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

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

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Uncle $crooge

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.

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Uncle $crooge

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

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

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Uncle $crooge

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

Attached Images

kw5.png (14.0 KB, 10 views)