Hello, I have a problem and until now I was not capable to solve it.
I designed two circuits. The first one is to receive the audio signal from a mic and amplify the mic. And the second one is to filter the high frequencies using a low-pass filter. When I simulated them individually the frequency response work as expected, but when I simulated together the frequency response is a huge mess.



And the last one occurs when they are combined.



Can someone give me a clue, I can't understand why that happens.

 

Hello,

The simulation won't run.
The lm358.txt is missing.

Bertus

 

Here's my take on the circuit:
I used the similar LM324 models I had.

Look closely at the value of R8 in your combined schematic to see the problem.

I changed the circuit so both op amps use the same offset virtual ground.
This eliminates the need for C2 and R5.

Note that if you use 1VAC for the source voltage, the output is then given in dB gain, since the dB values are referenced to 1Vac.
There's no concern about overdriving the circuit, since the AC analysis uses linear models that are insensitive to signal levels.

 

In your original circuit the input to the filter is 2.37k. In the combined cct its 2.37ohm? So 60nF C4 is having big impact on input stage...

edit: crutschow types faster than me!

 

Hello,

The simulation won't run.
The lm358.txt is missing.

Bertus

Sorry, I forgot to attach the lm358.txt

 

Here's my take on the circuit:
I used the similar LM324 models I had.

Look closely at the value of R8 in your combined schematic to see the problem.

I changed the circuit so both op amps use the same offset virtual ground.
This eliminates the need for C2 and R5.

Note that if you use 1VAC for the source voltage, the output is then given in dB gain, since the dB values are referenced to 1Vac.
There's no concern about overdriving the circuit, since the AC analysis uses linear models that are insensitive to signal levels.

View attachment 214996

I really appreciate your help, now I understand the problem and the improvements done. And thanks for the tip for the source voltage, I will use for other simulations.

 

In your original circuit the input to the filter is 2.37k. In the combined cct its 2.37ohm? So 60nF C4 is having big impact on input stage...

edit: crutschow types faster than me!

I realized that now, thanks Irving

 

NOBODY uses a lousy old LM358 dual or LM324 quad opamp for a mic preamp because:
1) It is very noisy (hisssss).
2) It has a slew rate so low that it has trouble producing frequencies above 2kHz.
3) It produces crossover distortion.

Why is your filter cutting all the important high frequency consonant sounds in speech? The conversation will be, "What did you say? What did you say?" over and over.

 

using unlabelled nodes may be confusing . . . as you change your circuit the node-numbers may be re assigned so your graph goes for not the initial target anymore -- put the node label on what you want to plot or -- confirm each time your plot corresponds to grid

 

NOBODY uses a lousy old LM358 dual or LM324 quad opamp for a mic preamp because:
1) It is very noisy (hisssss).
2) It has a slew rate so low that it has trouble producing frequencies above 2kHz.
3) It produces crossover distortion.

Why is your filter cutting all the important high frequency consonant sounds in speech? The conversation will be, "What did you say? What did you say?" over and over.

Thank you for your response. I'm using that particular one because I want something low-cost. Do you recommended something else? And I would like to cut 6kHz above because I'm working with 4 array mic and I have to split 48kHz sample rate into those 4 mic. That's why, but I do not need something really good, the purpose it's for embedded system

 

using unlabelled nodes may be confusing . . . as you change your circuit the node-numbers may be re assigned so your graph goes for not the initial target anymore -- put the node label on what you want to plot or -- confirm each time your plot corresponds to grid

Thanks for the tip

 

something else?

LM318 ? https://www.diyaudio.com/forums/solid-state/349054-lm318-distortion.html vs https://www.diyaudio.com/forums/analogue-source/154905-phono-preamp-2.html (never used the chip myself)
LT1115 ? no-idea
both require a good PCB design

 

I'm using that particular one because I want something low-cost.

Cheap means maybe a fake and low quality.

 

TL071? I have this, but haven't built it yet:

 

I think AG has previously recommended the TLO7x series, The MC3317x series, and the MC3407x series for low noise audio applications.

 

Here is the awful crossover distortion buzz caused by an LM358 or LM324 when it has 100% negative feedback to try reducing it. The crossover distortion is worse when the opamp has some gain reducing the negative feedback.
Which sounds worse, the buzz or the hissss? Both.