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First, I'd like to say i'm not experienced on circuits, but i'm doing my best to learn.

I'm trying to create an audio equalizer which is capable of amplifying low frequencies (20Hz-200Hz), mid frequencies (200Hz-2kHz) and high frequencies (2kHz - 20kHz).

At the same time it should be capable of amplifying the overall output frequency \$\pm10\$ Hz.

I used a high pass filter and a low pass filter connected through a voltage buffer (LM741 Op Amp) to create a band-pass filter with different values of R and C to achieve the desired frequency to the low, mid and high frequencies.

I'm having problems with the LTSpice simulation, which doesn't output frequencies it is supposed to yield.

This is a screenshot of what I did and the output:

enter image description here enter image description here

I wasn't sure if I had to add every voltage source to the Op Amp, so I just made one for every Op Amp.

V(n002) is for the one on the top (low freq band-pass filter) right before R13. V(n013) in the middle right before R14. V(n018) is the one on the bottom (high freq band-pass).

I calculated the high pass filter and low pass filter with the cutoff frequency:

\$f_c=\dfrac{1}{2\pi RC}\$

And checked the results using \$\textit{Mathematica}\$ software which showed the correct band pass filters.

Can any one explain me why it isn't working?

Thank you!!

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  • \$\begingroup\$ If you want to learn how to design , learn how to search 1st. Find the best designs and learn how they work better. Don't guess and make something inaccurate and redundant. Baxandall filters is the most common for 2 bands in 1 Op Amp and there are 3 band filter, then octave EQ, 1/2 Octave graphic EQ , 1/3 octave EQ, gyrator EQ filters. Your choice uses 6 Op Amps instead of 1 or none makearadio.com/tech/tone.htm Why make a square wheel and ask why it won't roll very smooth? e.g. learn what GBW really means for filters \$\endgroup\$
    – D.A.S.
    Commented Jul 1, 2019 at 4:22
  • \$\begingroup\$ Recommended reading: ti.com/lit/an/snoa387c/snoa387c.pdf \$\endgroup\$
    – Peter Smith
    Commented Jul 1, 2019 at 11:24
  • \$\begingroup\$ Thanks @PeterSmith! @SunnyskyguyEE75 I'm not trying to learn design yet, just learning transfer functions and op amps and trying to make a use of what I know. \$\endgroup\$
    – FelipeMedLev
    Commented Jul 1, 2019 at 12:28

1 Answer 1

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But it IS working, at least within the limitations of your opamp. The peaks in your response curves fall about where they should.

In fact, if you draw the asymptotes to your response curves, they hit the peak response level exactly where they should, as shown here for the low band filter, V(n002):

marked-up frequency response chart

You can also see that the lower -3 dB point for V(n013) also falls at 200 Hz, as does the lower -20 dB point for V(n018) — all exactly as expected.

There are many Reasons not to use a 741 op-amp?

You're running into its gain-bandwidth limitations specifically.

And no, you don't need a separate power supply for every opamp. A single positive supply and a single negative supply would be sufficient.

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  • \$\begingroup\$ But I see for example the low frequency band pass filter V(n002) working from 20 to 2000 Hz instead of 20 to 200 Hz. Which other op amp is recommended for this? Thanks \$\endgroup\$
    – FelipeMedLev
    Commented Jul 1, 2019 at 2:27
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    \$\begingroup\$ Look again. The -3 dB points on V(n002) fall close to 20 and 200 Hz, just as they should. This would be easier to see if you constrained the frequency range of your sweep to maybe 1 Hz to 100 kHz, rather than 1 mHz to 10 MHz, which is excessive. \$\endgroup\$
    – Dave Tweed
    Commented Jul 1, 2019 at 11:16
  • \$\begingroup\$ Ohhh you're right. Thanks! Apparently there's something wrong with V(n018) tho. \$\endgroup\$
    – FelipeMedLev
    Commented Jul 1, 2019 at 12:29
  • \$\begingroup\$ Yes. That was what my original point about the gain-bandwidth limitation of the 741 was all about. If you picked a higher-performance opamp, it would work just fine. \$\endgroup\$
    – Dave Tweed
    Commented Jul 1, 2019 at 12:33
  • \$\begingroup\$ I just realized there was a mistake with the capacitance on the high frequency low pass filter, it was 0.9 nF. I also realized that to amplify by 10dB I needed to have a transfer function of \$\sqrt{10}\$. \$\endgroup\$
    – FelipeMedLev
    Commented Jul 1, 2019 at 15:53

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