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Active audio filter doesn't work

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Hi,

I used the 7660 or equal with very high sensitivity circuit...OPAMP gain of 20 and a 16 ADC with 3VRef. Makes a resolution of 2.3uV/LSB.
A 3kHz analog signal filter and sampling with 10kHz with gives stable readings down to +/-1LSB.

At the 7660 output I have simple LC filters. Low ESR capacitors.
Four layer optimized layout hand routed).

****
So I´m sure there is a solution for your problem, too.
As said before. Capacitors are the one thing, PCB layout is the other. Unfortunately the worst of both will dominate.

Klaus
 

Maybe there is a voltage converter IC available that operates at a frequency higher than our hearing? Have you ever heard the switching from a high power car amplifier or from a class-D amplifier?
 

Audioguru said:
Maybe there is a voltage converter IC available that operates at a frequency higher than our hearing? Have you ever heard the switching from a high power car amplifier or from a class-D amplifier?
Click to expand...

Yeah, ones like this dual supply from linear that use frequencies between 100 KHz and 2 MHz are pretty typical. Most of the designs that the RF guys in a previous company I worked at used >>100 KHz switching frequencies, that's pretty far outside of hearing range.

Or this one from Analog.
 

KlausST said:
Hi,

One important question is: Do you need DC performance (input to output)?
If you can live with a lower cutoff frequency of - lets say - 10 Hz, then this makes all easier.. Then we use capacitors for coupling...
Click to expand...

Could you please specify where and which the capacitor should be? (on post 13)

...

I would like to get it at least working, before I buy another thing. I worry how will the filters sound, because what I have heard from it was not what I was expecting...
 

The MAX1044 voltage converter should be used instead of the ICL7660 because it can operate at 35kHz and its whistle will not be audible. It costs a small fortune but it is only ordinary money that grows on trees in many places.

I do not know why you are boosting the bass like a boom-box and chopping off the highs like an AM radio with those filters.
 

Hi,

For single supply you need to bias the signal with about half of the supply voltage.
At least at input, but most probably at output, too.

C = 1 / (2 × Pi * f * R)
f = cutoff frequency
R = here the impedance (input or output)

Klaus
 
Earlier I thought that the coupling capacitors can be used instead of the biasing circuit, but I was probably wrong?
(Klaus said that if I am okay with lower cutoff frequency, then we use coupling capacitors, it would be all easier)

I am really having trouble finding those C,R values, should it be calculated? I would be very glad if someone did it for me. The instructions are in english and thats bad for me.

Should the inverting op-amp bias circuit look like this?

Snímek obrazovky (11).png
 

When the opamp has a single supply then its (+) input must be biased at about half the supply voltage and the (-) input must be capacitor-coupled.
Your signal generator is shorting the (-) input almost to ground then the opamp amplifies it with its internal gain of a few hundred thousand and causes its output to be saturated at a DC voltage as high as it can go.
DOO it like DIS:
 

Attachments

  • lows boost filter.png
    lows boost filter.png
    12.4 KB · Views: 62
I should also ask about the low-pass filter. Is this correct? I really don't know about the values.

lowpass.png
 

Correct in which regard? I don't remember that you ever mentioned a specification.

- - - Updated - - -

At second sight, the Sallen Key filter parameters seems to indicate instability. In so far the design is not correct. How did you arrive at the component values?
 

You show a 3rd Order Sallen and key Lowpass filter. There are many tutorials and electronic calculators in Google. I sim'd it:
 

Attachments

  • 3rd Order Sallen and Key Lowpass filter.png
    3rd Order Sallen and Key Lowpass filter.png
    27.2 KB · Views: 62

Yes, looks reasonable. I don't actually understand why my filter tool decided that the circuit would be unstable.
 

I was asking if the biasing components are correct. Those are R2,R3,R4,R5 and C3. And the crossed wire was there originally.
 

Hi,

And the crossed wire was there originally.
Click to expand...
If there was a wire... then don´t change it. (It confuses your filter characteristic)
Then probably in the orignal plan was no R4, R5, C3...You don´t need it.

R2 and R3 are correct as far as i can see.

***
Your output voltage is now biased to VCC/2. Maybe you need a serial resistor at the output.

Klaus
 

So I've finished both circuits. The volume is good with clean sound.
However the problem with the, somehow trimmed frequency response, persists. I will fully describe it when I have more time, but in very short, it seems that something doesn't let the filters to fully boost or damp the sound. Though I can hear that the high freq. roll-off starts at the right frequency very steeply, the high-end is still there very much.
And this is exactly the same with the bass boost. The 100Hz is, let's say +4dB, and all frequencies below it STAY at the same level.
I am very disappointed. What could be the problem?
 

David Kosman said:
I can hear that the high freq. roll-off starts at the right frequency very steeply, the high-end is still there very much.
Click to expand...
Of course the high-end is still there very much. The response of the lowpass filter in post #73 shows that 12kHz to 15khzx is cut slightly, 20kHz is cut a little and higher frequencies that nobody can hear are cut more.

And this is exactly the same with the bass boost. The 100Hz is, let's say +4dB, and all frequencies below it STAY at the same level.
Click to expand...
That is what the response of the bass filter shows in post #7 except the boost begins at about 120Hz and peaks at 35Hz with about +14dB down to below earthquake frequencies.
 

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