Audio filter and non-linear output waveform

Thread Starter

noweare

Joined Jun 30, 2017
112
Hello All,
I am building an audio filter to try and make 8 bit wav file a bit less
crappy sounding. The pdf shows a six pole audio filter using sallen-key topology
that I am hoping will help.

I was using the LM324 OP AMP in the design but it doesnt seem to like
capacitors as it causes a very non-linear output when driven with a sine wave.

I don't know if I got a batch of bad chips but they all do the same thing.

For this application I would think any general purpose op amp would work.

I changed the op amp to a LF353 OP AMP and it took care of the non-linear problem.
With the LF353 OP AMP I added some gain in the last stage and that also caused
non-linear output. Not nearly as bad as the LM324. In both cases I was driving the
stage with a sig generator with a d.c. bias halfway between the VCC and ground.

Attached is a pdf of my circuit. Does any one know why these op amps would
act this way.
 

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Thread Starter

noweare

Joined Jun 30, 2017
112
I do not have a load on the filter right now. I am just looking at the output with a scope.

I will put a small 1 watt amp between the filter and speaker. Right now using a powered speaker until I am happy with the sound. I am kind of wondering why the non-linearities from the op amps, especially the LM324. I already spent quite a bit of time on this as I didn't think the op amp could case this problem. I do not consider myself a hardware guy, more comfortable with software. I want to get more competent with hardware. But this is usually what I run into when I dabble in it. : )

I will be using a 1/2 watt, 8 ohm speaker when all is figured out.

Thanks
Joe
 

Thread Starter

noweare

Joined Jun 30, 2017
112
Hi Dana,
Right now I am using a 1vpp from a sig. generator to drive the filter to make sure everything looks good before hooking it up to real audio.
I am biasing the signal half way between ground and Vcc.

What I think i will do is just remove the gain resistors since I need to add the 1 watt amplifier to drive the speaker.

My audio data are 8 bits each and centered at hex 7F which would be 2.5 volts (0xff is 5 volts, 0x00 is 0 volts). I looked at the file and the values do not go near 255 and I guess based on what you found out, I don't care about notes being close to 0 volts.

Thanks
Joe
 

Audioguru

Joined Dec 20, 2007
11,249
The LM324 quad opamp and its sister the LM358 dual opamp are the first low power opamps made. Therefore they do not have their outputs biased into class-AB so they produce crossover distortion. The low power also causes hiss and a poor high frequency response above 2kHz.
 

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Thread Starter

noweare

Joined Jun 30, 2017
112

Thread Starter

noweare

Joined Jun 30, 2017
112
The LM324 quad opamp and its sister the LM358 dual opamp are the first low power opamps made. Therefore they do not have their outputs biased into class-AB so they produce crossover distortion. The low power also causes hiss and a poor high frequency response above 2kHz.
That's what I was getting on the scope! Since I was only applying 1 volt to the filter the distortion was even worse than your pic. Thanks for that.
 
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