Audio amp circuit with low pass filter

Thread Starter

abu1985

Joined Oct 18, 2015
22
I put together a pcb using a TI TPA3110D2 audio chip. The board works fine, sounds good. I'd like to start playing with filtering circuits and tried putting together a simple low pass active filter on my solderless breadboard. The output signal from the active filter looks okay. But when I feed that signal from the breadboard into my TPA pcb, the chip looses it's mind. Nasty sounds without signals, and hard pops when removing power. (it TI chip works perfect if I just input both channels from the iPad)

I was wondering if it had something to do with the filter cap already going into the chip? That would be C4 on the drawing attached. Did that low pass circuit create a bandpass or notch filter? I questioned how the grounds work being on two separate boards as well.

If anyone is bored enough to throw some knowledge my way with this, I enjoy learning this stuff..

I appreciate your time, thanks!
 

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MrChips

Joined Oct 2, 2009
19,280
There are some issues with your LPF design.

1) LM741 opamp is a classic design used in text books and teaching labs. The design is 50 years old.
Time to move on to more recent designs.

2) LM741 is designed to be used with ±15V supplies which you are not using.

3) If you are using a single supply as opposed to a dual supply, you need to create a common reference voltage halfway between your supply rails. I see your Vcc is 12VDC. Hence you have to create a common node at 6VDC and this becomes your pseudo-ground for your opamp and its input and output signals.

There are ways to create a pseudo-ground:

1) use two resistors to form a voltage divider
2) use the voltage divider followed by an op-amp buffer circuit
3) use a commercial split supply IC.
 

Thread Starter

abu1985

Joined Oct 18, 2015
22
There are some issues with your LPF design.

1) LM741 opamp is a classic design used in text books and teaching labs. The design is 50 years old.
Time to move on to more recent designs.

2) LM741 is designed to be used with ±15V supplies which you are not using.

3) If you are using a single supply as opposed to a dual supply, you need to create a common reference voltage halfway between your supply rails. I see your Vcc is 12VDC. Hence you have to create a common node at 6VDC and this becomes your pseudo-ground for your opamp and its input and output signals.

There are ways to create a pseudo-ground:

1) use two resistors to form a voltage divider
2) use the voltage divider followed by an op-amp buffer circuit
3) use a commercial split supply IC.
Okay, pseudo-ground... I'll look that up and get that working.

Off subject here. I hit reply to say thanks, then decided I'll come back later. But I couldn't find a way out of the reply; to cancel it. Do you know how?

Moderator edit: Simply select and delete the quoted text.
 

Audioguru

Joined Dec 20, 2007
11,251
I tried a lousy old 741 opamp 50 years ago and its sounds were so bad that I replaced it but it also sounded bad so I never used one again.
The datasheet for an LM741 or a uA741 does not show its high noise and distortion and poor high frequency response. An application for audio is not mentioned.

The pseudo ground biases the + input and output so that the output can swing up and down with the input signal. When a plus and minus power supply is used then the input is biased at 0V which is half the total supply voltage and allows the output to also swing up and down with the input signal. The datasheets for a 741 opamp show only a 30V total supply voltage and nothing less.
 
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