Operational Amplifier not working?

Hello,


I'm trying to use the KA324 IC (which consists of four opamps) for amplifying an audio signal.

I've tried almost every schematic from this website: http://sound.westhost.com/dwopa.htm
and also from lots of other websites, but none of them work.

If the output volume is Really low, the sound is pretty good, but very quiet.
When I turn up the volume, the sound is really bad, I almost can't recognise the song I'm playing.
The output volume is also not that really high. If I keep increasing the input voltage, the output stops increasing after a while. It's not really 'loud' at that moment too.

I've connected the IC to 12V DC (between the two middle pins). The + input goes to the ground connection, the - input goes to my POT voltage divider of the input voltage. The output pin goes straight to the speaker.
I also added a 100kOhm resistor between output and - input. Without that resistor, I hear nothing at all but I think that's normal.

I've also tried the three other op-amps from that IC but no luck.

Is this opamp just not ment for audio amplification? or am I doing something wrong?


Greetings,

Electorials

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AC
gmoon6 years ago
Sure, no prob. Using a capacitor on the output of any of your amps is probably a must (look at the datasheet for the LM386 to see what I mean).

Without a cap, the offset DC bias voltage becomes "standing DC" on the speaker, which increases current draw and ruins the audio. Capacitors pass AC and block DC...
gmoon6 years ago
Cool, I'm glad it's progressing. Looks like you're getting a "crash course" in op amps. There's lots to learn, but you're well on the way.

No problem chaining op amps together. If you're using them in "inverting amplifier" configuration, each stage inverts the previous one. So even numbers of inverting stages result in the same phase as the original signal.

Normally you would want to capacitively couple the stages together, but direct coupling can work too. If you "direct couple" two op amps, you'll need a resistor between the two--without it the gain is "infinite." I've used 4.7K with good effect (but it depends on the op amp and the application).

I'm always more comfortable with capacitive coupling 'cause they form filters (and the theory is easy to understand), but I've used direct for at least one op amp project and it worked fine. If you have a strong input signal there would be less headroom with direct, however.

RE: inverted signals--unless you're mixing signals together, having an inverted output isn't a real big deal for audio.

(Also--RE: your "op amp/transistor" amp above--you'll need to block DC offset somewhere in that circuit after the op amp with a cap...)
You are running before you can walk. The input bias current, and input offset voltage of an LM324 class amplifier is quite high. Reduce the value of your feedback resistors, but keep them in the same ratio
READ UP !

You have made an inverting amplifier. For many purposes - like AC work, it doesn't matter.

Look at how to wire a non-inverting amplifier.

Steve
"Internet" does not have all the knowledge you need. You need to read books too.

Steve
Get some books - like the "Art of Electronics" by Horowitz and Hill

Steve
But that would allow you to use the original board which already had all the engineering done and worked. If you want to do something else with it, then by all means, do so. I was under the impression you wanted to use the original board.

Qa
Put some feedback resistors in !!! At the moment its got close to infinite gain.
gmoon6 years ago

The power amplifier would be an additional stage, after the op amp preamp. It wouldn't utilize the divider voltage reference as ground. The preamp stage is strictly "small signal," so little current is necessary to shape the waveform.

If the main power supply "sags," then the divider as ground might be a problem. But that's usually only a characteristic of cheap, poorly designed amplifiers w/ underrated power supplies (or of old tube amps ;-).

Interstage coupling would take care of any bias issues between the stages. If you're using the same 9V power supply for multiple stages/devices, you just need to be sure everything is capacitive coupled.

But if the TEA2025B worked for you before, you shouldn't need the op amp preamp at all. Just find a similar "general purpose" audio amp IC.

I was only making a comment about bipolar power supplies and op amps in general...
If both inputs are connected to your reference ground, the output will be at the same potential as the reference ground, which is 0V relative to that, or Vcc/2 relative to the - terminal on your battery.
Get it working like this, and then you can use an opamp to MAKE a ground.

Steve
It will certainly going to help, though your voltage divider values should be << your feedback resistor values.

Steve
gmoon6 years ago
Hey, we've all "bricked" a few ICs in our time...

There you go--that's really what we've all been saying ;-). It's an audio amp; one that only requires a few extra discrete components. And it's specifically designed to drive a speaker.

You shouldn't need an extra op amp preamp, unless you're using it for a low-power, high impedance signal like a guitar pickup. But if it worked with your setup before, then you already know that.

Incidentally, a bipolar (split rail) power supply isn't necessary with many op amp circuits, unless they are drawing higher current, or they are old op amps--or sometimes if they are "instrument amps" and require a very stable voltage/reference.

Instead, most op amp audio preamps (and audio effects) just use voltage divider to create a "signal ground" reference--i.e., they place two resistors (maybe 10K or 20K) between the pos and neg power supply. The voltage in the center is midway--for example, for 9V power supply the ground reference would then be 4.5V. Then ALL the ground references within the op amp circuit would use that 4.5V as the ground.

Of course, the V+ and GND pins of the op amp would still be connected to the full voltage. And one needs to be careful with a divider "ground reference" if the power supply is being shared with multiple devices in the signal chain...
No, you just bias ground to be at Vcc/2

This STILL won't give you any power,

Steve
Since steve and g have pointed out that this is NOT a power amplifier - rather it would be better suited for a preamp (that's to bring the signal up to a usable level FOR the power amp) - I would also like to point out that it sounds as if you have connected the power backwards. Positive should go to pin 4 (VCC, middle on the left as you are looking down on the top) while negative should go to pin 11 (GND, middle on the right as you are looking down on the top. What purpose does the voltage divider perform? 12V is within it's range.

For further help, you should post a pic or actual schematic of how it is currently hooked up. Just draw the opamp as a block (with pin 1 marker) and the pins coming out and show how things are physically hooked together. This would tell us a lot about your build.

Qa
Yes,. absolutely right. Electrorials: Don't TALK circuits, draw them.

steve
Here's the PDF of the datasheet. It does not need a split supply, though it can use one. However, It has been pointed out that it is a small signal device, not a power device. Follow the advice given elsewhere in these answers and look for a stereo audio power amp IC that uses 12V. Do you still have the original? If so, what are the numbers on it?

Qa
Don't keep talking about a circuit, post a diagram of what you have. An amplifier isn't built by slapping in a transistor without any design.

Steve

Well, you are in luck as a quick search shows that Hobby Engineering has the original TEA2025B in stock for a paltry $2.00 US. I'm not sure what that is in Euros at the moment. That would be the best thing to do if you are still trying to repair the original board.


Qa

sshuggi6 years ago
By the sounds of it, your opamp is only operating in the positive... I've connected the IC to 12V DC (between the two middle pins) The opamp needs something like a +6V and a -6V in the power terminals as apposed to +12V and 0. That way, i can actually amplify sounds when the signal swings into the negative. And it does with audio.

Another reason it might be sounding bad is you're clipping the signal. Say your gain(amplification) is x15, and you have a waveform input that reaches+/-1V. With your current configuration, it will try and amplify the signal to 15V, but all it has to work with is 12V on the positive side. So, the signal is amplified normally up to 12V and then flat lines from there until the input goes back down. With any negative voltage signal, all it can do is output 0V... This is possibly what is giving you poor sound, especially when you try and turn it up.

Like everyone else is saying, this configuration only gives as much power as whatever is powering the opamp. If that's a battery, don't expect much more out of it that mini speakers.

Something to think about: Before you go hooking everything up to a massive power supply, check the ratings on you parts. The opamp has a limit to the current that it can output and the speaker has a max wattage.
P=I2R   The standard minispeaker has R≈10 Ohm and P≈0.5 W. Plugging this into the equation, the max current is something like 220 mA. There's probably other things to consider, but just know the limits of your parts.
"Audio" is not the same as "power amp". The old NE5534 op-amp is a superb AUDIO signal amplfier, but it is NOT a power amplfier.
Then as I suspected you are trying to use an opamp as a power amp.: It won't work. It won't work as a signal amplifier without split rails either
gmoon6 years ago

Probably wasn't an opamp you found, but an audio amp IC.

There are lots of dedicated amp ICs: LM386, LM389, TDA2003, TDA2005, TDA7294, etc. Several ICs in the TDAxxxx line--some are decently high wattage.

(Lots of online experimenters refer to LM386 amps as "op amps," but they are not).
gmoon6 years ago

More than likely the KA324 isn't designed to drive a speaker load--it has a different (higher) output impedance. And low current handling...

I took a quick look at the datasheet, and in the specs it gives the two values (under Voltage Swing) for load resistance--2K and 10K.  You're using a speaker as a load--what's that, approx 8 ohms? Pretty far from the load.

In short, a general opamp isn't a power amplifier. This would be an appropriate chip for a preamp, or a mixer or something similar. Something early in the signal chain. But it's not the chip to drive a speaker load.
An op-amp is NOT suitable for "power" amplification, and it sounds like you are operating it "single ended", when it really needs either split supplies, or special input biasing arrangements.