Hey guys

I'm doing an audio project which requires a few op-amps. I started using LM342's. With a basic voltage follower circuit (circuit 1 in the pic below), without any resistors, the output is perfect. However, as soon as I add resistors, a glitch appears on the output. A little spike just above ground when I'm inputting into the non-inverting input, (circuit 3) and just below ground when I'm inputting into the inverting input. (circuit 2) The rest of the signal is fine, but every time it passes that specific voltage, there's a small spike.

I'm running it off the + and - 12V from an old PC power supply. Tested with two different inputs - A pretty old oscillator outputting a sine around 2V, and my tablet outputting a sine wave from the audio jack (here I had to change the resistor values to amplify the signal enough that the wave passed the point at which the glitch occurs) I even changed the IC out to make sure it wasn't a faulty IC, but the same thing happens with the new one as well.

I'm pretty new to electronics. Am I missing something really simple?

Circuits: http://users.silenceisdefeat.net/~djbksa/Tests.png

 

Maybe a crossover distortion? Try placing a resistor from the + rail to the output, 10k might do it.

 

Yes, an LM324 is an old "low power" quad opamp. it has low power because its output transistors are missing bias current so they produce crossover distortion.
The LM324 and its sister the LM358 dual low power opamp also have too much noise (hiss) and have trouble above only 2kHz so they should never be used for audio.

 

Thanks guys

I tried the 10k resistor, and that did get rid of the glitch. Can you explain to me what this actually does?

I also pushed up the frequency of the oscillator and at 20kHz the output was basically a triangle wave at higher amplitudes, but looked ok when I reduced the amplitude (below about 1.5V if I recall correctly).

The main function of the op-amps will be for unbalancing audio signals from a professional microphone, and then balancing it again at the other end, with some adjustable gain somewhere along the line. This will go out into a mixing desk, which will expect a mic level, so the voltages will be pretty small. Probably around 20mV. Does this make the noise from the op-amp more of a problem?

I'm not going for audiophile quality in this build, but obviously telephone quality is not ideal My aims for this project are to keep cost and physical size down, hence why I started with the 324. Will it be ok at these low levels or is it preferable to use a higher quality op-amp? And if so, do you have any suggestions on which one?

Thanks in advance!

 

Read this post
http://www.electro-tech-online.com/...ignificant-data-sheet-errors.html#post1044382

 

I tried the 10k resistor, and that did get rid of the glitch. Can you explain to me what this actually does?

It makes the one of the output transistors on at all time, making the amplifier run in class A instead of class B,

I also pushed up the frequency of the oscillator and at 20kHz the output was basically a triangle wave at higher amplitudes, but looked ok when I reduced the amplitude (below about 1.5V if I recall correctly).

Look for slew rate.

I'm not going for audiophile quality in this build, but obviously telephone quality is not ideal

Then don´t use a 40 year old part and use something more suitable for these days, like for example NE5534.

 

I built an audio circuit with an LM324 once and was shocked at how noisy it was! I would only use the 324 as a noise generator.

 

The LM358 dual opamp is very noisy (hiss), has crossover distortion (buzz) and has trouble above only 2kHz (muffled). Today at Digikey it costs $.45US and the LM324 quad opamp has the same problems and costs $.53US.

The low noise, wideband and very low distortion TL072 dual audio opamp costs $.47US and the TL074 quad opamp costs $.55US.

So the excellent opamps cost only 1 or 2 cents more than the garbage opamps.

 

Thanks guys, really appreciate the help!