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I'm trying to simulate an audio amplifier circuit in LTspice. I made this circuit based on the circuit I found here: https://circuits-diy.com/how-to-make-audio-amplifier-circuit-using-555-timer/ .

I'm able to discern the sound at the output, but there's an irritating high pitched sound along with the amplified sound signal. I tried adding a low pass filter also, at the output, but that ringing sound is still there. How do I remove this?

Also, why is this high pitched sound coming at the output?

Circuit diagram: audio amplifier circuit

This is the audio file generated at the output: https://soundcloud.com/karun-mathews/test4

This is the sample input audio file I used: https://www2.cs.uic.edu/~i101/SoundFiles/PinkPanther60.wav

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  • \$\begingroup\$ Did you actually build it, or just simulate it? \$\endgroup\$
    – JRE
    Commented Dec 4, 2020 at 15:22
  • \$\begingroup\$ I built the circuit (in LTspice) found on that website by myself. Then I added those filters at the output because they seemed to make it better. \$\endgroup\$
    – karun mathews
    Commented Dec 4, 2020 at 15:24
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    \$\begingroup\$ Ok. You've only simulated it. It could be an artefact of the simulation, or it could be a problem with the circuit. \$\endgroup\$
    – JRE
    Commented Dec 4, 2020 at 15:25
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    \$\begingroup\$ Hmm. I can't decide if that thing is trying to be a class D amplifier (at a too low switching frequency) or if it is trying to misuse the internal parts of the 555 as a regular class A or AB or something amplifier. \$\endgroup\$
    – JRE
    Commented Dec 4, 2020 at 15:41
  • \$\begingroup\$ Make C2 smaller and see if the frequency of the whine goes up. If it does, then it's probably trying to be a class D amplifier. \$\endgroup\$
    – JRE
    Commented Dec 4, 2020 at 15:42

2 Answers 2

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I've played with the circuit a little in LTspice. It is a class D amplifier - if we're being polite and not calling it names.

It varies not only the pulse widths, but also the frequency.

When CV is at a higher voltage, the frequency goes down. The frequency in simulation varied from way over 100kHz to 22kHz. The closer CV is to Vcc, the lower the frequency. If CV is pinned up to Vcc by the transistor circuit, then the frequency could be even lower.

I think your transistor amplifier is messed up, so that CV stays way up close to Vcc all the time.

Remove the transistor, and couple V1 in through just C1, and I think you'll get much better results.

You might also need to attenuate the audio signal somewhat.

It's also possible that you are getting aliasing. I don't know how well LTspice converts signals to .wav files, but if the 555 frequency is above the Nyquist frequency for the sampling rate, then you might be getting the switching frequency aliased down in to audible frequencies.

I reconstructed your entire circuit, including the BC107. That caused the switching frequency to drop to about 7kHz. The amplifier or the BC107 model is messed up - and I think it's the amplifier.

I added a capacitor between the collector of Q1 and CV, and the switching frequency went back up to where it belongs.

Like this:

enter image description here

I think you'll find the squeal is gone with that small modification.

Regardless, this is a lousy "amplifier."

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  • \$\begingroup\$ ok thanks. Do you think it's advisable using a 555 timer in an audio amplifier circuit? Because none of the sources i checked use it. \$\endgroup\$
    – karun mathews
    Commented Dec 4, 2020 at 17:35
  • \$\begingroup\$ I had added it, because i needed to include it in this circuit (which is an assignment). \$\endgroup\$
    – karun mathews
    Commented Dec 4, 2020 at 17:36
  • \$\begingroup\$ No, I don't think it is a good idea to use the 555 as an amplifier, at least not with this circuit. \$\endgroup\$
    – JRE
    Commented Dec 4, 2020 at 17:43
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Modulate the Duty Cycle, with the voice or music, and run the 555 output thru an LC filter.

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