At this point I think we can all agree that yesterday's amplifier was pretty much terrible. It had way too much signal distortion and not enough common sense. Now that I've learned what I did wrong, I can apply this knowledge to future projects. Or current projects, as the case may be.
In Analog Systems Design last week I built a PROPER amplifier. As it turns out, a real amplifier has multiple stages designed to preserve and filter the audio signal in the most controllable way possible. Here's a flowchart for the one I built last week:
As you can see, a proper audio amplifier actually contains two amplifiers. And a few other components in between that I'll get in to a little later. To analyze this circuit, let's start from the end and work backwards:
- The Power Amplifier: This is very similar to the circuit I built in the last update. It takes a voltage signal and makes it louder. Nothing too fancy. A potentiometer attached to the feedback loop of this amplifier serves as a volume control knob. The input voltage comes from…
- The Equalizer: This summing amplifier takes the signals from each of the audio filters and combines them all into one single signal. Usually the gain on this amplifier is 1, or unity, indicating that it does not amplify or dampen the signals. It only combines them. Depending on the number of filters in the amplifier, the equalizer could have any number of knobs to dampen each frequency range. The frequency ranges are determined by…
- The Audio Filters: Low-Pass, High Pass, and Band-Pass filters can be employed in this section to split the sound in to bass, treble and mid-level chunks. My personal amplifier was designed to isolate bass notes with a (low-pass filter) and high notes (with a high-pass filter). A third, unmodified signal, was also sent to the equalizer for mixing purposes. The high- and low-pass filters only function correctly when the voltage input is large enough to interact noticeably with the capacitors. Most audio signals are not loud enough to do this on their own, so you've got to use…
- The Preamp: This amplifier doesn't have quite as much power or gain as the power amp. Its job is to take the tiny AC signal produced by the audio device, and amplify it before it gets sent to the filters. Some audio devices (like Carbon Microphones, for example) need a DC current in order to produce any kind of signal at all, but sending DC into this preamp would be bad-news-bears. High-DC currents could saturate the preamp very easily, which would result in a totally gross sound. So we need to remove the DC but preserve the AC with…
- The Coupler: Fortunately for all of us, DC voltage can be thought of as an EXTREMELY low frequency AC voltage. Imagine it as an alternating current signal with a period of ∞ and a frequency of 1/∞ (a.k.a. zero). The coupler is just a high-pass filter that passes almost every frequency except for extremely low ones. My coupler let every single frequency in the range of normal human hearing pass right on through.
This amplifier actually sounds pretty good! For testing purposes, we had to output the signal through a really small, tinny speaker that was about an inch in diameter, but it was still a lot nicer sounding than the mess I made with that op-amp so many years ago!
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