Simple audio preamp

Ever since Alexander Graham Bell, we have fundamentally had a single equation relating sound to electricity: P=kE. That is, the sound pressure being either felt by a microphone or being imposed on the air by a speaker is proportional to voltage. However, the value of k varies depending on what stage of the process you're talking about. For microphones, k is quite small, and for speakers, k is (relatively speaking) quite large. How we get from one place to another is with amplification, where in Eo = k*Ei.

Where this gets interesting is when different manufacturers want to interconnect equipment and have expectations on what the value of k is supposed to be. That brings us to the definition of "line level," as that is what stereo components expect as inputs and are expected to present as outputs.

Naturally, historically there have been two values for this - one for professionals and a different one for consumer products.

The professional standard is +4 dBu, which is about 3.472 Vp-p. 0 dBu is historically defined as the voltage required to generate a milliwatt on a 600Ω load. That 600Ω value is a holdover from telephony. Modern stereo equipment is much higher impedance than that. Still, that voltage level remains.

The historical consumer standard, by contrast, is -10 dBV. That's a Vp-p of .894 volts. When you look at it that way, it turns out that Belkin actually is doing the right thing with the analog output of their SoundForm Connect box. The trouble is, things changed in the 80s and it appears Belkin missed the memo.

CD player manufacturers wanted to highlight the extreme dynamic range (by comparison) of digital sources, so they cranked up the output levels to raise them higher above the noise floor.

Digital levels simply represent the input value to the DAC. Typically the very highest amplitude signal possible is called 0dBFS - that is, 0 dB away from full scale. Historically with analog audio, however, 0 dB has been regarded as a "nominal" maximum level, which is occasionally exceeded by transient signal excursions. Allowing for these excursions allows peak sounds to be rendered properly without clipping. That means that amplifiers have to allow for that. In the digital realm, you have to scale back the nominal level to make sure you have headroom for the peaks. Increasing levels gives you more resolution, but less headroom.

It turns out that going from -10 dBV to +4 dBu is about a 12 dB gain. The little amplifier I built for our patio speakers works much better with a higher level of input power (with a lower level, you have to turn the volume up until you hear a lot of noise). Thus, this pre-amp project.