Simple audio preamp

A simple amplifier to boost audio by 15 dB

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The Belkin SoundForm Connect AirPlay 2 is a nice little box to let you stream audio to a pair of powered speakers or a stereo or what not. The only problem with it (other than it being somewhat overpriced) is that the analog audio output is about 15 dB too low. This simple preamp is a workaround for that.

The heart of the preamp is an LM386 dual op amp. Since the goal is simply line level outputs, we don't have to worry about output impedance a whole lot. Just about any op amp will work just fine for this.

Audio signals are bipolar. There are two ways you can amplify them. You can either use a bipolar power supply to power the amp or you can shift the audio upwards so that the maximum negative excursion is at or above 0 volts. While you can AC couple audio to achieve the latter, this requires a virtual ground configuration for the amp. This can really only be done in the inverting configuration, which means that if you don't want the audio to be inverted on the output, you have to use two inverting amplifier stages.

In addition to that, proper line level output is about 6 volts peak-to-peak. While USB-C can supply higher voltages, our current requirements are low enough that PD negotiation or a boost converter would be overkill. You could simply use a barrel connector instead of USB-C power with an arbitrary voltage, but that's a bit retrograde. Using a charge pump to invert the 5 volt supply to make a ±5v supply solves all of the problems and is easy to do.

Since we have a bipolar supply of sufficient voltage, we can use a simple non-inverting configuration. The gain is 6x (1 + 100k/20k), which for audio is 15 dB (20log₁₀[voltage gain]). The input is AC coupled with a cap to eliminate any input DC bias. But the problem with that is that it means that there is no DC bias on the op amp's positive input. This won't actually work in practice - you have to tie that input to ground with a bias resistor. The DC blocking capacitor and bias resistor form an RC low pass filter, so it's important for bass fidelity to pick values that result in a sufficiently low roll-off. 10 µF and 10 kΩ result in a nominal roll-off frequency of under 1 Hz.

The USB-C power supply starts with a special power-only USB C receptacle. There are no data pins connected - just the two Vbus, ground and CCx pins. Since we require 5v at less than 1A we can just use two 5.1kΩ pull-down resistors on the CCx pins. The shell is loosely coupled to ground in an effort to prevent ground loops. VBus is passed in through a ferrite bead to try to filter any supply noise.


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  • What really is line level?

    Nick Sayer11/03/2021 at 21:07 0 comments

    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.

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