Low THD PWM audio amp, DIY friendly

It's all about the Shannon Nyquist sampling theorum.

"Strictly speaking, the theorem only applies to a class of mathematical functions having a Fourier transform that is zero outside of a finite region of frequencies. Intuitively we expect that when one reduces a continuous function to a discrete sequence and interpolates back to a continuous function, the fidelity of the result depends on the density (or sample rate) of the original samples. The sampling theorem introduces the concept of a sample rate that is sufficient for perfect fidelity for the class of functions that are band-limited to a given bandwidth, such that no actual information is lost in the sampling process. It expresses the sufficient sample rate in terms of the bandwidth for the class of functions. The theorem also leads to a formula for perfectly reconstructing the original continuous-time function from the samples." [1]

But, HOLD ON: "Practical digital-to-analog converters produce neither scaled and delayed sinc functions, nor ideal Dirac pulses. Instead they produce a piecewise-constant sequence of scaled and delayed rectangular pulses (the zero-order hold), usually followed by a lowpass filter (called an "anti-imaging filter") to remove spurious high-frequency replicas (images) of the original baseband signal." [1]

In case that was too verbose.....

"sample rate that is sufficient for perfect fidelity".... "such that no actual information is lost in the sampling process"... "theorem also leads to a formula for perfectly reconstructing the original continuous-time function from the samples"

PERFECTLY!!! Zero information loss between sampling and reconstruction. Still WTFs me to this day.

So yeah, you can feed bits to a PWM. But that not music.

[1] https://en.m.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_samp...

The good class D amplifiers are closed-loop control systems. They don’t just create a output a fixed sequence of highs and lows for a given piece of music — they determine when to switch based on some combination of the input signal, the observed output voltage and current, and various state variables (for example, for protection). And they operate at frequencies of several megahertz, possibly with rather high precision pulse widths needed.

So one could do this with an ADC, a microcontroller, and a single output bit, but it would need to run the ADC at quite high frequency and possibly have multiple input channels (for current and voltage). This sounds expensive.

A TPA3255 costs $12.50 (qty 1 from Digikey). Add a dollar or two a nice DAC. I don’t think a 20-bit ADC, a DSP running at a couple hundred MHz, and most of the same driver and power circuitry that’s already in the TPA3255 would come out at the same price.

A good amplifier has better than 100 dB SNR. Your ear may have better than 100 dB dynamic range. Your sound system may have a bit less useful range, since your goal isn’t actual to destroy your ears. But 100 dB is a vaguely credible target.

So somehow you’re supposed to get a preamp and an amp, and you’re supposed to connect them, usually with RCA (unbalanced) cables. You’re supposed to get both devices to agree as to what voltage is full scale. And you’re supposed to get a noise floor, on the interconnect, better than -100dBFS. And you are supposed to do this with at least two separate cables plus whatever ground (or neutral) connections exist to both chassis, and this is all done near 60Hz currents that induce a voltage around every loop in question.

This is a tall order. At least the pros use balanced connections.

Or the industry could move to digital interconnects, where the DAC and the amp are connected by short traces on a PCB with a proper ground plane. And full scale on the DAC could be matched to the amp’s input.

Note: the best tweeters ever made have rather strange electrical characteristics, in the same vein that some headphones do. The Dynaxx family tends to handle anything, high or low ohm (even speakers that are 4ohm nominal or headphones that are 1000+ohm), but also ones with ridiculous impedance curves. People have realized that due to its exceptional tolerance of weirdly designed speakers, it makes for a good speaker amp too. 25w is enough to drive these tweeters loud enough, and some mids as well; woofers, no, not with a single board.

Kevin Gilmore also has sequels to his Dynaxx family (these being, themselves, improved versions of the KG Dynamic), due to some parts being replaced as they've fallen out of production and are hard to buy now.

The catalog I'm recalling was cover to cover populated amplifier circuit boards accompanied by specs and prices. Lots of huge toroidal transformers... maybe it was a local thing back in IL.

The cover was mostly black with glowy green text IIRC, nothing like the vintage Crutchfield catalogs I can find via GIS/ebay. Oh well.

"the electromagnetic signal of course rides half outside the wire in a world increasingly filled with RF/EMF noise. Be sure to keep your cables off the floor or there WILL be impedence changes. Also, the Frequency Response is largely not relevant as 95+% of audiophiles don't have effective bass traps or ways to remove excess energy from the room. Intelligibility is the better metric, and no one uses it because it requires bass trapping and that's just to ugly and expensive."

Are you ready to rip my head off yet for all these largely unknown but completely true scientific facts? :)

It’s a shame because Class-D has come a long way, there seems to be a natural opposition to anything digital.

Class-A is great but is expensive, heavy and a big electricity bill.

Or price, although that would raise new questions. Another is availability of components. Or perhaps the form factor (these capacitors seem slightly less tall than the ones on the other board in the picture).