Comment by klodolph | Hacker News Reader

Comment by klodolph | Hacker News Reader

The article covers some of the alternatives below, so take this rant with a grain of salt.

I don't understand the need to connect a massive linear power supply to a class D power amplifier. That seems completely backwards to me. If you're going class D, then why not eliminate what is often one of the biggest and most obvious expenses and replace the linear power supply with a switching power supply?

Typically in these projects you spend some chunk of the BOM cost on a few nice transistors or an amp chip, another big chunk on the transformer, smaller chunks of money on capacitors, then miscellaneous ICs/amps/hardware/etc. There's some variation here between components, but it just seems bonkers to pay for a big hunk of metal for a 1960s era solution for your power supply, when you are already using super fancy high-speed switching circuits in the amp.

There is no reason that these devices should have any appreciable mass beyond the heat sink. If you've got a massive transformer, you're building an amp like you've traveled in time back to the 1980s, before class D amps existed (well, before they were feasible for these applications). Nothing wrong with that, it's just that the linear power supply adds a lot of cost and mass for no reason.

At my last company, we tested tons of combinations of class D modules with various power supplies, and the best sounding results tended to be with linear power supplies. Don't get me wrong, there's tons of tricky stuff when it comes to the subjective side of audio, so it may be at least partially placebo effect. But we pretty consistently found that off the shelf switching PSUs sounded worse subjectively, even if they measured the same on the bench.

I haven't been doing audio for a while, so I don't remember more of the details. But audio signals tend to need all their power in short bursts all at once, when a low frequency bass note hits for example, and so the transient current tends to be much more important than a stable voltage rail. A lot of switching supplies do not optimize for this. Well designed versions can both sound great, but a cheap linear supply is going to sound much better than a cheap switcher. Audio is stuck with 1960s technology for the most part anyway though.

You could add the massive capacitors if you needed the bursts of energy, it just seems like the mains transformer itself is out of place.

> so it may be at least partially placebo effect

That really sounds like a kind of thing that should be tested blinded.

We did a lot of blind testing, but blind testing takes more effort and we were always swamped. Usually the engineers would just test things ourselves, and then I would give my boss a blind test for the final approval.

>I don't understand the need to connect a massive linear power supply to a class D power amplifier.

Generally from what I understand, linear, unregulated (gasp) supplies are preferred over anything else. I think the reasoning is that the PSRR is great at lower frequencies so having a bit of 120Hz ripple isn't a big deal.

Though this is a bit of antiquated advice, IMO, since you can get amazing performance out of SMPS supplies with a bit of filtering.

Although they are conceptually related, it is harder to engineer switching power supplies than it is to engineer class-D amplifiers, so I could sort of understand a hobbyist drawing up a design that uses 1960s technology to make DC to go in to 1990s technology.

It's hard to engineer a good class D amplifier, but you don't have to do that, you just buy a chip and read the application notes. Same reason why you don't need to engineer an SMPS, you buy a module (if you are a hobbyist) or buy a chip and read the application notes (if you are making a product).

If you are just gluing modules together, then just get the SMPS module and save yourself the weight. If you think you need a linear power supply, then why do you need a linear power supply if you don't need a linear amplifier? Just seems odd.

If you are just gluing modules together... then buy an amplifier. ;)

Because gluing modules together you can spend less money than buying an amplifier with similar specs, and because it's your hobby.

In the past, when I've built amps, it might have cost me something like $300 in materials to make something which would have cost me $1000 to buy. Back when I was a high school student and didn't have much money, that was a good deal.

Perhaps because you'd have to synchronize the switching of the supply with the switching of the amp to prevent aliasing effects.

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