Final month, an fascinating Class AB audio amplifier was within the highlight, a monoblock-configured pair of which I’m truly listening to (Widespread Panic’s Gentle Fuse Get Away, to be exact) as I sort these follow-on phrases:

This time, it’s Class D’s flip within the highlight. Right here’s a abstract hyperlink to EDN’s voluminous protection of the know-how over the previous few many years, and a Wikipedia’s abstract:

A category-D amplifier, or switching amplifier, is an digital amplifier during which the amplifying gadgets (transistors, normally MOSFETs) function as digital switches, and never as linear acquire gadgets as in different amplifiers. They function by quickly switching backwards and forwards between the availability rails, utilizing pulse-width modulation, pulse-density modulation, or associated methods to supply a pulse practice output. A easy low-pass filter could also be used to attenuate their high-frequency content material to supply analog output present and voltage. Little vitality is dissipated within the amplifying transistors as a result of they’re at all times both totally on or totally off, so effectivity can exceed 90%.

 Tripath, which branded the know-how as “Class T”, was the primary mainstream-volume provider of Class D know-how. My first publicity to Class D stretches again almost 30 years, to 2007 once I first met with (and auditioned prototype gear from) D2Audio and its at all times entertaining (meant as a praise), to not point out well-informed, Chief Know-how Officer Skip Taylor at CES. I subsequently sic’d Skip on my then-colleague, EDN then-Analog Editor Joshua Israelsohn, who had quite a few gratifying mind-melding (possibly additionally melting?) conferences with Skip and his colleagues. Intersil purchased D2Audio a 12 months later, preceded by Texas Devices’ 2000 acquisition of Burr-Brown and adopted by Infineon’s 2018 buy Merus Audio and Analog Gadgets’ 2021 Maxim purchase (all as pattern examples, not meant to be a complete checklist)…and Class D know-how was off to the races.

Class D vs Class AB

Right here’s one other tackle Class D versus the Class AB (and A, for that matter) precursors, from Paul McGowan, the co-founder and CEO of high-end audio gear provider PS Audio, whose “Ask Paul” ongoing video sequence is each entertaining and academic, due to this fact extremely beneficial:

That this video comes from Paul (and PS Audio, positioned simply “up the highway” from me in Boulder, CO, for that matter) is very revealing, I believe. Audiophiles, the nexus of PS Audio’s buyer base, are usually talking each change-adverse and perversely choosy in terms of perceived high quality. That they, who D2Audio was particularly concentrating on with its demos means again in 2007, have been among the many first to undertake Class D amplifier know-how tells me a couple of issues:

• With all due respect to Schiit co-founder Jason Stoddard, his diatribe about Class D’s tendency to “hiss like a demon cat, drilling slowly into your synapses and draining your soul” is dated and overstated, IMHO no less than. As I famous final month, “he may need been proper about Class D a couple of years in the past, particularly within the near-field configurations he’s particularly advocating for Rekkr, however not.”
• Extra usually, the know-how has for some time now been ok for audiophiles, so I’d wager it’s additionally ok for the plenty.
• And why was it interesting to audiophiles? Value for them is a secondary-at-most concern, proper? Nicely, their listening rooms are likely to comprise large audio system, requiring formidable quantities of energy to drive them. And, as Wikipedia notes, “The main benefit of a class-D amplifier is that it may be rather more environment friendly than a linear amplifier, dissipating much less energy as warmth within the lively gadgets…additionally, given that giant warmth sinks usually are not required, class-D amplifiers are a lot lighter weight than class-A, -B, or -AB amplifiers.”

Schiit’s Class-AB-based Rekkr

Lighter…and far smaller, too. Within the spirit of “an image paints a thousand phrases,” listed below are some examples. First off, right here once more is the Class AB-based Rekkr (Web Archive cache hyperlink), which is sound-spec’d as follows:

• Stereo, 8 Ohms: 2W RMS per channel
• Stereo, 4 Ohms: 3W RMS per channel
• Mono, 8 Ohms: 4W RMS

with the next kind factor-related specs:

• Measurement: 5” x 3.5” x 1.25”
• Weight: 1 lbs

Schiit’s Class-AB-based Gjallarhorn

Subsequent up is its Gjallarhorn “huge brother”, additionally Class AB-based, additionally talked about (however not proven) in final month’s piece, and sound-spec’d as follows:

• Stereo, 8 Ohms: 10W RMS per channel
• Stereo, 4 Ohms: 15W RMS per channel
• Mono, 8 ohms: 30W RMS

with the next kind factor-related specs:

• Measurement: 9” x 6” x 2.5”
• Weight: 8 lbs

Rekkr vs Gjallarhorn dimension

Right here’s a visible comparability of their respective kind elements:

Foreground, then background, focus:

Ditto:

Eager-eyed readers could have already observed that the comparability’s not fully truthful, for the reason that Gjallarhorn integrates the AC/DC conversion circuitry that’s alternatively positioned (no less than partly) within the Rekkr’s exterior AC/AC “wall wart”. However as you possibly can see, the Rekkr PSU is fairly tiny, so…:

Enter Class D opponents

Now for the Class D opponents (and, I’d usually argue, successors). Earlier this 12 months, forward of the looming tariffs, I acquired three totally different “Chi-Fi” producer/mannequin combos…not counting the Class D circuitry contained in the powered audio system I already owned…or my newest community audio streamer…or my newest sound bar (from Yamaha, changing the Hisense unit I complained about in Could)…all of which you’ll hear extra about in different weblog posts to return…

The primary was a monoblock-only unit, the Fosi Audio V3 Mono:

Its (once more, mono on this case) output specs, together with these of the opposite two gadgets I’ll be showcasing at the moment, range relying on the capabilities of the facility provide linked to it:

• Rated Energy Output : 48V/5A–240W@4Ω ; 32V/5A –100W@4Ω

And listed below are its kind issue particulars:

• 6 x 4.2 x 1.6 inches (142mm x 105mm x 35mm)
• 06 kilos (0.48)

The second is Douk Audio’s A5, a dual-channel (stereo) Class D amp:

Once more, its output specs are energy provide voltage-and-current, in addition to speaker impedance, dependent. Douk Audio offers extra granular element on its web site than Fosi Audio does even within the V3 Mono consumer handbook, sadly. However the common pattern is comparable in observe, on condition that they’re each primarily based on Texas Devices’ TPA3255 chipset:

And its kind issue particulars? Right here you go:

• Dimensions (W*D*H): 95*92*50 mm/3.74*3.62*1.97 in
• Internet weight: 506 g/1.12 lb

Final, however undoubtedly not least, is a newer Douk Audio machine improve, the A5 Professional, including each a Bluetooth receiver and a separate headphone output amplifier:

Similar TI TPA3255-based audio energy amplifier subsystem as with the bottom A5, so identical output specs as proven earlier. The shape issue is tweaked a bit (however solely a bit, and sure mostly-to-completely to only make room for extra knobs-and-such on the entrance panel), nevertheless:

• Dimensions (W*D*H): 130*112*33 mm/5.12*4.41*1.30 in
• Internet weight: 525 g/1.16 lb

Schiit Rekkr (AB) vs Fosi Audio V3 (D)

That’s the final of the inventory photographs, no less than for some time; now for some extra “actual life” ones. First off, right here’s how the Schiit Rekkr stacks up (actually) in opposition to the Fosi Audio V3 Mono, which is able to as much as (speaker impedance- and energy supply-dependent) greater than 50x the mono output energy at comparable distortion:

Schiit Gjallarhorn (AB) vs Fosi Audio V3 (D)

How in regards to the Schiit Gjallarhorn versus the Fosi Audio V3 Mono? Glad you requested. Once more, as a reminder, the latter has ~8X the mono output energy on this case, with its beefiest energy provide possibility and when driving the identical impedance and at related (inaudible) distortion ranges. To not point out being half the worth (and even much less, relying on the place it’s sourced and the way it’s kitted):

Once more, a two-photo foreground-then-background focus shift:

As soon as once more, the detail-oriented amongst you’ll level out that the Gjallarhorn chassis additionally encompasses AC/DC conversion circuitry, exterior to (and never proven in) the Fosi Audio case. You’re proper, though there was a way to my insanity. I didn’t wish to present three units of vs-V3 Mono images, one set with every of the three PSUs I’ve in my possession: 32V/5A, 48V/5A and 48V/10A. As an alternative, listed below are the standalone undersides of the facility provides, able to getting used with any of the three Class D amplifiers I’m masking at the moment. 32V/5A first:

Now 48V/5A:

And eventually, the “Huge Kahuna” 48V/10A model:

And right here they’re stacked on prime of one another, with the 32V/5A one on prime and (clearly) the 48V/10A one on the underside:

The DC Energy Filter

Unbiased opinions recommend that the incremental energy output of the Fosi Audio V3 Mono tails off past the 48V/5A level…the Douk Audio items’ additive output efficiency will increase extra linearly at 48V/10A, however that’s to be anticipated as there are two audio energy amplifiers—one for every channel—inside. However there’s one other motive to run the Fosi Audio V3 Mono—two of them, truly—with a 48V/10A supply. In such a configuration, the corporate additionally sells what it calls a “DC Energy Filter”, which (together with an acceptable cable possibility) splits the 10A enter present evenly amongst each of its outputs, enabling a single PSU to concurrently gas two amplifiers:

I personal two DC Energy Filters. One got here bundled with a two-amplifier set I purchased off eBay. The opposite was a standalone buy from Fosi’s on-line retailer, used with two different V3 Mono amps I received individually (for causes I’ll clarify additional in a teardown to return!):

Schiit Gjallarhorn (AB) vs Douk Audio A5 (D)

What about these two Douk Audio items? Right here’s the A5, alongside the 48V/5A energy provide it got here bundled with, on prime of the Schiit Gjallarhorn:

Schiit Gjallarhorn (AB) vs Douk Audio A5 Professional (D)

Now for its A5 Professional “huge brother”:

Schiit Gjallarhorn (AB) vs Douk Audio A5 + A5 Professional (D)

As a result of I couldn’t resist, the next photographs show that (after dishing out with the PSUs), I may match each Douk Audio gadgets on prime of a Gjallarhorn:

All of the Class Ds stacked

And in closing, listed below are all three of at the moment’s Class D gadgets stacked on prime of one another, showcasing their kind issue similarities:

Notable observations

Wrapping up, there are a few different factors I needed to notice. First off, all three of the Class D amps assist (consider it or not) user-accessible op-amp swapping:

analogous to the “tube rolling” of occasions previous (and current, for some people, and probably others, too…there may be, in any case, a Vali 2++ now sitting on the prime of the Schiit stack on my desk):

Lastly, what’s with the “PFFB” promotion outstanding on each producers’ web sites?

It stands for Submit-Filter Suggestions, and understanding what it’s and does first requires a step (or few) again. Though, because the Wikipedia Class D definition I shared in the beginning of this piece famous, “A easy low-pass filter could also be used to attenuate their high-frequency content material to supply analog output present and voltage,” in observe the output filtering circuitry tends to be notably extra complicated than this; to render inaudible the in any other case distorting aforementioned “hiss like a demon cat”, for instance, to suppress part shift artifacts, and so on. To reiterate on this circuit’s robustness significance, I’ll flip you over to Paul McGowan once more for extra on the subject:

PFFB, applied in TI’s TPA3255 (and others), is the most recent evolution on this output filtering scheme. Quoting from Google’s AI Overview abstract of the search subject:

PFFB, or Submit-Filter Suggestions, is a secondary suggestions loop in Class-D amplifiers that takes a portion of the sign after the LC output filter and feeds it again to the enter to enhance audio high quality. This method reduces distortion and improves the linearity of the facility stage and output filter elements, significantly the inductor, which is a major supply of distortion. PFFB additionally will increase load independence, that means the amplifier’s efficiency is much less affected by the precise loudspeaker linked.

Right here’s a visual instance of PFFB’s advantages. First off, an output level-vs-frequency plot from Audio Science Evaluate’s analysis of the Fosi Audio V3 Mono:

Requoting extremely beneficial content material professional (and long-time private collaborator) Amir Majidimehr, “There may be primarily no influence as much as 20 kHz between the 4 and eight ohm, indicating very low output impedance, albeit with a little bit of peaking. Evaluate that to non-PFFB amps corresponding to Fosi Audio V3 stereo amp:”

The TL;DR (on the finish of one other lengthy writeup…sorry!) abstract of Amir’s findings (and PFFB’s advantages): it suppresses an amplifier’s perceived loudness from in any other case various with output frequency, not that this arguably was perceptible a lot if in any respect beforehand, candidly (particularly as a result of the variability tended to happen at excessive frequencies, exhausting for all however the “golden ears” amongst us to discern, anyway). However because it now “comes alongside for the trip” with fashionable Class D amplifier designs anyway, at little if any incremental value and with no sonic draw back…

With that, encroaching on the two,300-word threshold, I’m going to log out for at the moment. Extra on this subject, together with the earlier-promised Class D amplifier teardown, to return quickly. Till then, “hold forth” along with your ideas within the feedback!

—Brian Dipert is the Editor-in-Chief of the Edge AI and Imaginative and prescient Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the corporate’s on-line publication.

 Associated Content material

• Audio amplifiers: How a lot energy (and at what tradeoffs) is basically required?
• EDN Class D amplifier protection
• Class D audio energy amplifiers: Including punch to your sound design
• The reality about Class D amplifiers

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