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Expanding the Class D Conversation: How Would You Characterize Their Differences?

Expanding the Class D Conversation: How Would You Characterize Their Differences?

I'm currently trialing the NAD M23 (1st gen. Eigentakt-based), and I find it intriguing enough to want to understand it better — which means understanding the broader sonic landscape of class D. So I'm crowd-sourcing.

In a recent exchange, the estimable Ralph Karsten (Atma-Sphere Music Systems) made two comments that stopped me cold. For those who missed it, here's what he said:

"IME, class D amps vary in sound more than tube amps, which is to say, quite a lot."

"IMO there is a bigger difference between various class D amps than you hear between various tube amps. IOW just because you heard one class D amp says nothing about how the next one might sound."

Link: https://forum.audiogon.com/posts/2885828

As I think through this more carefully, these are genuinely important claims. My own experience with tube amps confirms that they produce audibly distinct characters across topologies and designs. If Ralph is right and class D exceeds that range, then generalizing from one class D experience to another is even more hazardous than I assumed.

One specific question for Audiogon members:

If you have a Class D amp or have compared class D amplifiers, how would you describe their character(s)?

Here are some criteria I use:

  1. Frequency balance — Is the tonal response even across bass, mids, and treble, or does it favor certain regions?
  2. High-frequency texture — Are the highs extended and smooth, or edgy, grainy, and fatiguing?
  3. Bass definition — Is the low end tight and articulate, or loose and bloated?
  4. Midrange character — Does the midrange feel present and natural, or recessed and thin?
  5. Transient speed — Does the amp respond quickly to dynamic attacks, or does it sound sluggish and rounded?
  6. Dynamic range — Does it scale convincingly from quiet passages to loud ones, or compress the difference?
  7. Soundstage width and depth — Does it create a convincing three-dimensional image, or sound flat and narrow?
  8. Image specificity — Are instruments and voices placed precisely, or do they blur and wander?
  9. Background noise floor — Is the silence between notes actually silent, or is there grain, haze, or hash?
  10. Long-term listenability — After an extended session, do you want to keep listening, or has something been quietly fatiguing you?

If you can include relevant system context — room, speakers, preamp — please do. Those variables will help me interpret what the amp itself is contributing.

I'm less interested in rankings than in understanding what Ralph mentioned, namely the [vast] range of sonic signatures class D is capable of. Eigentakt, Hypex, Pascal, Purifi, GaN-based, etc. — all fair game.

Price is no constraint here — I'm interested in the full range of what's out there.

hilde45
atmasphere's avatar
atmasphere
12,404 posts
 

The point I question is whether low impedance alone can cause amplifiers to distort, as long as amps are similarly matched with their loads.

@devinplombier All amps make distortion. When driving a lower impedance load, all amps make more distortion; ’matching to the load’ has nothing to do with it, although there are better matches for some amps and some speakers. 

When you compare apples and oranges like that it can be confusing. To understand how this works, the 800W amp driving 2 Ohms will make more distortion doing that than it will driving 4 Ohms, and will be even less distortion driving 8 Ohms, all while making the same power.

The 800 Watt amp might be lower distortion than the 40 Watt amp or it may not. That depends on the design of the two amps; an entirely different subject (I think we can agree that a 40 Watt amp is likely to have a very different design from one that makes 800 Watts). But in either case, to minimize distortion low impedances should be avoided. 

bartsw's avatar
bartsw
232 posts
 

@hilde45 

I'm getting my hands on an AGD to try against the NAD M23.

Do you use a switcher?

hilde45's avatar
hilde45 OP
5,998 posts
 

@bartsw No, I don't use a switcher. Don't want it in the signal chain, plus I want to use the same exact cables.

bartsw's avatar
bartsw
232 posts
 

@hilde45 passive switchers don't add noise. Sometimes I truly think two amps sound different but they are not.

hilde45's avatar
hilde45 OP
5,998 posts
 

@bartsw 

Good point.

There are plusses and minuses. Based on some research just now:

I realize there would be no coloration from active circuitry and the switching speed would be good because auditory memory is notoriously short.

There could be electrical challenges from the following issues:

  • If there is any contact resistance and oxidation from relay contacts and rotary switches with low-impedance it could matter at the margins.
  • More important, the when you switch between two amplifiers, the *inactive* amplifier’s output stage still presents an impedance to the speaker terminals. This can subtly load the active amp and cause issues, I’ve read. It’s likely the switchers address this in their design but I’d want to verify that. I don’t want both amps connected simultaneously even briefly.
  • I’d be alert to grounding and hum loops if both amplifiers are connected and share a chassis ground through the switcher, especially with mixed solid-state/tube topologies.

As for the plusses and minuses from the audio/critical perspective:

Advantages:

  • The switching speed advantage is decisive for level-matched comparisons and would allow focus on a specific attribute.
  • Eliminates the "cable swap" time delay. The switcher removes that variable.

Disadvantages

  • Rapid switching can prevent the kind of extended, immersive listening that reveals what an amplifier actually does to music over time — how it handles dynamic architecture across a whole movement, how fatigue accumulates or doesn’t, how your relationship to the music shift A switcher optimizes for *discrimination* at the cost of *acquaintance*. The experience of "listening for differences" is different from the experience of "listening to music."
  • The inactive amp’s electrical presence is a genuine presence — not just electrically but sonically. The active amp is operating into a slightly different effective load than it would standalone. How much this matters depends on the specific output impedances and topologies. 

Overall: If I had a switcher I knew would be benign, I would use it as another way to compare, not a replacement for the way I compare now.

Thanks for the suggestion!

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