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

I have the AGD Duets, the Boulder 866 and the Pass INT-25 (same design as the XA-25 but with a built in preamp).  My house does not have air conditioning, so my primary motivation for getting the Duets was to have lower power amps that I can use in summer. 

I compared the AGD Duets and Pass INT-25 with the Songer S1x speakers. I think that any comparisons of amplifiers is going to depend on how good of a match they are for the characteristics of the speakers, so my comparison might not be relevant to others.  In the thread on the Songer S1x speakers, Ken has a description of the characteristics of his speakers and the types of amps that match well with the S1x.

The Duets differ from some of the other AGD amps in that they use a Gallium Nitride Power MOSFET design for both the output power stage and the power supply.  I also took the Duets to YG's listening room to hear them with the Haley 3, and the Duets compared well to the much more expensive Burmester 909 monoblocks that we also listened to.   

I'm not a reliable reviewer - I rely mostly on whether the music is emotionally engaging, and that can be affected by a lot of other variables, like my stress and attention level. For what it's worth, this was my comparison of the Duets and the INT-25 using the S1x.  I was using the Grimm MU2 with both amps, and with the Duets, I used the Grimm as a preamplifier directly into the Duets.

I replaced the AGD Duets with the Pass INT-25 with very high expectations, and a lot of the magic I was getting with Duets was gone. It still sounded very good, but not emotionally engaging in the way the AGD Duets were, which surprised me. I hadn't run the Pass for 6 months, so I thought it needed some time to warm up, so I left it in for 3 days, and that might have helped, but for most of the music I listen to (a lot of neo-classical piano & strings, alt and indie rock, world music and some jazz) the INT-25 lacked the emotional engagement. Some difference are that the INT-25 is RCA, the Duets are XLR, and the INT-25 is designed with some "pleasing second order distortion", while I think the AGD has lower distortion and also has a higher rated power (300 W at 4 ohms, 150 W at 8 ohms).

The Duets were richer, more detailed. more natural sounding, and very dynamic with very well defined, tight and engaging bass. Kronos Quartet's Pieces of Africa, which has a lot of dynamic range, was just stunning with the S1x and Duets, the best I've heard it.

@kirkwallace Thank you!

@tonnesen 

Thank you for this — really appreciate the care and detail you brought to your impressions. We have quite a bit in common: I've heard all those amps as well, and I've also visited the YG showroom, so your reference points resonate with me.

I think you've actually put your finger on exactly why it's so difficult to draw firm conclusions from comparisons like ours: room acoustics, amp-speaker synergy, and perhaps most importantly, what specific sonic characteristics produce an emotional response in a given listener — all of these vary enormously. Your Songer S1x pairing is quite different from my system, and what makes the AGD "click" emotionally in your room with your speakers may involve entirely different mechanisms than what I'm hearing in mine. Two listeners could hear the same two amps in different systems and come away with opposite rank-orderings, and neither would be wrong.

So rather than seeing our impressions as contradictory, I'd treat them as independently confirming that these are both serious contenders worth attention — and that the AGD in particular seems to hold up across a surprisingly wide range of contexts. The YG visit is a good example of that. Thanks again for sharing this so thoughtfully.

Rowland eventually gave up and abandoned the technology as he could never get fully satisfied with the end result; he could not get the sound to be up to his high standards. Today, if you look at the brands offering Class D, it seems to be many of the mass marketing manufacturers, because they can be made affordably. Now of course there may be exceptions to that, with some specialized boutique firms still trying to make it work. I think that speaks for itself: the sound of Class D is still evolving, but when a well respected high end amplifier engineer leaves it aside, I will too.

@cooperdude6 Its easier to control variables when you design and build the class D module in house, rather than buying one off-the-shelf. Its also very easy to screw up the performance of a good module if there are problems with the input buffer or the power supply. Finally, switching noise is a big deal; it can mess with other parts of your system. IMO/IME, GaNFETs allow the designer to really keep the noise floor down because they have no leads like MOSFETs do. So you can control stray parasitic inductances in a way that is very difficult with the older MOSFETs. 

I really don't think the sound of class D is even a thing. Some class D amps are very good and others are junk. That is why there are such big differences between the 'sound' you get out of them. 

@atmasphere --

Ralph, I'm wondering: Do you have any inclination to develop a higher-powered version of your Class D amps?

@mike_in_nc Yes, some. 

As I keep pointing out to people though, if you can't make the speaker fly with 125 Watts (or 250) you'll need an amp that makes 1200 Watts (or 2500) because that's 10dB higher, which to your ears sounds twice as loud.

What that means is the speaker you have might be too inefficient to be practical. Inefficient speakers have three other problems:

1) speaker cables are far more critical!

2) the amplifier driving a hard to drive speaker will make more distortion; i.e. will not be sounding its best

3) the speaker itself will have a phenomena known as 'thermal compression' which will limit dynamic impact. 

There is no way to get around these problems other than using a speaker that is easier to drive and more efficient. There's no reason at all why anyone should have less resolution on that account; quite often easily driven speakers have more resolution rather than less. This is unsurprising as its easier for the amp to move the speaker cone to do its job. 

Ultimately (at least in my case) the goal is to get the best sound possible. That is done with an easier to drive speaker. I've not found an exception to this in 50 years but you never know...