GaN-based Class D power amps


The use of GaN-based power transistor tech is now emerging for Class D audio power amplifiers. Seems appropriate to devote a forum thread to this topic. At least 3 companies have commercial class D amps in their books:

Merrill Audio, with their model Element 118 ($36k per monoblock, 400 W into 8 ohms, 800W into 4 ohms), Element 116 ($22k per monoblock, 300 W into 8 ohms, 600W into 4 ohms) and Element 114 (coming soon). 
https://merrillaudio.net

Review of Element 118 at this link:
http://www.enjoythemusic.com/superioraudio/equipment/1018/Merrill_Audio_Element_118_Monoblock_Amplif...

ADG Productions, with their Vivace Class D amp ($15k per monoblock pair, 100W into 4 ohms). (The designer emailed me indicating he has another product in the pipeline.)
http://agdproduction.com
Review of the Vivace Class D moniblocks at this link (warning: link might not work (1/11/2019)):
https://positive-feedback.com/reviews/hardware-reviews/agd-production-vivace-gantube-monoblock-ampli...

Technics SE-R1 Class D stereo amp ($17k per stereo amp, 150WPC into 8 ohms, 300WPC into 4 ohms) 
https://www.technics.com/us/products/r1/se-r1.html
Preliminary review of the Technics SE-R1 at this link:
https://www.stereophile.com/content/technics-se-r1-digital-amplifier
Technics also has a lower priced GaN-based class D integrated amp in their catalog:
https://www.technics.com/us/products/grand-class/stereo-integrated-amplifier-su-g700.html

Anyone listened to or own any of these amps?


Previewcelander
Thanks, I've been reading a bit on phase shift or angle, same thing ??  Still trying to digest it, from one article it appears they're saying not to worry to much about it in an amp. 

In one sense, you can consider the effects of phase angle being built into the frequency response (which represents voltage sensitivity over the full bandwidth): whether the phase angle is 0 degrees or 60 degrees, the voltage demanded from the amplifier remains the same. As a result you don’t have to worry that an amplifier is going to have to swing loads of extra voltage and current in order to cover the effects of a difficult phase angle.

Thanks, I’ve been reading a bit on phase shift or angle, same thing ?? Still trying to digest it
"phase shift or angle"
Two TOTALLY different things. Phase Shift can be caused by a few things, in Class-D case it’s caused the by the output filter that needs to get rid of the 600khz switching frequency noise, without that filter your speakers and ears would fry up.
This filter HAS to be low order because it needs to absorb a lot of wattage And because of it’s low order roll off trying to get rid of the switching frequency, it continues to create it’s effect in (degrees) down into the audio band as 70!! degrees of phase shift (red trace https://ibb.co/jfd6tqy) and it’s sensitive to the human ear because it in the upper/mids and highs, which too many complain about in Class-D to be just a coincidence.

All this is related to the one smart thing atmasphere said about phase shift down into the audio band, but it was on another thread and the direct opposite to what he's trying to sell you here.
atmasphere
"If you really want to get the soundstage right, the amp needs to have minimal phase shift in the audio regions so it will need bandwidth past 80KHz.


The FIX= As Technics did in the SE-R1 because of the use of GaN technology, is to raise the switching frequency nearly 3 x higher to 1.5mhz, and you can then raise the filter up with it, and then the phase shift effects are also raised along with it out of the audio band. to above 20khz Simple isn’t it. This comes at a cost of having to use small heat sinks, nothing major.

Phase Angle also expressed in -degrees, common in bass measurements of speakers, it combines with the speakers impedance to create an amplifier loading measurement call EPDR (equivalent peak dissipation) resistance. It’s a long read but all explained here. 3 pages of it here, this is the 2nd page.
https://www.stereophile.com/content/heavy-load-how-loudspeakers-torture-amplifiers-page-2

Cheers George
1) @atmasphere , this would only be true if there was a difference in phase-shift between the left and right channels:

The ear uses phase information for echo location, so it can alter the soundstage too

2) @georgehifi , I am still waiting for you to explain how this 1KHz square wave for an ICE_Power module with "theoretically" large phase shift is really quite a nice square wave. If there was a large phase shift between all the harmonics that build up this square wave, then the square wave would be a mess. Hint .... the graph doesn’t show what you think it does.
If you believe this is the actual phase shift of the output of an IcePower based amplifier, can you explain this near perfect 1KHz square wave?

https://www.stereophile.com/images/archivesart/555Belfig2.jpg

3) @georgehifi , EPDR only impacts devices operating in the linear region. Class-D devices switch hard on/off, hence EPDR is e meaningless term for Class D.

4) @georgehifi , at 500Khz, for dynamic speakers, a typical tweeter may be around 100-200 ohms. Given a magnetic storage element, but no LC output filter, the harmonic content at 500Khz, given the high impedance of the speaker at that frequency is not going to damage the tweeter (and that’s not even taking into account inductive properties of crossover capacitors). Other speaker technologies may have issues obviously.

5) @georgehifi , the filter does not need to "absorb a lot of wattage". The elements are reactive and not used in a manner where the energy is stored, then dissipated (like a logic gate). Losses would be due to parasitic resistances which would be kept to a minimum.
I guess I'm doomed to not understanding this. I thought a phase shift like that shown in the plot where it dips is a negative shift which means a lag in the signal. Something not audible just that the whole signal is delayed my a few microseconds. I don't know . 🙄
All this is related to the one smart thing atmasphere said about phase shift down into the audio band, but it was on another thread and the direct opposite to what he's trying to sell you here.
atmasphere
"If you really want to get the soundstage right, the amp needs to have minimal phase shift in the audio regions so it will need bandwidth past 80KHz.
Actually I'm saying the same thing here as well.  But the above statement is in regards to amps lacking feedback. I am **also** saying that amps with **enough** feedback (plus 35dB) can have 0 phase shift in the audio band even without that bandwidth.
this would only be true if there was a difference in phase-shift between the left and right channels:

The ear uses phase information for echo location, so it can alter the soundstage too
@roberttdid  Absolutely correct. But that difference has to come from the signal, and the amp has to otherwise not mess with it. When we are talking about amps that have no feedback (which can be an SET or a class D amp) minor differences between the two channels can result in slightly different bandwidths and so a difference in phase shift between the two channels. To avoid this both channels have to have enough bandwidth that this is off the table.