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GaN Systems Amplifier Eval Board Measurements

amirm

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#1
This is on overview of new type of transistor called GaN (Gallium Nitride) and an evaluation package from GaN system which kindly sent it to me last year(!). I lost touch with them for a while and hence the lateness of this write up.

This is a shot of the eval board (GS-EVB-AUD-xxx1-GS) which is rated at 200 watts/channel into 8 ohm:
GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS review.jpg


Notice the absence of any heatsinks. Here is the accompanied 400 watt switching power supply:

GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS Power Supply review.jpg


GaN transistors are to the left of the transformer on the bottom with yellow stickers on them. Again, no heatsink due to high efficiency.

Current switching power supplies and amplifiers (Class D) use a type of transistor called MOSFET. These transistors are used in digital systems and are designed to be operated at their extreme operation point (fully conducting or not). This provides high efficiency because the transistor is not kept in less conductive state that generates a lot of losses. What I just described is an ideal transistor. Practical MOSFETs take time to switch and during that time, they generate losses in the form of heat and limit how fast you can switch them. GaN transistors aim to solve this problem as these few slides from GaN systems show:

GaN Systems Class-D Audio Technology  Silicon FET Lossess.jpg


The capacitors (Cxx) are implicit part of the characteristic of the MOSFET transistor and need to be charged and discharged to lower or raise in the input voltage to a silicon MOSFET. This slows down the part causing that less than ideal ramp to full on. There is also an implicit "body diode" that causes slow reverse recovery which results in more losses. GaN transistors don't have reverse body diodes and lower capacitances:

GaN Systems Class-D Audio Technology  GAN FET Lossess.jpg


Here is a measurement showing the full cycle (turn on and off):

GaN Systems Class-D Audio Technology Switching Waveform.jpg


The faster switching results in better efficiency as noted:

GaN Systems Class-D Audio Technology -  Class D Amplifier Efficiency.jpg


The graphs are not the same which makes the interpretation hard. But if you draw a line at 50 watts on the left, you see that the GaN based amp has already achieved 90% efficiency whereas the Class D is around 80%.

There is also an important aspect to fidelity. Because of the slow switching time, there needs to be a gap between complimentary pair of MOSFET transistors switching (or else they fight each other resulting in destruction or power loss). This gap directly results in distortion in the output of the class D amplifier. Without mitigation, this would result in very poor performance. The impact here is lowered by using feedback, and a lot of it in the case of Hypex and Purifi amplifiers. High feedback amplifiers are difficult to design and keep stable. GaN transistors switch faster and hence don't create as much distortion to be corrected. You are fixing the problem as the source.

Back to our eval board, this is a very complex device with digital input, processing, etc. There is a mechanism to change level of feedback to show the goodness of GaN without a lot of feedback. All of this makes the board expensive (over $1,000). So I am not sure how many of you want to go and buy it so this is more of a technology overview of what to look for in design from others.

Note that this board was commissioned to be designed for GaN Systems. This is a company that specializes in both low and high voltage GaN transistors. Much of GaN transistor market is focused on very high voltage parts for Electric Vehicles (EV) and such. Those transistors do not bring much to the plate here in audio amplifiers where voltages are below 100 volts. This focus on lower voltage GaN transistors allows these to have the efficiencies I explained above.

GaN Systems Class D Amplifier Measurements
As I noted, this module accepts both types of input, analog and digital. Let's start with analog:

GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS 29 dB Gain Analog In Audio ...png


And digital:

GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS Audio Measurements.png


Performance here is above average compared to median of all amps I have tested (which lands around 78 dB SINAD). But it not stellar. Speaking with the designer, his focus was to keep the level of feedback low to show the advantage of the GaN transistors not needing much. He was not aiming to produce the best performance possible. Personally I wish he had targeted that given that is what we like to see here. :)

SNR lands in the same range as our total distortion:
GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS SNR Audio Measurements.png


Here is power into 4 ohm:

GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS Power into 4 ohm Audio Meas...png


With a 300 watt power supply, there is not enough juice to drive the amplifier to max power with two channels.

GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS Max and Peak Power Audio Me...png


Here is the same using 8 ohm load:
GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS Power into 8 ohm Audio Meas...png


Here is an interesting measurement comparing feedback to no feedback:

GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS Power into 4 ohm Closed vs ...png


We see that the distortion is still under control instead of shooting through the roof.

Most impressive is this measurement:
GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS Power vs THD+N vs Frequency...png


I don't think I have ever tested a switching amplifier with this level of frequency independence. Even Class AB linear amps struggle to produce such an absolutely clean response. Amplifiers with high amount of feedback run out of gain at higher frequencies and hence distort more. Not this design.

Finally, here is the switching spectrum:
GAN Systems Class D Amplifier Evaluation Module GS-EVB-AUD-xxx1-GS 1 kHz FFT Audio Measurements.png


Conclusions
MOSFET transistors have revolutionized computing and switching power designs. It is nice to see a new type become available after decades of refinement of MOSFET. GaN transistors offered by GaN systems show a path to cooler running, more efficient and better performing amplifiers and power supplies. The eval unit unfortunately doesn't show the improved performance due to different target for it. So best to look at commercial implementations that shoot for state of the art implementations.

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YSC

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#3
that's impressive to me! the flatting out of distortion vs power and frequency looks to me that it's achieving some almost ideal efficiency and hence the levelling off of distortion after certain threshold. by this is looks like with more feedback etc. in the design of products we are going to see IRL will give stellar performing amps running cool
 

Francis Vaughan

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#5
GaN has been leading a quiet revolution in RF systems, really enabling a whole new paradigm in cost effective capability. I had never really though about applicability to audio, it seems such a long way away. But it makes much sense.

Perhaps the next trick will be to see a combination of Axign's AX5689 and these transistors in a reference design. That might be the next step on the path to peak power amp, to go with peak DAC.
 

EJ3

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#6
If this gets implemented in the way that us here at ASR would like to see, that would likely be the turning point for me to start taking CLASS D seriously. I have been paying attention to it like I would something in my peripheral vision but this could be the item that makes it a truly viable improvement and brings it to center screen.
 

voodooless

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#7
If this gets implemented in the way that us here at ASR would like to see, that would likely be the turning point for me to start taking CLASS D seriously. I have been paying attention to it like I would something in my peripheral vision but this could be the item that makes it a truly viable improvement and brings it to center screen.
Why start only now? What's not serious about the current implementations?

On a totally different note: wouldn't those new transistors not need additional effort into the output filters? Faster rise times, sharper edges should mean more harmonics?
 
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#8
THD+N looks much better than most amplifiers out there. If only we could have great SINAD as well, GaN is definitely gonna stomp the market.
Great efficiency and less dependence on heatsinks mean smaller and lighter designs too. Maybe it'll take off in the PA market first?
 

Francis Vaughan

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#9
On a totally different note: wouldn't those new transistors not need additional effort into the output filters? Faster rise times, sharper edges should mean more harmonics?
Interesting point.
More harmonics, but not necessarily harmonics of greater amplitude, just more energy at even higher frequencies. This may simply not matter that much, or just require more care in choice of materials and construction of components in the filter. You get into trouble when things like inter-winding capcitance of inductors or self inductance of capacitors start to cut in. So the resultant filter probably looks identical, but might just require more tight specification. One place it could become interesting is if there is a reliance on component losses at very high frquencies - then energy disspation in the filter might become an issue. Picking a ferrite core with the right RF losses for instance.
 

voodooless

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#10
THD+N looks much better than most amplifiers out there. If only we could have great SINAD as well, GaN is definitely gonna stomp the market.
Great efficiency and less dependence on heatsinks mean smaller and lighter designs too. Maybe it'll take off in the PA market first?
Orchard Audio has GaN-based amps. They seem to be performing exceptionally well. He's a member here (@orchardaudio), so can probably comment :cool:
 

EJ3

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#11
Why start only now? What's not serious about the current implementations?

On a totally different note: wouldn't those new transistors not need additional effort into the output filters? Faster rise times, sharper edges should mean more harmonics?
Because, until this, there has not been the possibility for CLASS D to be enough better to cause me to desire to replace my triplet NAD 2200's. But if this is implemented properly, that possibility will exist. That is just me.
Your opinion may, of course, be different.
 

voodooless

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#14
Because, until this, there has not been the possibility for CLASS D to be enough better to cause me to desire to replace my triplet NAD 2200's. But if this is implemented properly, that possibility will exist. That is just me.
Your opinion may, of course, be different.
I think that is a bit of a different line of thinking. For sure, there is no immediate reason to replace a perfectly fine working AB amp :). SINAD difference is about 10dB to the best Class D’s though. I doubt a GaN amplifier will add another 10 dB.
 

EJ3

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#15
Longevity, Reliability, etc have not been answered to my satisfaction. A power surge took out a 45 amp relay with one of my NAD's. It did not harm anything else and was a simple fix. I wonder about if it had been a CLASS D, would it have taken out all three? & left me with no music on a small island in the Pacific? Perhaps you live in a place where you have less than a 6 week wait to get something (perhaps double that if you need to send something out for repair). I don't. And don't want to. Average daily change in out door temperature is only 8 degrees, it doesn't go below 73F or over 96F & when you throw an anchor into 100 feet of water, you can see it hit the bottom. We have 40 or so square miles of land and 975 people. I'm happy here. It's a 4 mile boat trip (or airplane ride) to the next island (which has about 40,000 people) and 130 miles by boat or air to the next larger populated island of 155,000 people. This suits me just fine.
I hope that you are happy were you are at. We all make choices based on our likes & dislikes. My choice is to be here because I like it here.
 

Koeitje

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#16
Stellar performing amps running cool already exist.
Just sayin'...
I don't know what kind of differences in heat we are talking about with GaN vs NCore, but my NC400's get pretty warm during use.
 

EJ3

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#17
I think that is a bit of a different line of thinking. For sure, there is no immediate reason to replace a perfectly fine working AB amp :). SINAD difference is about 10dB to the best Class D’s though. I doubt a GaN amplifier will add another 10 dB.
True, but it may make the CLASS D's as whole better, more reliable and have more choices in the "amenities" side of the designs. My system is an eclectic design of separates that are easily swapped out, part of the reason that it is tri-amped by 3 NAD 2200's, is that I can reconfigure the system to operate on two of the triplets, while I repair the 3rd one. The rest of my system is set up the same way.
As Voodooless said: 'For sure, there is no immediate reason to replace a perfectly fine working AB amp :)". The quality of my NAD's sound, this is from Amir's test of one of my personal triplet NAD 2200"s:


Response now (in green) as it should be, ruler flat to below 10 Hz, and well extending past the 40 kHz limit of this measurement.


And signal to noise ratio:








Wow, we have one kilowatt of power coming out of this amp in short duration!
Conclusions
Nice to see innovation like this from equipment that is over 30 years old! Shame on manufacturers that produce amplifiers for much less power, more distortion and higher prices these days. No, you don't get a fancy case here and sheet metal is strictly budget category. But you are not going to sit on the amp. The guts are where it matters and NAD 2200 delivers.

Overall, I am happy to recommend the NAD 2200. I almost gave it the highest honors but given the upgraded nature of the test unit, and the fact that used amps may have issues, I avoided that. But you could have easily pushed me to give it the golfing panther.

It's been good enough for me since the 1980's. Now something has come along that may change that. Maybe.
 
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boXem | audio

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#19
I think that is a bit of a different line of thinking. For sure, there is no immediate reason to replace a perfectly fine working AB amp :). SINAD difference is about 10dB to the best Class D’s though. I doubt a GaN amplifier will add another 10 dB.
I doubt it too.
While @orchardaudio made very nice stuff with GaN, this demontrator is nothing else than "meh"
 

Offler

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#20
Wow. This can be beneficial even to powersources used in computing - CPUs and GPUs.

Buck convertors use standard silicon mosfets, but GaN transistors seems to be a better option for those as well.
 
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