My thoughts on Class D and a short review of the KJF SA-01

orangeart

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Super review, thank you!

I have more than a passing interest, as my Primare I32 is just coming up to its 4th birthday, and earlier this year I bought a Nord from a fellow Wammer.  I agree with the comments that these designs get surprisingly close to ‘perfect’ and definitely sound nothing like some attribute to class D. 

Perhaps Stefan @orangeart might be able to kindly explain the power ratings - Hypex emphasise the peak outputs, which is understandable because (a) the numbers are bigger, and (b) peak power is arguably more relevant when playing music.  However, what I don’t understand is how does a nominally 50 watt amplifier yield the 136/256 into 8/4ohms shown in the Hifi World review?  That’s double the rated continuous power, but not as high as the peak output claimed by Hypex.
That is a biggie, I was asked about input sensitivity yesterday elsewhere which is a related question so I will paste that below, any more questions just ask.

I was asked about amplifier input sensitivity in another thread and as it isn't as simple as it first looks I thought I would add my answer here for anyone else who was interested. 

OK here is amp gain in a little more detail. As I said earlier the input sensitivity is a function of  the rated power available and the load (speakers) on the amp. Some manufacturers will massage the rated load upward and some will be more conservative. I'll explain the maths and then give an answer to your last question about how the measurement by HiFi world can be different. 

Input sensitivity of am amp is given by the following formula. √(Rated power Wrms* Load Ω)/10^(25.6/20)= sensitivity V RMS. To plug in some numbers for real life situation, the NC500MP has a rated power of 500Wrms into a 4 ohm load, therefore we get √(500* 4)/𝟏𝟎^(𝟐𝟓.𝟔/𝟐𝟎)=2.35V RMS. That is to drive the system to full power into 4 ohm load we need to input 2.35 Vrms. Of course real world loads are not a flat impedance, they generally rise as frequency rises and have a spike at resonance and maybe some dips here and there as well so the above only gives a rough outline into an average impedance load.

Now we can see that if we change some of those numbers, the input sensitivity changes, so for the same amp module into an 8 ohm load it is rated at 270Wrms, this same formula gives us a value of 2.43 Vrms to drive to a full signal.

Into a 2 ohm load it is only rated at 400Wrms which gives us only 1.48 Vrms before we hit the limit.

Now manufacturers will take different things into consideration when providing a max rated power. It might be based on the thermal properties of the output devices, get to close to thier thermal breaking point and bang. It might be based on the maximum voltage available in the PSU for the output stage. The first part of the formula - √(Rated power Wrms * Load Ω) gives us the voltage needed to drive a load to the power we want for any amplifier, 500Wrms into a 4 ohm load requires 44.72Vrms available from the PSU. Let us assume there is some headroom and the PSU can produce 45V from the 230V supply available from your house wall socket. Now what happens if that supply is 210V or 254V? If that means we get less voltage from the PSU we may not be able to drive the speakers to the same level. 

The second part of the formula is the amplifier gain part of the formula where 25.6 is the gain in dB of the amp, this figure is made up of 2 sections of gain in most amps, the input circuit (ususally some sort of op-amp) which provides initial gain and the output devices which provide the rest. Ideally they would be completely linear, load agnostic, heat agnostic, manufaturing varience agnostic, etc. These modules have a really tight tolerance, however minimum gain would be expected to be 25dB and max gain 26dB for these particual modules, changing that number in the formula gives you a different voltage that you need to supply to get the full rated output power.

Made more difficult is that some pre amp manufatures provide Vrms capabilities for thier output and others provide peak voltage. The formula above is for Vrms which is not the peak V. Peak volage is the voltage at the top of the sine wave, the RMS voltage is a mathematically derived average voltage along the length of a whole sine cycle. To get to peak voltage from the RMS voltage we need to multiply by √2 or 1.414, to go the other way you would multiply by .707. Mathmaticians amongst you might recognise those numbers, they crop up a lot in maths.

Now when an amp is tested by the magazines, they are pretty ruthless. They use a dummy load which is a bank of high power resistors and apply a signal at the input. These are provided by signal generators that can produce a high magnitude sine wave without clipping it's own signal. A scope is attached to the speaker output/load and the input sine is increased until just before the output looks like it is clipping (flattening on the top of the waveform). In other words the maximum they can manage, pretty brutal. Remember this 500Wrms into 4 Ohms is not the continuous expected operation, average level is expected to be down at 100Wrms, Music is dymanic so amp continuous operation is usally given at about a fifth of full signal, and also usually where stuff like distortion is measured. The recent HiFi world measurment show the amp exceed the rated continuous power by a large margin as these tests represent a continuos load, the distortion figures are also given at full load, showing just how impressive these Hypex modules are. As Noel put it 'almost startingly good measured performance'

To answer more directly though, assuming a 4 Ohm load the 125W modules require 1.17Vrms to manage full power, the 250W modules need 1.66Vrms and the 500W modules require 2.35Vrms The amp will display clipping on the button ring if you are pushing to hard :)
 

uzzy

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This amp has got me interested too .. been talking to Stephan on Facebook (nice chap) .. the question is for a tight old git like me is "shall I or shan't I"  at the moment I am smarting are replacing the dishwasher and helping daughter out with her first house purchase lol

 

orangeart

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This amp has got me interested too .. been talking to Stephan on Facebook (nice chap) .. the question is for a tight old git like me is "shall I or shan't I"  at the moment I am smarting are replacing the dishwasher and helping daughter out with her first house purchase lol
I'm not that nice, I've told my kids when the leave ( at 18) they are on thier own...

 

ZZag

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Can i get a demo of this amp please i am thinking of buying one or a pair.  

 

Tarzan

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This amp has got me interested too .. been talking to Stephan on Facebook (nice chap) .. the question is for a tight old git like me is "shall I or shan't I"  at the moment I am smarting are replacing the dishwasher and helping daughter out with her first house purchase lol
Gender neutral up and purchase the amplifier- important things like dishwashers and helping kids out can wait. :yeah: :geek: :)

 
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audio_PHIL_e

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This amp has got me interested too .. been talking to Stephan on Facebook (nice chap) .. the question is for a tight old git like me is "shall I or shan't I"  at the moment I am smarting are replacing the dishwasher and helping daughter out with her first house purchase lol
divorcing the dishwasher could be expensive enough let alone replacing her  ;)

 
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audio_PHIL_e

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FWIW I've got an Amptastic. It's a 10W class D job that I bought because a workmate bought one and raved about it. I think his recommendation was accurate, as I also liked it. Shame the company has gone bust - I would not have wished that upon them as they had a good product. Anyway despite my preference for valves, I'm cool with the idea that class D can sound good. The amount of current that a valve amp draws doesn't bother me as I'm in the minority of people that use them; so few people use them that not using one would make naff-all difference to global warming, but for them that think it makes a difference, I'd suggest that going class D is not a bad move. 

 

dannybgoode

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FWIW I've got an Amptastic. It's a 10W class D job that I bought because a workmate bought one and raved about it. I think his recommendation was accurate, as I also liked it. Shame the company has gone bust - I would not have wished that upon them as they had a good product. Anyway despite my preference for valves, I'm cool with the idea that class D can sound good. The amount of current that a valve amp draws doesn't bother me as I'm in the minority of people that use them; so few people use them that not using one would make naff-all difference to global warming, but for them that think it makes a difference, I'd suggest that going class D is not a bad move. 
I can’t remember the technicalities however apparently there is a reason valves and Class D can sound very similar.

Given I like Class A as well I can’t really talk about the environmental side or things either :D

 
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DavidR888

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I was incredibly fortunate not so long ago to be invited to beta test a new Hypex based Class D amplifier from KJF Audio (of Pensil speaker kit fame). I believe I was one of the first people in the world other than Stefan to hear it as well. I have been intrigued by Class D for some time and the various options out there so this was a great opportunity to hear a decent one in action.

Full disclosure - I was sent the beta unit, had it for 3 or 4 weeks and then sent it on to the next tester. No money changed hands either way although I will, when funds permit, be buying one :) . Note also I write these reviews for no other reason than hifi is a hobby and I like words so I tend to write them for kit I demo from time to time on the off-chance someone is interested.

My system; analogue front end - Custom one off ARB TT and arm with Rega Ania for stereo and a Leak 2001 Transcription Unit with AT MONO3/LP for mono both feeding a Cyrus Phono Signature. Digital front end - Topping D50s fed by a Raspberry Pi 4 with Roon and HQPlayer all powered by an ifi iUSB3.0, preamp - Khozmo Acoustics Passive Pre, usual power amp - Baldwin HiFi 405C. Speakers - Rega RX3 with a BK Electronics XLS200 sub. Van Damme interconnects and Atlas speaker cables.


I have a set demo track list I always use as follows and then I meander off listening to whatever. My music taste cover just about every genre under the sun so I try to cover as much as possible in a track list which means I won't get evicted for squatting when in a dealers!


Sophie Zelmani - Oh Dear
Agnes Obel - Cameras Rolling
A Man Called Adam - Easter Song (Cafe del Mar mix)
Tony de Vit - Are You All Ready
Miles Davis - So What
Ray Lamontagne - Barfly
Alison Krauss - Down To The River To Pray


The sublime Ms Zelmani. I adore this album and the first track has moved me to tears before. Her haunting voice has a slight reverb in parts and there’s a mighty bass line that, whilst restrained, moves organs in the right system. However it is easy to have the bass overblown at the expense of the guitar and vocals. The SA-01 is ruthlessly revealing though teasing every last detail and every last drop of emotion from Sophie's voice. I don't usually like ultra revealing systems though but this is different of which I will explain more in a bit.
On to Agnes Obel then. A new artist to me but I’ve been listening to her album a lot (Myopia) and had listened to my current system for an hour or so before leaving to calibrate my hearing so know it well. Again ruthless detail and in I could hear more texture to Agnes’s voice in parts and more space between the different elements that make up the track but still. The first track actually breaks into a duet and only on a sorted system can you really hear the male vocal contributing.


Ah Easter Song, my chill out track of choice and has been since I first heard it on Cafe del Mar vol Dos all those years ago. There is a long repeating bass note during the build up at the start of the track. An ok system will deliver it as a nice chunky lump of monotone bass woomph but a really good system will show just how much the bassline modulates and fades in and out. The SA-01 is in the really really good category here. There is also a harp that can be a bit bright, particularly when juxtaposed over the bassline but again, whilst the SA-01 is ruthless in putting out what's on the track it has an unerring delicacy as well. The other aspect of this track I have been trying to reproduce is one I heard in the demo room at Moorgate Acoustics when listening to a pair of Spendor D7 fed by a Sugden A21SE - just a particular note was as holographic as I have ever heard - floated in a part of the room no speaker has the right to project sound and was quite magical. I didn't quite get this with the SA-01 and have been chasing it it with various amp / speaker combos but it did a fine job of at least resolving the note in question if not quite left it floating in the air.


The legend that is TdV and perhaps his seminal hard house anthem next - Are You All Ready? A track I have heard hundreds of times in clubs, have mixed into my own sets dozens of times and have heard my friends mix dozens more. A track I know intimately. And yet, I now realise just how much more there is to the production. A little leading edge to one of the bass loops, a 'swing' to the hoover that is more staccato on lesser amps and countless little cheeky noises and effects chucked in. I find a good well produced house track is a great system check as there is so much to them that just gets lost on a good club system. Get up close and personal though and the detail is revealed. Pace is important as well, a good hard house track should drive along at a constant yet unrelenting tempo such that you cannot but nod your head and tap your foot in time. Doesn't necessarily have to be at a massively high BPM but it does need to be totally metronomic and the SA-01 passed this test with flying colours. It had an iron grip on the bass beat and didn't deviate or hesitate although the repetition was bang on.

Let’s see what Miles could bring to the party. At last I was getting excited. The double bass at the start of So What has so much texture and timbre. I could visual the size of the instrument given the energy the notes were giving out. And a double bass should sound like this, you should get an impression of the string tension and just what a powerful instrument it is. There should be real attack to the leading edge of the note. And then, being a traditional Blue Note recording the instruments should all be clearly and definably in their own space. Drums to the left, piano to the right with the double bass next to it and the sax and trumpet in the middle. And this is what you get with the SA-01 - the perfect 60's jazz soundstage. Nothing more and nothing less. Oh, and the timing - just perfect. Again more on this shortly.

Ray Lamontagne. His control of his voice at the beginning of Barfly had a real sense of restraint and he almost breathes the words out and the SA-01 presented this far better than anything I’ve heard before. Again there are some powerful bass notes but these need to be properly controlled so as not to become bloomy and again the SA-01 grips the speakers and doesn't let them stray.

Finally Ms Alison Krauss. My favourite female vocalists and one of the best there is. I have seen her sign Down to the River live, solo and A Cappello. The recorded track she has a gospel choir supporting her, starting softly but growing louder. They harmonise beautifully and on my current system it comes across as a really well sorted choir but with the SA-01 I can pick out individual voices and follow them through the harmonies - something I simply haven't been able to do before. Again - that ruthless attention to the very finest of details that I don't usually like.

So to wrap up - why do I like the laser like focus on detail of the SA-01. Simply because it is the only amp I have heard, at any price (and I've heard some high end Naim through some high end speakers, have enjoyed ownership of 25W of pure Class A loveliness of a Sugden A21L etc) that combines this ruthlessness with pure musicality. You want to sit back and let the music wash over you, you can. Want to get lost in the detail, you can. Want to just enjoy the finest audio reproduction you've ever heard? This is the amp to do it.
However, the kicker for me is the timing this thing brings to any genre. Hard house needs a certain tempo and the SA-01 is metronomic totally nailing the chugging beats. Softly miked vocals require an amp that can be delicate and let the vocals and emotion flows and again that's what you get. Ditto the timing nuances required by classical, jazz, indy rock, metal, rock n roll, country etc - it's as if the amp resets its internal clock to suit the music being played. It's unerring at first, then jaw dropping and then you realise just what really well sorted, really well thought out Class D can do. It has truly come of age in the KJF amp.

Finally a note on the construction. Personally I wouldn't mind if it looked like something you'd more usually leave lying in a skip if you saw it poking out but luckily the case is very fetching. Very similar dimensions to the standard Cyrus shoebox but very pronounced curves to the corners and I really like the walnut finish. It matches the sides to my Khozmo pre just beautifully. The current production run of them have an aluminium faceplate though and not wood but also look the business.

So in summary - I have already made it clear I want one and I will be buying one, no question at all and I will sorely missed the amp when I had to pass it on to the next tester. If Class D is tempting you then would recommend a listen :)

EDIT - If anyone is interested in a 'proper' review this is what Hifi World think of it https://kjfaudio.com/wp-content/uploads/2021/03/HiFi-World-April-21.pdf
Thanks for the review. I had my ears opened by class D when I heard the Aavik amplifiers in the past year. I think the key, whether it's class A, AB or D is in the design and implementation.
 
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Jubal

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Thanks for bumping this and the review in the first place. Most enjoyable interlude just now to listen to the same tracks with the analysis in front of me and see what I thought on a modest system. Bookmarked for reference.
 
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