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Hifisonix kx-Amplifier
Hifisonix kx-Amplifier
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Old 10th November 2018, 05:57 PM   #1
Bonsai is offline Bonsai  Europe
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Hifisonix kx-Amplifier
Default Hifisonix kx-Amplifier

Here is the newest Hifisonix class A amplifier. It is a development from the original sx-Amplifier design published in 2012 - so class A and CFA topology. This design features 28W peak class A output, and will deliver about 50 Watts class AB into 4 Ohms.

The kx-Amp features dramatically higher loop gains, and as a result, about 60x less distortion than the original sx-Amp - slew rate remains the same at 140 V/us.

If you follow the link below, there is a write-up you can download all the details.

Hifisonix kx-Amplifier

As per the sx-Amp and the nx-Amp, Jim's Audio is supplying very high quality through hole plated, silk screen, gold flash PCB's - link is on the page above.

There is also a Ripple Eater PSU upgrade available for the kx (or sx) amp - through hole plated, gold flashed etc - but you will need to order this separately - details on the website.

Disclaimer: I make no financial gain from any of the products on the Hifisonix webpage - this is purely for fun and DIY enjoyment

Any questions/assistance needed, please feel free to post up here or message me

Happy building!

(picture of my one below!)


December 2020 Update



As of December 2020, 107 kx-Amplifier PCB sets have been sold by Jim’s Audio. 6 builders have reported 15-20 MHz oscillation at between ~1.5mV and 80 mV. Since the oscillation is at very high frequency, it is not loop related, but more likely parasitic oscillation (loop related problems are usually <5 MHz, and normally below 2 MHz even in a CFA amp like this).
Builders reported that touching Q3 or Q4 collector stopped the oscillation, or, placing a scope probe on the collectors also seemed to stop the problem whilst monitoring the output.
The probable cause of this issue appears to be parasitic oscillation in Q2 and Q1. These are emitter followers (called ‘beta helpers’ when used in an amplifier in this way) and have a tendency to oscillate at HF due to the formation of Colpitts oscillator structures arising from the parasitic L and C components around the devices. Modelling showed it was easy to get oscillation in the 30-40 MHz range with the probable parasitic L and C on the PCB – not as low as that reported, but a sure indication that there was potential for problems.
If you can see this oscillation with a scope, or you can hear a clearly discernible hiss (‘Shhh’) from your amp with the inputs unplugged then do the following:- (note: a well-built kx-Amp must be absolutely silent on 90 dB sensitivity speaker with the inputs unplugged on both hum and hiss)
1. Change R4 and R5 from 10k to 1k (this will BTW improve the square wave response as well)*
2. Raise Q1 and Q2 up off the board and insert a 1k resistor in series with the collector (see the post #479 – thanks to Polsol for the technique)
3. Across each of the 1000uF capacitors under the board, solder an SMD 1206 1uF 50V X7R decoupling capacitor (example part from Mouser 187-CL31B105KBHNFNE)
4. As a general improvement with signal sources with 50 Ohm impedance (most CD players and modern preamps), replace R31 with a 1.8k resistor. This reduces the slight overshoot which is quite normal for a TPC compensated amplifier (See Cordell ‘Designing Power Amplifiers’ 1st Edition pages 177-180)
*if you leave the R4 and R5 10k resistors in situ, you must instead place a 68 Ohm in series with 330pF capacitor network across R4 and across R5. This has the same effect as the 1k resistor in that it dramatically lowers the impedance at the emitter of Q1 and Q2 at HF, suppressing the oscillation.

My thanks to diyAudio member Rallyfinnen for identifying this issue, and for the patience of the affected builders.
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File Type: jpg IMG_5492 (2).jpg (323.0 KB, 2950 views)

Last edited by Bonsai; 23rd December 2020 at 10:35 AM.
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Old 10th November 2018, 07:24 PM   #2
maty tinman is offline maty tinman  Spain
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Default KEF LS50

http://hifisonix.com/wordpress/wp-co...r-Oct-2018.pdf

page 20

Quote:
Most of my assessment has been done with a pair of Kef LS50’s and a B&W sub-bass to augment the LS50’s low end (down by 3 dB at 79 Hz), which in an small listening environment, make for a wonderfully immersive experience. The kx-amplifier is very smooth and euphonic, but above all, its also a very accurate amplifier. On B&W 703’s (nice easy load to drive), the kx-Amplifier has a wonderfully relaxed sound with plenty of beautifully articulated bass...
page 34

Quote:
I listened to a variety of classical and jazz music (vinyl and CD) on my B&W 703’s and my Kef LS50’s – Ella Fitzgerald, Leonard Bernstein’s George Gershwin recordings, Steve Gadd, Fourplay and the Berlin Philharmonic’s Swan Lake and Sleeping Beauty in amongst others. The kx-Amplifier retains the warmth and ‘liquidity’ of the sx-Amplifier, but my impressions are that the top end is cleaner (cymbals, strings) and better bass performance – its almost as if it was slightly more boosted than the sx-Amp and the notes better articulated. The mid range to my ears is cleaner – and I think this has to do with the dramatically lower (measured) distortion. On 4 ohms and higher at full power, the kx-Amp produces no more than about 0.01% distortion, which is about 60x lower than the sx-Amplifier and it will happily drive a 2 Ohm load - obviously in class AB mode - with commendably low distortion.

Imaging remains outstanding, and particularly enjoyable on the KEF’s which really excel in this department. The left to right and sound stage depth are the best out of all the amplifiers I use. This characteristic seems to be a particular feature of class A amps – I have no explanation as to why – it’s a matter that needs some investigation in my view. I used a B&W sub-bass with the KEF’s on some material but as I remarked elsewhere, this was simply to augment the lower end of these speakers which drop off rapidly below about 70 Hz. On the 703’s the bass is superb and extends very low with fabulous articulation.

Used with the appropriate speakers (either 90 dB+ efficiency, or if lower, in a small listening space), the kx-Amplifier delivers an outstanding listening experience. The classic class A smoothness, lack of any glare and the effortless mid-range are available in bucketloads. The bass performance in my view is a significant improvement as well and I put that down to the more capable EF2 output stage.
KEF LS50 has a true sensitivity of only 83 dB. Minimum impedance of 4 Ohms.

How does orchestral music sound with them -without sub? Presumably they require more peak power. Many of my musics have a DR15.
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Last edited by maty tinman; 10th November 2018 at 07:44 PM.
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Old 10th November 2018, 07:39 PM   #3
maty tinman is offline maty tinman  Spain
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[Polish] KEF LS50 - Zespoły głośnikowe | Testy w Audio.com.pl

to English: Google Translate

Almost 4 Ohms.

Quote:
The jubilee product obliges ... But apparently not necessarily to reliable information. The nominal impedance, according to the bad practice of large British companies (a large slow more? British is allowed more?) Is presented as 8 ohms, here with quite an insolent note that the minimum is 3.3 ohms ... Maybe I am imprudent, writing such things? Well, we are a guest.

Click the image to open in full size.

And we still like ... On the characteristics measured in our laboratory, I do not see such a low minimum, slightly lower than 4 ohms we are falling at the very bottom of the band, but I would rather take into account the minimum at 200 Hz, there it has a value of 4 ohms, and hence the "nominal impedance" should be taken into account.


Click the image to open in full size.
Quote:
The sensitivity is not high, but decent - 83 dB, which despite appearances is a standard result for a small substitute, although usually the value of this parameter overstates, even considerably. KEF gives 85 dB, and here I will not be defiled, because even though 2 dB is quite a bit, on the other hand, differences between catalog and measured data are usually larger...
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Last edited by maty tinman; 10th November 2018 at 07:51 PM.
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Old 10th November 2018, 07:54 PM   #4
poseidonsvoice is offline poseidonsvoice  United States
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Hifisonix kx-Amplifier
Quote:
Originally Posted by Bonsai View Post
Here is the newest Hifisonix class A amplifier.....
Andrew!

Welcome back! It's great to see you here again and hopefully more builders will try the Kx amplifier. I was saddened when you had left!

I have got all the parts, boards, etc...for mine and I just finished rebuilding/rewiring my room so the Kx should get into the build log hopefully by Spring 2019.

Your measurements on the Hifisonix web page look promising, and I hope my corrections weren't too forthcoming.

Thanks again for your contributions and particularly for the Ripple Eater PS which can be used on many other Class A and Class AB solid state designs as well. It saves a bunch space compared to having inductors and large caps! Much lighter chassis' are always a big plus!

Kind regards,
Anand.
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Old 10th November 2018, 08:13 PM   #5
Bonsai is offline Bonsai  Europe
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Hifisonix kx-Amplifier
Thanks Anand! Good job proof reading the BOM and presentation :-)

Last edited by Bonsai; 10th November 2018 at 08:18 PM.
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Old 10th November 2018, 08:16 PM   #6
Bonsai is offline Bonsai  Europe
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Hifisonix kx-Amplifier
Maty
My sensitivity figures for the LS50’s were taken from the KEF specification. Indeed, they may well be much lower - you often see big discrepancies between manufacturers and independent tests. Nevertheless, the kx-Amp drives the LS50’s very well. They sound good on orchestral music.

Last edited by Bonsai; 10th November 2018 at 08:31 PM.
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Old 10th November 2018, 08:40 PM   #7
maty tinman is offline maty tinman  Spain
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Default Class AAB -> 50 watts at 8 Ohms, 100 watts at 4 Ohms

My questions is with kx-amplifier working only at class A.

I have a KEF Q100 (true sensitivity almost 85 dB, minimum 4.7 Ohms) cheap tweaked by me and really I LOVE the sound (after my last tweak -more mass- they sound much better than new!!!).

Your amplifier can works like class AAB -> 50 watts at 8 Ohms, 100 watts at 4 Ohms.

page 13

Quote:
About the kx-Amplifier ‘mode selection’ option and Heatsink requirements

J8 in the circuit is a 2 pin link that increases the standing current from ~300 mA per output pair to around 600 mA per pair when closed. This allows the amplifier to be switched from class A to class AAB mode, retaining some of the characteristics of the class A sound but at much reduced power consumption.

Further, in the class AAB mode, you can run the kx-Amplifier at +- 35V rails which will provide 50W RMS into 8 Ωs, and close to 100W into 4 But do note, with the higher supply rails you will still need substantial heatsinks even in class AAB mode, where the standing dissipation will be around 50 Watts.
Quote:
The kx-Amplifier requires a heatsink of at least 0.3 ̊C/Watt or lower per channel. Each channel heatsink will therefore weigh a minimum of 1.5 kG

So, either run on the +- 22-26V and have the option to switch between the two modes, or link out J8 permanently and run the amplifier on +- 35V rails in class AAB mode.
Click the image to open in full size.
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Last edited by maty tinman; 10th November 2018 at 08:50 PM.
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Old 10th November 2018, 09:02 PM   #8
maty tinman is offline maty tinman  Spain
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Last question.

Class AAB -> standing dissipation will be around 50 Watts. One module or two?

You know, summer in Spain is hot (and humit in Tarragona).
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Old 10th November 2018, 09:10 PM   #9
maty tinman is offline maty tinman  Spain
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Default JLH 10 watts - Hiraga 20 watts - sx 15 watts. At 8 Ohms, 20 kHz.

page 21

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Old 10th November 2018, 10:22 PM   #10
Bonsai is offline Bonsai  Europe
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Hifisonix kx-Amplifier
It’s 50 W per channel with the original sx amp supplies - about 60 watts on the slightly higher rails for class A. If you are worried about heat, build it as a class A amp, and just switch to class AAB in summer (half the dissipation).
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