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Hifisonix nx-Amplifier: A High Performance 100 W Class AB Current Feedback Amplifier
Originally published in December 2012.
This power amp design is based on the sx-Amplifier topology, but with the TIS and output stage re-configured for class AB operation, enabling it to deliver 100 W into 8 Ohms. I have kept the design very simple, retaining the CFA topology, providing a very wide -3 dB bandwidth of 570 kHz, high slew rates (140 V/us) and low distortion. Since it was first published in 2012, about 400 board sets have been sold world-wide (the sister sx and kx2 class A current feedback amplifiers have had combined sales of around 450 board sets). The nx-Amplifier is a perfect introduction to current feedback amplifiers, showcasing the topologies legendary open midrange, fantastic bass performance and delicate, silky top end.
Amplifier write up with circuit description, wiring diagram and construction details:- The-Ovation-nx-Amplifier-V2.10 (updated November 2014 to V2.0 PCB’s incorporating all feedback and updates). Please IGNORE the BOM in the build document and instead use this latest BOM, released in December 2021 that uses currently available semiconductors nx-Amp BOM December 2021
You can get a complete set of double sided THP Gold Plated V2.0 PCB’s for the above from Jim’s Audio on ebay here:
Hifisonix nx-Amplifier PCB Set
(2 off nx-Amplifier boards + 1 off PSU +Protect excl. shipping).
Here is a link to the diyAudio forum discussing this amplifier and the sx-Amplifier. If you have any questions, you can post them up here or on the diyAudio forum.
1. The nx&sx-Amp PSU board is a simple unregulated PSU board with no muting, output short circuit or DC offset protection. The only protection is the amplifier module fuses
2. . The nx-Amp_PSU+Protection board option couples an unregulated PSU with speaker muting, output short circuit protection and DC offset detection protection.
See the nx-Amplifier article above for details. If you have any questions, feel free to post your questions up here.
If you are a manufacturer, kit supplier or DIY shop, and you want to use the designs (circuit or PCBs) you need to contact me.
I’d be happy to hear your comments/feedback.
Notes and Addendums
1. Please note, there in no R2 on the PSU +Protect PCB or on the schematic although the BOM calls for it.
Comments
77 responses to “Hifisonix nx-Amplifier: A High Performance 100 W Class AB Current Feedback Amplifier”
Hello Al,
which amp are you building? Can you give me a component reference number on the board?
On the boards the diodes are symbolized as a broken box with one end rounded and the opposite end straight. Could you advise me what the orientation is for anode and cathode?
Cheers,
Albert
Hello Bonsai,
Other projects have taken a lot of my time but now it’s time to start ovation nx build finally. I was wondering do you have most recent and complete bom in .xls form? It seems I can’t copy&paste from .pdf and it’s quite a job manually type everything to mouser IF you happen to have .xls saved somewhere? 🙂
Thank you!
Yes, you can bridge them. Note that if you bridge an amp, the calculated additional power is 4x the power non-bridged power. However, the output stages are only sized for 100W into 8 Ohms or ~180W into 4 Ohms. So my view is you can safely deliver about 150 Watt continuous with peaks of 200 Watts.
If your source preamp uses a balanced output, then you simply take signal ‘hot’ to one channel and signal ‘cold’ to the other channel. You then use the XLR screen to make the signal return back to your preamp (normally this is connected to the amp chassis in a balanced input system – you have to change this for bridged operation). note you will need to modify the source so that the XLR screen is connected to signal 0V rather than the chassis. You must insert a 22uF 35V non-polarized cap on the input to each nx-Amplifier module.
If you do not have a balanced output on your preamp, you will need to put an inverting stage with a gain of -1 (use for example a good quality opamp like a LME47910 or an NE5534) in front of one of the amplifiers.
You will have to take very good care with the wiring to avoid any hum pickup with either approach.
My personal view is that even with 30-0-30 VAC trafo, you will still get plenty of power . . .maybe no need to go to the trouble of bridging 😉
Thanks Bonsai!
One more stupid question; I have 2pcs of those trafos, so I could do 2 more channels. Is it possible to bridge this amp and how?Hello Esa,
Yes this will still work – you should get about 80 W RMS output. After you have built it, just check that the front end Zener diodes are each measuring 10V
Hi,
Just ordered pair of NX pcb’s to replace my Aleph 5 – build, for JBL 2225H duty class-AB is better choice IMO. My question is about psu, I have 2x30v 545va toroid reserved for this build, 30v is sufficient voltage to drive NX-amp properly?Hello Damir,
No – I would definitely not recommend it for 2 Ohm rated speakers. The reason is it only uses 2 output pairs so there is likely to be a dissipation problem. Secondly, it uses an EF2 output stage and not an EF3, so its likely that distortion will be higher than with a nominally 8 ohm rated load.
The nx-Amplifier is certainly ok to drive conventionally rated 8 ohm speakers that have a normal low impedance dip in the upper bass region. Typically the speaker impedance dips to 3 (or in some rare occasions 2 ohms) between about 200 Hz and 400 Hz. You have a high impedance bump at the LF resonance point (50-100 Hz) and beyond 500 Hz the speaker impedance ruses due to crossover impedance increasing with f. As far as I know (but please correct me if I am, wrong), the apogees have a very low impedance right across the frequency response below about 500 Hz.
Hi,
pls, advice if nx-Ampli can drive 2 ohm speakers (Apogee Duetta)
Tks for support
BR
DamirI did not measure or simulate it – but it’s between 100 and 200. High DF is not critical for great performance – the cabling and cross over make getting low damping favors in practice quite difficult.
Yes, you can get them st Jim’s Audio – just follow the link on the 2 nd page of the write-up
in your specifications there is no damping factor , did you not measure it or perhaps left it out of the specs
Hi
Great amp design which I can’t wait to try out. I don’t see the PCBs on ebay now. Do you know if they are still available?
Andrew
Hello Sjoerd, I think the Zeners will be a much easier solution – and cheaper as well. The Zeners produce little noise in practice (3 to 5 uV wide band) and the decoupling gets rid of a lot of this.
The Hypex products are very good – but I think they are really for a different application.
Hi Andrew, I am going to build this amp coming winter. It has been referred to me by Struth on DIYAudio.com.
My question is if I could exchange the 10Volt Zeners with these:
https://www.hypexshop.com/DetailServlet?detailID=2014Ofcourse positive and negative ones. I think this will do much with the sound quality because this is very clean!
Any changes need to be made if I put these on instead of the zeners?
Thanks!
Sjoerd
Good luck with your build!
Bonsai.
ok thanks, ill make sure that all connections are properly grounded,
thanks for the info regarding the PCBs,after i posted asking about the files,i realized that it was unnecessary.
Jack, you must always ground DIY amplifiers. Registered commercial amplifier companies have to pass stringent certifications (e.g. UL Certification) to be able to ship double insulated products that DIY builders cannot do.
Always earth (i.e ground) the chassis of your amplifier.
For PCB’s please see the link in the nx-Amplifier write-up to ‘Jim’s Audio’ ebay store where you can buy ready made Double Sided THP Gold Plated PCB’s.
hello
i read your very informative article on proper grounding/hum and if i understand correctly if the transformer is double insulate, no grounding is necessary. is this correct? if so then avel transformers are double insulated …………also im trying to find the gerbers on this amp.will they be up soon as i really want to get started on this fantastic build………thanksDavid,
Firstly, thank you for your kind comments.WRT the transistors, it’s unfortunate that you cannot get the same gain grades. However, this will not stop your amplifier from working well and sounding good.
I would replace the driver transistors with the improved (higher Ic rating, higher power and larger SOA) types below
KSA1220A (this is a PNP)
KSC2690A (NPN)Then, just use the 1381 and 1302 devices you can get for the VAS devices.
Any other questions, let me know – good luck with your build!
Hello Bonsai,
Firstly, thank you for making your designs available to the masses and providing such great documentation.
I intend to construct the nx-Ovation amplifier but have a couple of questions.
I have scoured the internet but am unable to find the KSA1381/ksc3503 in the same gain grade from a reliable source. Fairchild only seem to manufacture the KSA1381 in the E grade and the KSC3503 in the D grade. Do you have any suggestions/alternatives?
Some years ago I built the Leach Low TIM amplifier and have been happy with it’s performance, do you have any views on how the nx-Ovation compares to the Leach in terms of sound.
Thanks
Best wishes
DavidVinnie,
yes, it could be adapted for sx-Amplifier. you would have to do the following
1. link the base to the emitter of the driver transistors
2. change the bias controller so they are the same as the sx-Amp
3. change compensation capacitor to 220pFThis should do it!
Regards
Andrew
Hello Andrew,
I wrote you on december last year asking about youramplifier with CFA topologgy
I bought two pcb sets from jim’s audio.
My question is: it is possible to use one set to build the 15watts class A version? I read the schematics and I found the 100 watt class AB a bit more complicated. Which are the modofications adopted in the 100 watt class AB that I should ignore to use the double sided pcd alsp for the 15 watts class A?
Thanks
Vinnie
Hello Rado, you must only do this if your total power supply voltage from V+ to V- is LESS THAN 80 VOLTS. If it is above this,mother use the PSMN4R3-100PS.
Anton I have an arrangement with Jims Audio to supply these boards so will not put the Gerbers up at this stage. The price is very reasonable: $25.99 for a set of 3 silk screened , gold flashed DSTHP boards (2 amplifier boards and 1 PSU plus protect board). Here is the link: http://www.ebay.com/itm/100W-100W-Ovation-nx-Amplifier100-W-100W-Current-Feedback-Amplifier-PCB-set-/321256536930?pt=US_Home_Audio_Amplifiers_Preamps&hash=item4acc61bb62
Dear Mr Russel,
Can I have the dual layer gerber file for Ovation nx-Amplifier.
Thank YouI use these PSMN4R4-80PS instead PSMN4R3-100PS
thanks
Unfortunately that’s far too high for this design. The whole front end would have to change and the TIS stage would also need a redesign because of power dissipation issues.
The maximum safe supply voltage is 50 V
I think + – 70V ….
Thanks
Radoslav, do you mean +- 35 Volt power supply?
The front end Zeners and their associated bias resistos set the front end stage and TIS currents.
For +-35V DC Supply R18 and R19 become 2.7 k Ohms
For +-25V DC Supply R18 and R19 become 1.8 k Ohms
I do not really recommend +- 25V DC rails – the output power will be severely curtailed – probably only 12~15 W into 8 Ohms.
I use 70V power supply ????When changing some resistance ???
Thanks
Vinnie, thanks for your kind comments – and best wishes for the new year as well.
You can use BC560/550 without any problem.
+- 35 V is quite low, so you are going to get a lot less power than with a +-50V power supply.
I will take a look at the circuit tomorrow and leg you know what the resistor value changes are.
Good luck with your project!
Hello Andrew,
I’m writing from Italy.
First of all congratulations for the design of this accurate CFA. I experimented several times the incredible sound of this topology, due to the fact that me and a friend of mine built different versions of the Mark Alexander amplifier. The last version has a new layout, with a relay that is a standard one instead of the relay reed in the original design and the output stage with three pairs (per channel) of IR IRFP240/9240. This version is driving a couple of not-so easy-to-drive Magneplanar 2.5, instead of a good (but not so good if compared with MA CFA) Aragon 8008. A good result, I think.
However I’m impressed for your paper, for your ability to design efficient PCBs and for your knowledge. That’s why I’m waiting for the PCBs bought from Jims Audio, and I will build and I will experiment different configuration of your superb amplifier.
My first question is: I want to use a power supply of +35/-35 continuos (I have a toroidal transformer of 2X24 alternate): which resistors I should change (I thimk R18 and R19)? A value of 2,2k could be correct? Which other changement?
Another question: you decided to use BC547/Bc557 instead of the usual BC550/Bc560, that is a no noise BJT: may I ask you why? Is it possible to use the BC 550/560 (I own hundreds of this BJT and I can select the best for VBE and hfe). A last paranoyd (from audiophile point of view) question: what do you think about using the supermatch couples SSM 2210/2220 or MAT02/MAT03 of PMI?
Thnaks again, congratulations and … Happy New Year!
VinnieYes, I have the spread sheet. I will post it up tonight along with the Rohm cross reference data for the RETs (resistor equipped transistors).
Thank you for the quick response. What I meant was an excel file that can be copied and paste into mouser which I think will make ordering parts much easier than typing them individually.
I have just received the PCB’s which I ordered at jim’s audio and will now order the parts need for this build. Anyways any ideas on increasing the output of this particular amp? I know it can do 180w @ 4ohms as per your documentation but somehow it would be much better if you have ample power to drive low impedance and inefficient speakers such as magnepan’s or esl’s.
Thank you for sharing your ideas and knowledge to the DIY world.
Hello Raffy,
If you down load the PDF file from my website, the BOM is detailed at the back of the document.
Hope you enjoy building and listening to your nx-Amp!
Any questions just let me know. Please also post your questions on DIYAudio on the sx and nx thread.
Regards
Andrew
Sir,
I’ve read a lot about your work especially the nx and sx at diyaudio and because of this I wanted to try myself to build the Ovation nx power amplifier. Just recently I have ordered the pcb’s at jim’s audio on ebay which still hasn’t arrived yet. Do you happen to have a worksheet (excel) file on ur BOM for the Ovation nx amplifier? It will make easier for me to order parts at Mouser and I will give you thanks for that.
Any feedback will be highly appreciated.
Raffy
bimi, yes another guy contacted me to tell me this already – but thanks for pointing out as well. The link to the article and the nx-Amplifier PCB Gerbers is broken – I will fix it on Thursday when I get back from my business travels.
In the meantime, you can read about the sx-Amplifier 😉
Thanks for your patience!
I can not find Ovation nx-Amplifier V2.0_6.pdf file.
Dan, you could try this. Just make sure that the SMPS output is clean under load and there are no HF spikes. CFA amplifiers are not noted for their outstanding PSRR, so make sure the SMPS is well filtered.
I am using B&W 703 speakers. They are specified at 89 dB.
Hello Bonsai, i was thinking to power this amp with a SMPS, do you think it’s ok ? Also What sensitivity your speaker have ?
Boban, yes a few other people picked up on this on DIYAudio.com.
1. If your gain is simming at 18x it’s because you have included the 15 Ohm ground isolation resistor in your sim circuit. Short it out and the gain will be correct at 37x (=32dB).
2. In the circuit, the front end buffer transistor load resistors are shown as 4.7k. This wrong – the correct value is 10k. Note that the PCB layout is correct.
3. Further, if you measure a good working unit, the VAS current is actually higher than the sim suggests at 43mA (sim shows 28 mA). The reasons for this are the low value of the VAS transistor emitter degen resistors (15 Ohms) and the in accuracy of the VAS transistor models. A higher emitter degen value, say 27 or 33 Ohms will help to stabilize the VAS current, but in practice 15 Ohms works just fine which is where I have left mine.
4. I will update the documentation in a few weeks to cover these points.
5. For the time being, there is a very nice thread on DIYAudio.com under solid state called ‘sx&nx amplifiers’ where some forum members are building these amps. Here is the link http://www.diyaudio.com/forums/solid-state/236522-sx-amp-nx-amp.html
Thanks for your feedback – I appreciate it.
Hi Bonsai,
im interested to build nx amp. I try to simulate, but something i do wrong. Can u please look: http://s19.postimg.org/f9are95ab/Screen_Shot_00039.jpg
I get current trough VAS 68mA and gain is 18x. I use LT Spice.Regards,
BobanHello Matteo,
Thanks for your kind comments. You could probably raise the supply voltage to +-60V and it would still be ok. But, you would need to add another pair of output devices to handle the reactive load dissipation and also to minimize loading of the VAS stage.I have written in the back of the nx-Amp write up my feelings on the sound quality compared to the other amplifiers. This is a very smooth sounding amplifier with great bass performance. The e-Amp is a big complex amplifier aimed at delivering very low distortion (<10 ppm at 20 kHz @ 180 W into 8 Ohms). The e-amp will deliver about 250 W RMS into 8 Ohms and > 40 A pk into the load – so it is designed to drive any kind of speaker. I would describe the e-Amp as sounding forward and crisp. All four of my designs (sx, nx, e-Amp and Ovation 250) have very good imaging on my B&W 703’s – for me this is a critical aspect of an amplifiers performance.
Additional Comment on 19th July 2013: I just realized that if you do raise the supply voltages, you will have to change Q9 andQ11 for higher voltage types. The original design used BC547C and BC557C which are 45V Vce max high hFE devices and too close for comfort to the specified max 50V supply railes of the nx-Amp. You can use BC546 and BC556 which have Vceo of 65V with the same high hFE that you get with the 547/557 types http://www.fairchildsemi.com/ds/BC/BC560.pdf
Congratulation.
is it possible to extend power to 150+150w ?
Do you have any comments to add regarding the sound quality and to compare with e-ovation amp?
Thank you in advance.Thanks for the interest Peter. I am actually working on a new preamp now with tone controls and a full set of line features. I will be posting up in a few months – still a lot of work to do though.
Neat design nicely implemented!
Would be interested to hear about the PreAmp with tone controls when you have a moment. Are you thinking of including remote control as part of the spec?
PeterBeauty in simplicity,
Excellent design choices, beautiful . . .
My sincere congratulation on Yours achievements 🙂
Waiting for the PCB and BOM files … Will give it a try 🙂
Thank You for all Yours effort and generosity to make this public available.
With my best wishes to You Andrew,
Andreas
Thank you anonymous! Glad you liked it!
Fabulous read – a fabulous write up with great photos n a meticoulsly detailed build – the magazines would pay $$$ for this sort of quality work – thanks for putting it out for the love of it !!!
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