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LuFo Amp - 39w SE Class A from 28v Rail
LuFo Amp - 39w SE Class A from 28v Rail
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Old 23rd May 2021, 03:31 AM   #1
xrk971 is offline xrk971  United States
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LuFo Amp - 39w SE Class A from 28v Rail
Default LuFo Amp - 39w SE Class A from 28v Rail

Update Oct 19, 2021 - Final manufacturing files (Gerbers, BOM, schematics) can be found here. A big thank you to JPS64 for making the beautiful layout.

In the course of testing the new aluminum IMS PCB adapter board for the LU1014D group buy by member Wg45 (with LU1014D's provided by Mr Pass, and the testing is being performed by Woofertester), I realized that the LU1014D can carry a lot of current and can now be cooled easily and effectively. The ability to mount and cool it easily, combined with the use of a MOSFET cascode stage to increase the rather low 20v limit gave this little transistor new life as an audio output stage. Inspired by Michael Rothacher's (and his alter ego, the “Dude”)excellent MoFo amp, which I built several years ago when it first came out, I saw that something simple and similar could be done here. I wanted to make something that folks could easily make using scraps from their bench without resorting to a high dissipation MOSFET like an IXYS, or fancy CPU coolers and fans. No, just conventional IRFP240's can be used and at safe levels of dissipation of only 25w each - suitable for mounting on a passive radiatior heatsink like a 4U x 300mm one from Modushop.

Here is the resulting schematic from LTSpice doodling. It has a minimal parts count, yet the performance is quite nice. With a 28v supply and about 3 amps of bias current, the simulations predict that it can get up to 39w into 8ohms before clip sets in. The THD at full power is a little over 0.13% but only 0.01% at 1w into 8ohms and 0.03% at 8w into 8ohms. Of course, all this is with a nice SE Class A harmonic profile that is dominant second order and monotonically descending higher orders.

Here is the schematic:
Click the image to open in full size.

Here is the predicted FFT for 1w into 8ohms (8vpp) - it gets 0.01%THD and a beautiful harmonic profile:
Click the image to open in full size.

At an average and rather loud level of 8w into 8ohms - it gets about 0.03% THD and still maintains a nice harmonic profile:
Click the image to open in full size.

For the purposes of a proof of concept build, I am using CPU coolers and a Noctua fan since I have a pile of them and only 1 conventional 4U x 300mm chassis from Modushop, currently occupied by an Alpha Nirvana (which, btw, has the same output power and dual rails). But the PCB that JPS64 will be doing the layout for me will be a large conventional design with widely spaced MOSFETs (probably UMS compatible). Here is how I am stacking the three MOSFETs (some NOS Phillips PSMN020 150w's that Hugh Dean gave me a while ago) on the Dell CPU cooler, to which I will directly solder a veroboard and build the amp on:
Click the image to open in full size.

Here is how the LU1014D is mounted onto an IMS PCB adapter that lets it be used like a TO-247:
Click the image to open in full size.

Here is a closeup of the completed amp - all of it fits on the little veroboard - except the LU1014D and the 67mH (0.5ohm DCR) microwave oven transformer (you can get surplus (MD 803AMR model) ones similar to this one on eBay), which are attached with 18ga silicone wire flying leads and pin end crimped tips for the screw terminal blocks:
Click the image to open in full size.

I am using an active bridge (LT4320) rectifier, with a CRC, and a BJT cap multiplier single rail PSU (I measured ripple under 3A load at less than 1mv rms). Although, I think this amp has decent PSRR due to the cascode feedback - so a conventional CRC might just work swell without audible hum:
Click the image to open in full size.

Here is the first power on test, the amp reached 3A bias current, the voltage at the LU1014D source pin was around 1.5v, and the voltage at the LU1014D drain pin was around 4.2v - pretty much spot on from the simulations. No magic smoke was emitted, and everything was stable. I adjusted the trimpot slightly to achieve about a 3v difference between the drain and source pin of the LU1014D:
Click the image to open in full size.

I then connected the Pass ACP+ as the preamp - knowing that this will not achieve the full power since it does not have the ability to put out 50Vpp swing - but it will let me test the sound a reasonable volume:
Click the image to open in full size.

Here it is playing music - it sounds great! I also tried it with a Melbourne preamp that can swing 55Vpp and it does indeed play loud. The bass impact is particularly good for a SE Class A amp. The combination of the low Rdson of the LU1014D and the reactive load of the 67mH microwave oven transformer (MOT) combine to give this amp a predicted damping factor of 157 at 50Hz:
Click the image to open in full size.

This amp sounds very nice - smooth and similar to the MoFo, but with a different - more muscular feel, perhaps a bit less distortion. It certainly is easy to build and if you already have a MoFo, you are 80% there. It works fine at 20v or 24v too - just lower output power.

It is a simple and easy amp to build - you will need either a Hammond 193V inductor or pull one from an old microwave oven (or buy those on eBay for about $25) and a powerful preamp if you want to reach the full 39w potential.

We now wait for JPS64 to design us one of his masterpiece PCBs - this should be really cool.

Edit May 26, 2021: max power before onset of clipping is 17.77vrms into 8ohms. That’s 39.5w.
Click the image to open in full size.

Edit May 24, 2021: Measurements for 1kHz 2.83vrms into 10ohms
Click the image to open in full size.

8Vrms into 10ohms:
Click the image to open in full size.

Mouser shopping cart here (thanks to Vunce):
LuFo Amp - 39w SE Class A from 28v Rail

Prototype LuFo UMS compatible production board built by Vunce:
Click the image to open in full size.

June 25, 2021 - Vunce’s verification build in stereo:
Click the image to open in full size.

June 28, 2021 - Note that use of non-audio specific chokes like MOTs or Hammond power supply filter chokes may not provide optimal performance when playing sustained bass heavy tracks - this will deplete the magnetic energy stored in the choke and may cause intermittent clipping of high crescendos when preceded by deep bass lines. More info here:
LuFo Amp - 39w SE Class A from 28v Rail

Edit July 10, 2021: LuFo Lite - a 5.6W 12vdc supply LuFo without a cascode:
Click the image to open in full size.
Attached Images
File Type: png LuFo-Triple-IRFP240-v1.png (118.7 KB, 4714 views)
File Type: png LuFo-Predicted-FFT-8Vpp-8ohms.png (33.0 KB, 4324 views)
File Type: png LuFo-Predicted-FFT-22.6Vpp-8ohms.png (27.7 KB, 3739 views)
File Type: jpg LuFo-Build-01.jpg (287.5 KB, 3571 views)
File Type: jpg LuFo-Build-02.jpg (366.4 KB, 4514 views)
File Type: jpg LuFo-Build-04.jpg (436.6 KB, 3693 views)
File Type: jpg LuFo-Build-05.jpg (408.7 KB, 3748 views)
File Type: jpg LuFo-Build-06.jpg (506.5 KB, 4303 views)
File Type: jpg LuFo-Build-07.jpg (503.5 KB, 3581 views)
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Last edited by xrk971; 19th October 2021 at 06:03 AM.
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Old 23rd May 2021, 03:41 AM   #2
twitchie is offline twitchie  Canada
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LuFo Amp - 39w SE Class A from 28v Rail
This is so ******* cool!!!

Great job XRK!
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Old 23rd May 2021, 04:18 AM   #3
mazzer is offline mazzer  United States
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Fantastic!
Love that build.
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Old 23rd May 2021, 04:46 AM   #4
koja is offline koja  Canada
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LuFo Amp - 39w SE Class A from 28v Rail
how does measured distortion compare with the sim?
what to expect into a 4R load?
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Old 23rd May 2021, 04:52 AM   #5
xrk971 is offline xrk971  United States
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LuFo Amp - 39w SE Class A from 28v Rail
Thanks, guys!

@koja: no measurements yet. I will listen for a few days first to make up my mind subjectively. Usually, the shapes of the harmonic profiles are accurate for SE Class A amps, if the model of the actives are good. Distortion doubles for 4ohms unless bias current is increased.
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Old 23rd May 2021, 04:56 AM   #6
zman01 is offline zman01  Bangladesh
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LuFo Amp - 39w SE Class A from 28v Rail
X,

Looks like a cool project and relatively simple build. Look forward to updates. Happy listening with the proto build!
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Old 23rd May 2021, 07:17 AM   #7
peppennino is offline peppennino  Italy
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LuFo Amp - 39w SE Class A from 28v Rail
Superb!
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Old 23rd May 2021, 09:55 AM   #8
Extreme_Boky is offline Extreme_Boky
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Very low distortion and very nice spectrum. Well done!
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Old 23rd May 2021, 10:29 AM   #9
marconi118 is offline marconi118  Germany
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Superb and smart to use the DC voltage across the choqué to bias the gate of LU

I suppose the 3x irfp240 can be replaced with one irfp150?!
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Old 23rd May 2021, 11:14 AM   #10
analog_sa is offline analog_sa  Europe
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LuFo Amp - 39w SE Class A from 28v Rail
Really impressed!
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