LuFo Amp - 39w SE Class A from 28v Rail

[von Ah]

Thanks, xrk. I‘ll get a couple of those to start. 2 from the same vendor is hard to ignore.
Cheers.

I’m scheming this build out and want to do something a little different. I’ll probably go with forced air cooling and repurpose a nice vintage power amp chassis.

 

[mbrennwa]

I would try to get setpoints to match what is in the simulation here:

These setpoints only apply if your LU1014 has caracteristics identical to those in the SPICE simulation. Very unlikely.

Found this on Mouser: https://www.mouser.com/ProductDetail/Bel-Signal-Transformer/CH-4?qs=0/2GmT7MRM9RsVUzZKxOJw==
Would that work, or is it the wrong kind of inductor?

Looks good to me!

 

[von Ah]

I’m looking at reasonable options for SMPS to power a stereo LuFo build.

Are Meanwell RPS series SMPS ok with extra capacitance at the output? Anyone have that experience?

Would two RPS-200-27 be preferred versus one RPS-400-27?

 

[xrk971]

I don’t know if Meanwells can handle the startup in rush current. Maybe with soft start circuit like I used on VFET, or the SFP but for DC.
SMPS made by MicroAudio can handle large caps at their outputs and not go into hiccup mode.

 

[von Ah]

Thanks, xrk. Understood on the inrush current limiter. I’ll contact MicroAudio about options.
About how much capacitance should I be aiming for per channel with the LuFo (assuming SMPS)? Would ~20k uF suffice?

 

[mbrennwa]

Thanks, xrk. Understood on the inrush current limiter. I’ll contact MicroAudio about options.
About how much capacitance should I be aiming for per channel with the LuFo (assuming SMPS)? Would ~20k uF suffice?

You could use an NTC between the SMPS and the caps. I did that with a MeanWell to charge a huge cap bank.

 

[von Ah]

Thanks! I recalled there was a simpler NTC option. I may ask you for some specifics down the road when I get to building.

 

[xrk971]

The NTC option works too but has a voltage drop and burns heat. It may be fine for you though.

 

[von Ah]

I’m not fixated on maximum output of the amp, so a little voltage loss (~1.5 V?) and heat at the NTC is fine with me, especially for the sake of simplicity.

 

[jwjarch]

And don't forget. You probably can't switch the amp on and off without waiting for the NTC to cool off first.

 

[von Ah]

Good point, thank you. I'm always wary of quick off-on with any of my DIY amps.

Do I need one NTC per channel, or just one for the single output of the presumed SMPS?

Does this rudimentary diagram make sense for this conversation?

 

[xrk971]

That looks like a good idea as the extra R of the NTC between the two cap banks gives more CRC channel separation.

 

[von Ah]

Would two RPS-200-27 be preferred versus one RPS-400-27?

Any thoughts from from the LuFo experts on this question?

 

[xrk971]

Dual PSU’s make for a monoblock. In general better imaging and soundstage due to perfect stereo separation (limited by source).

 

[von Ah]

Is there any problem with using two in one chassis where the AC input is split to both SMPS?

 

[xrk971]

No, as long as they only common ground between the two side is the chassis dirty ground star connect point. Always use NTCs to separate clean and dirty ground.

This is the Alpha Nirvana amp with linear PSU but much of the same principles of keeping separated grounding and idea of monoblocks apply.

 

[von Ah]

SMDs conquered! First time soldering tiny things with paste and hot plate. Went perfectly as far as I can tell. Thanks to xrk971 for the great videos and suggestions.

 

[Plott]

Cool @von Ah, this step is waiting for me too, that's why I have such a delay
I was 17 years old as I soldered SMD thingies for the last time, in the school... 36 years ago
But if it works (I must try it)... the next one will be X's class D amp, with many SMD parts

 

[Vunce]

Nice work von Ah!
LU’s lined up and reporting for duty!!

 

[von Ah]

Thanks, all. I hope I didn’t cook any of them. I pulled the pan off to a damp towel soon after all boards were flowing and shiny. One took longer than the rest. I’ll have to improve my technique to get more even heating. Although the only other SMD task pending so far are the Lender preamp boards for the LuFo. So many little pieces.

I‘ll use the little transistor tester on the LD’s tomorrow. For now I’ll celebrate the success for not burning myself or the house.