1 hour ago, Pars said:

Now there is a HC approved overkill transformer!

The two trimpots on the GRLV by the output terminals are for balancing out the output voltages.

haha it seeems like a little bit overkill... i agreed but the price between 200VA/100VA is almost the same. same price more weight🤣

If you overdo it, you'd need a softstart circuit. Otherwise you're torturing the ON/OFF switch with inrush currents.

12 hours ago, RudeWolf said:

If you overdo it, you'd need a softstart circuit. Otherwise you're torturing the ON/OFF switch with inrush currents.

Thx for the suggestion. Seems like I real need a softstart borad to built.

Did anyone ever do a run of CFA3 boards?

I did my own. Pretty sure everyone else also did.

10 hours ago, Pars said:

Did anyone ever do a run of CFA3 boards?

I just completed it last week by setting it to ZF and 100ma bias ( +-20V rail).

Still waiting the chassis and soft starter.

Sounds better than my dynalo\hi with my HD800 and K1000.

 

 

Yeah, I've built the CFA2. Toss up between it and my DynaFET, prefer the dyna slightly. Probably because it took me so long to get it working...

finished today my cfa2, was a trouble free build beside that i like small case (think is a tetris disorder i caught in my teenage years ;-).

once again, these trafo covers look nice but the shielding effect is near zero, to get ride of a small hum i needed to add some grain oriented bands. heatsinkt of the case seems a bit overdesingned, gets only a bit warm. offset without servos was surprising low at around 10-15mv, could use nice matched mje15030/15031 from my previous dynahi project.

the amps sounds very nice, one of the few amps that are neural and speedy that i like, really a "wire with gain" type of desing. thanks a lot for the design kevin gilmore and all of you for the help from this forum :-).

That's quite a compact build, what is the size of that case?

Looks good! What VA is that transformer? It looks quite big. 

it's a BRZHIFI BZ2607  case from aliex..., size is  260*311*70mm.  i have to many silver boxes so i like compact sizes even if the layout isn't perfekt.

the trafo is a 100va /24volt from triad magnetic VPT24-4170 (mouser), with a bias of 200ma the amp needs only around 40va so a lot of reserve here, next time i would chose one that is a bit smaler.

Here is an attempt to make a “smd raal uberamp driver”. I followed Kevin’s schematic. The big resistor is for feedback/gain, I guess 1.5 – 2 W is required 

and a few components on “solder” side.

 The real challenge will be the power sections. 3A x * +/-24V * 8 * balanced - might be a bit extreme for headphone amplifier…

Nothing says *extreme* like uberamp!

On 11/27/2020 at 1:49 PM, rumina said:

it's a BRZHIFI BZ2607  case from aliex..., size is  260*311*70mm.  i have to many silver boxes so i like compact sizes even if the layout isn't perfekt.

the trafo is a 100va /24volt from triad magnetic VPT24-4170 (mouser), with a bias of 200ma the amp needs only around 40va so a lot of reserve here, next time i would chose one that is a bit smaler.

Don't forget that a transformer shouldn't work near max output current. Most recommendations for Class A amplifier are to use something with at least double the output current capability. 

So far only been connected to power supplies. Have had a couple of long nights in kitchen.

Here on ”test bench” or floor.  

Output board on heat sink powered by the box to the right containing two Mean Wells 2*24V/15A. Driver board is powered by AMB dual sigma set at 15V.

Upper DMM shows 0.166V cross one of the 0.47 ohm power resistor. 65.5 on the other meter is temperature in Celsius, prob has a special hole in the heat sink. I’ve run this at higher current… 95 degree Celsius - not got good for wooden floor.

 Sound quality – interesting. Try a balanced version. Two output boards on one (bigger) heat sink…

 

Completed a second channel of a Raal amplifier. 

Current through output boards are 2.5 A giving heat sink (Modushop 300 x 40 x 80 mm) temperature of 85 degree Celsius (185 dergee Fahrenheit).

Driver boards powered by +/- 15V. Original it’s +/-35V. I might try the higher voltage but my idea is to use mostly smd parts in driver boards and then you probably have to reduce voltage to something like +/- 15V.

There is no bass filter at the moment on the boards but Roon is taking care of that.

Rear plate is to short. But that was what I had at hand – shouldn’t affect the sound?

Edited December 23, 20205 yr by JoaMat

2 hours ago, JoaMat said:

Rear plate is to short. But that was what I had at hand – shouldn’t affect the sound?

Oh, it will have to sound more "open"

The Thd is going to be higher at lower driver board voltages. Not a lot higher, but the idea is that the driver board needs to be able to fully drive the output board, so the driver board needs at least a couple more volts than the output board.

 

Thanks for info, Kevin.

Now changed driver voltage from +/-15 V to +/-35 V and op amp to OPA445 (from OP27).

So far I’m happy with the outcome. 

I’ve reduced power board voltages to +/-12 V. Two Mean Wells 12 V capable to deliver 29 A. Heat sink temperature decreased considerable – of course. My old ears think the sound is as good as with +/-24 V

Next step is to make a few more boards to get a balanced set up.

+/-12v should be fine for a balnced setup. 50 amp power supplies would be perfect.

Cannot find a quad 10k RK50 pot anywhere, mouser have the 50k in stock... 
Is 50k pot ok for CFA2?

not ideal, maybe find 20k?

The RK50 is only available in 10K as a stereo unit, the quad's are always 50K AFAIK.

That makes sense now, and after re-reading the data sheet the default value is 50K, but it does state they can accommodate 10K, 20K and 100K... probably as a special order of no less than 200pcs haha.

Earlier today I reached out to Alps Alpine, I may get lucky, if not there's a 10K Goldpoint 47 stepper.