Pass XA25?

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#81
2 picoDumbs said:
I've been able to match all specs in simulation.
The only thing I'm considering is whether to FAB the biasing circuit or put together a thermally compensated biasing circuit for the output stage.
Probably try it without thermal compensation to start with.
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Well there you go, although it's worth a word of caution from lhquam who
has been exploring the failure of the semi models to accurately predict
distortion and such.

As to biasing, you can do it the regular way, or you can just use a big sink
and adjust the value until it settles into where you want it - just takes a
little longer warm-up time.
 
#85
Nelson Pass said:
Three stages, DF is about 700, high current, high slew, .00x% distortion,
40 uV output noise, big Class A envelope, 25 watts into 8, 100 watts into 2.

And it sounds great.
Click to expand...

Thank you mr. Pass for these, what a person you are 🙂 Some constructor would not say even a word of his construction, just that it sounds good 🙂

Ok, mine 2 cents: something with square law in output stage (gives high class A swing), input maybe singleton /vas stage with bootstrap...?
Just blubbering.. 😛
 
#88
#92
The hocky-pucks are a game changer for Class-A amps in several ways that I can elaborate on at another time. Nelson claims that the XA25 amp is very simple (I believe him), and if the released specs are (relatively) accurate then here is my guess:
  • JFET input stage with positive current feedback (PCF) of either Type-2 or my recently discovered topology described in the F7 thread.
  • Complementary, undegenerated common-source laterals for the front-end stage outputs. What laterals (NOS) might your suggest for the front-end?
  • Complementary, undegenerated IXYS output fets running at about 1.3A with 26V-28V rails. With the SOT-227 package the thermal coefficient is about .2C/W Junction to heatsink. With the devices running at about 35W each that means that the junction temperature is only about 7 degrees higher than the heatsink, and thermal runaway can be easily prevented with simple thermistor feedback to the output stage bias circuit.
Any suggestions for medium power complementary laterals that are available for the front-end stage outputs?
 
#96
generg said:
I could imagine that the "no degeration" statement in the advertisement is only meant for the outputstage.
Click to expand...
The XA25 description in Pass Laboratories XA25 says the following: "This also allows elimination of degeneration, “that other negative feedback” in all stages". If that is to be believed, then the front-end outputs would not be degenerated. That could be safely done with laterals, but I am not sure that would be a good idea with vertical MOSFETs.
 
#98
I addressed some the topic of degeneration in my BAF 2016, and I have
the raw video for that thanks to WBS. Just waiting for bandwidth to arrive
on the 1st....

In any case, the drift of Ids against temperature is quantifiable and can be
easily handled without degeneration if you have adequate sinking and are
willing to do lengthy adjustment and put up with longer warm-up times.

Just things that DIYers are good at and commercial manufacturers are not.
 
#100
Any chance we can get a sound comparison of XA25 against some similarly powered X products from Mr. Pass? I am curious how the "half X" amp compares to the "full X" models. My SPICE fiddling shows impressive figures for my guess at an XA25-like circuit, but the SPICE numbers don't tell us how it will sound in real life...
 
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