Pass Labs Int-25 review in Stereophile

[Manolo47]

a very enthusiastic review by Herb Reichert. He mentions that he likes it better than the HPA-1 - XA-25 amplifier combination. Smoother sounding or some
thing like that.... I wonder what topology is implemented in the front end to make that happen.

 

[MEPER]

We can have a look inside here:
INT-25

Looks like the output devices are quite "heavy".....are those some that are still produced?

 

[MEPER]

More info here:

Pass Labs XA25 Class A Stereo Amplifier - Positive Feedback

Big output power mosfets…….PP followers without degeneration......with some NFB. Almost a FW-amp?

 

[Zen Mod]

once you invent XA25, it's easy peasy to add selector and volume

 

[RobLK]

We can have a look inside here:
INT-25

Looks like the output devices are quite "heavy".....are those some that are still produced?

I think this it is a picture of the xa25 isn't it?

 

[Mark Johnson]

Notice the enormous inrush current limiter component at lower right. Cowabunga!

 

[MEPER]

Yes, I also noticed these "CL-60 big-brothers".

 

[MEPER]

I think this it is a picture of the xa25 isn't it?

I think the first link was Int-25 and second link was xa25.....the output stage seems quite similar. I was interesting in knowing the output devices…...and realized after posting that it was not the exact same amp.....but in close family I guess.

 

[RobLK]

Wow! they show a simplified schematic. I saw a review that said the xa25 clipped at 80 watts into 8 ohms. So could the rails be somewhere near +-40 volts?

 

[MEPER]

A Pass DIY amp in this forum with same output stage could be fun.

 

[wayne]

The INT-25 has a slightly different layout with all the power supply components on one board now and that has carried over to the XA-25. This way the front panel display board or just a switch on the XA-25 will turn it on.

Amatherm makes the thermistor for inrush, they have some big ones.

 

[cubicincher]

I think I see IXYS - Mosfets in SOT-227?


Greets
Dirk

 

[2 picoDumbs]

Same as xa25, hence the reason it is called int 25

 

[RobLK]

a couple of little things I didn't notice before, the xa25 has what looks like an scr on the power boards and all the big caps on the vas board are only rated at 16 volts.

 

[Zen Mod]

plenty of musings in F4 Beast thread

 

[Extreme_Boky]

No gate stoppers of any kind, anywhere... With likes of ixfn140n20p and its complement, there's no need for the load sharing drain/source resistors neither... single device (680W/200A) suffices

Power supply wire feeds to left/right amp PCB are... not even 10cm... Nice.

Attached is my attempt at making the photo clearer. Moderators, if deemed too revealing - pls remove. The last thing I want to do is to upset Mr Pass.

 

[Zen Mod]

how you can see that there are no gate stoppers?

not likely ......... and even if that's the case , ZM is Chicken

 

[Extreme_Boky]

"There are several unique things about the amplifier that make it a departure from the rest of the Pass Labs amplifiers. It's a fundamentally very simple topology made special by new parts and a unique approach to operating them in Class A without degeneration in the circuit (simplified circuit shown below). Years ago, we discovered that degeneration—that is to say placing resistors in series with FET source pins (or emitter pins for bipolar transistors)—has an impact on the sonic performance. My late business partner Joe Sammut was adamant about the qualitative difference, and could spot it in blind testing, so I took it seriously. The difficulty comes the fact that there is a reason why people use degeneration (the "other form of feedback") in gain stages—it stabilizes the characteristics of the parts so that you don't have to do precise matching and compensation to keep circuits stable. At the same time, it acts like the feedback it is. Routinely, your "no feedback" solid-state amplifiers depend on degeneration in the gain stages to control the performance, and so it does not achieve all the goals that make SET tube amps attractive."

Mr Pass

Source: Pass Labs XA25 Class A Stereo Amplifier - Positive Feedback

 

[Extreme_Boky]

No gate stoppers with MOSFETs would be indeed a tremendous achievement...

 

[Zen Mod]

can't see why and how?

you need gate stoppers for stability, and you can't hear them in circuit unless using several times multiplied optimal value