Pass XA25?

[MEPER]

M2(x) uses the voltage over the degeneration resistors as input for the autobias circuit based on the 4n35. Without degen resistors in the OPS, you would not have M2.

Yes, I can see that.......on schematic.....so we need a way to control bias without "degeneration".....

Then there should also be a good "sonic" reason to avoid degeneration.....like lower distortion, lower output impedance....larger class A envelope.....etc....

 

[SonnyMarrow]

I have a general question about the XA25. Are the two negative terminals both connected to circuit ground? As in, could you connect the two negatives as a common ground for a single ended TRS output?

 

[Zen Mod]

yes, common PSU, common negatives on back

when you buy one, confirm with ohmmeter, for your convenience



what is TRS?

 

[SonnyMarrow]

I have a multimeter, but I'm asking for a friend that owns the XA25. He wanted to use a 1/4" headphone jack adapter with the amp for very low efficiency planar headphones. I assumed the negatives were connected since it's single ended input only. Thank you!

 

[elwood625]

Tip, Ring, Sleeve. 1/4" stereo phone plug

 

[Zen Mod]

 

[OrbLink]

Biasing the output stage by using current sensing from Drain pins via Opto should work here as well.
I have a simple aplifier circuit using this technic and it work just fine.

 

[qpek]

Hello
Can somebody tell me what is current first Jfet stage ?

 

[Veysel]

Chat gpt usually says XA25 is very good amp when asking him a good DIY amplifier example

 

[TungstenAudio]

Pretty sure.ChatGPT wouldn’t be able to distinguish between an XA25 and a two foot section of RR rail. Let’s leave the discussion to humans.

 

[grataku]

ChatGPT says XA25 is 'safe and effective'.

 

[Justcallmedad]

Front Gain Amplifier shows only 16V lytic capacitors...

 

[Justcallmedad]

I mean the picture of the front end pcb.

 

[william2001]

I noticed that too. Maybe a quad in series per phase with leveling resistors? What is the rail voltage of this thing? I assume it's ... high-ish? (due to the pucks)

 

[Zen Mod]

capacitors sometimes can be used in other purposes, not just smoothing da rails

 

[william2001]

Ha! Well I guess there is that...

 

[Zen Mod]

Ha!



to help a little - Pa pulled some century old tricks from bag

nothing wrong with using collective wisdom......

 

[Adams_Leo]

Hold on to your hat... I think the circuit configuration is different than what we previously thought....

As you just said, there are no de-generative feedback resistors.
With this information at hand, I think the circuit is something like this:

View attachment 616955

this should be high output f5m version😁😁😁

 

[Justcallmedad]

After reading some of the threads on First Watt and Pass Labs designs, as well as the square law paper, this is my guess of the XA-25 front end, maybe it could be close… ? I have no clued how could be the output stage, optocoupler biased with runaway control, and DC coupled!

I'm gradually coming back to audio electronics, which I haven't worked on it for maybe 10 years, Since the Blowtorch threads and Charles Hansen designs. I lost a lot of electronics knowledge in the meantime, knowing that my job was more artistic as a sound engineer. When I retired I rather did amateur music as bass player with friends.

What are your thoughts, let’s go ;-)

 

[Russellc]

Check out zenmod's XA 252 clone of XA 25.