Pass XA25?

[lhquam]

You are omitting that the XA.8 is a balanced circuit.

Correct, I forgot to mention that. The open loop gain calculation is the same for each half.

 

[lhquam]

For completeness, here is the balanced XA.8 topology.

 

[lae2]

Reference System - The Real Mckay

Pass XA25 with Aerial 7Bs

 

[lhquam]

Put the XA25 inside the fireplace for better heat convection.

 

[dady]

Is winter time and help to the heater power..

 

[Alexnova]

Hey folks, hope everyone has had an enjoyable holiday and looking towards the new year.

I've searched this thread and the forum for a question I have regarding the Class A operation of the XA25. I'm reading conflicting information online.

It's my understanding that the XA25 is 25W@8ohm and 50W@8ohm in Class A.

However, the measurements done on this amplifier from a few places (including Stereophile) show much more output than what is spec'd. Stereophile reported the following: "an amp capable of putting as much as 50Wpc class-A (or 100Wpc class-AB) into a 2 ohm load "

How far does the class A envelope push into? I did read from Mr. Pass here that XA25 has a high class A bias. I actually did not think the XA25 ever went into class AB operation before reaching 1%THD.

Can anyone please help?

Thank you kindly.

 

[Zen Mod]

only Pa and Wayne can tell exact figures

 

[MEPER]

It seems to be this one used as the N-channel in the amp?
https://docs.rs-online.com/cc13/0900766b80a3c588.pdf

What is the "name" of the corresponding P-channel?

I wonder if the N-Channel can used for an "Ultra MoFo" amp?
In that case it may change name to "MoFu"?

But I guess it may require an additional driver to drive the big Mosfet?
A 10-20A bias MoFu.....I have the chokes for that current......but not the heatsinks....or PSU....but may be easy to get a SMPS for that current.

 

[woody]

Alexnova, I believe Nelson made a comment during the last Burning amp to the effect that his amps often deliver more power than is specified.

 

[carsten.witt]

@MEPER,
6moons shows this part but sometimes Nelson hide the mystery parts well(he had already written this several times) and i think it is it not the right part.

 

[MEPER]

OK, P-channels seems to be harder to find.....I can see IXYS makes four in SOT227 case. A best guess could be this one if it should match the IXFN60N80P best......
https://www.infinity-component.hk/datasheet/46-IXTN40P50P.pdf

....but the idea to try out the N-channel in a MoFo concept......could be interesting. But the problem is probably how to drive it......

 

[Nelson Pass]

the measurements done on this amplifier from a few places (including Stereophile) show much more output than what is spec'd. Stereophile reported the following: "an amp capable of putting as much as 50Wpc class-A (or 100Wpc class-AB) into a 2 ohm load "

How far does the class A envelope push into? I did read from Mr. Pass here that XA25 has a high class A bias. I actually did not think the XA25 ever went into class AB operation before reaching 1%THD.

The question as to what truly constitutes Class A operation has been
re-examined since 1975, and most of the designers I have talked to
agree that an amplifier is still Class A when both polarities of the output
stage have significant current and both are contributing to the output
signal. A figure of 10% minimum of the DC bias value seems reasonable.

The XA25 is unique in that it operates its push-pull follower output stage
without degeneration, that is so say no resistors between the Source
pins and the output. As the Fets involved are square law devices, this
gives them an extended Class A envelope, even though the bias voltage
is constant. We idle these channels at about 120 watts each.

Here is a scope shot of the current going through the "unused" transistor
at 100 watts into 2 ohms. The minimum current in this case is 0.2 amps,
which about 10% of bias.

 

[Zen Mod]

lemme see .......... 0.2 is 10%, which means 2A is 100%, 120W of heat means 30V rails

as we already didn't know basic numbers



in short - build it , set it as you wish, don't forget to enjoy entire trip

 

[MEPER]

What is the "secret" to avoid "degeneration" .....in amps......not as a "general term"?

E.g. what will happen if I removed "degeneration" in M2X output stage?

 

[2 picoDumbs]

For single pair of transistors, possibly bias circuitry with thermal compensation if required.
Some devices don't need it.

 

[Zen Mod]

already done in Babelfish M25

 

[MEPER]

Ok.....so maybe I need a closer look at Babelfish.....M25.
Maybe PCB is compatible with M2X PCB's....so holes are placed so it can be swapped and fits on heatsinks.....
Why was M2X not made like this....without degeneration?

 

[2 picoDumbs]

M2X uses a bias circuit that depends on source resistance to set bias.

 

[Bensen]

Ok.....so maybe I need a closer look at Babelfish.....M25.
Maybe PCB is compatible with M2X PCB's....so holes are placed so it can be swapped and fits on heatsinks.....
Why was M2X not made like this....without degeneration?

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.

 

[Zen Mod]

well, this thread is hardly proper one for that

we already beat that horse to full life (lhquam made his prototype) in Beast F4 something called thread

edit: F4 Beast Builders