Pass Labs X-Series

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dejanm

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Joined 2005
Hi,

I am thinking of buying X-150.5 or X-250.5. I know that X-series works in a class A and then switches to class AB. Does anybody know at which output level this switch happens ? This question refers to above mentioned models ...

Thanks in advance ...

Dejan

P.S. Does anybody have any experience with Pass Labs amplifiers and Focal Electra 1027 Be ? Any recommendations ... ?
 
BobEllis

BobEllis

Member
Joined 2003
I'll hazard a guess of ~20-25W RMS for the 150.5 and explain my thinking so you can do the calculations for the 250.5. Perhaps Nelson will enlighten us.

Looking at the owner's manual we see that the rails must be a bit over 25V so enable a swing of 50V with a bridged amp. For easy number crunching, let's use 50V rail to rail. At 125W/channel at idle, this implies 2.5A bias. (ignoring for the moment the PSU losses and front end dissipation)

This 2.5A is split between the two halves of the bridged amp, or 1.25A each side. Assuming that the output stage is a Source follower that implies that the output stages can supply double their bias while staying class A. Since the output current of each side of the amp is sourced or sunk by the other, we end up with a maximum of 2.5A PEAK output in class A.

This 2.5V peak output into 8 ohms results in 50 watts peak or 25 W RMS. Toss out a bit to account for the PSU losses, front end and its heavily biased VAS stages along with the slightly higher actual rail voltages and you're down to maybe 20W.

Not that 20W in class A is anything to sneeze at. With reasonably efficient speakers you will probably not exceed that except on the rare peak.

How'd I do, Nelson?

Edit: Note that the class A power output will be less than half what we just calculated in the parts of the spectrum where the impedance is near the stated 3.5 ohm minimum of your speakers.
 
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dejanm

Member
Joined 2005
Zen Mod said:



which model of Focals?
specs,link?

maybe smaller ,but sweeter Pass amp will suit them,if sens and impedance modul is adequate.....
Click to expand...

As already mentioned: Focal Electra 1027 Be ....

I was driving them with valve amps, even with 300B SE and it was working fine. Didn't had a slam and punch that I am looking for when listening to Massive Attack and similar, though. That is my concern. But maybe XA-30 can provide it along with finesse and transparency of a class A SE MOS Fet design .... That is actually my question ....

Larger XA models drop out of my budget - unfortunately ....
 
D

dejanm

Member
Joined 2005
BobEllis said:
I'll hazard a guess of ~20-25W RMS for the 150.5 and explain my thinking so you can do the calculations for the 250.5. Perhaps Nelson will enlighten us.

Looking at the owner's manual we see that the rails must be a bit over 25V so enable a swing of 50V with a bridged amp. For easy number crunching, let's use 50V rail to rail. At 125W/channel at idle, this implies 2.5A bias. (ignoring for the moment the PSU losses and front end dissipation)

This 2.5A is split between the two halves of the bridged amp, or 1.25A each side. Assuming that the output stage is a Source follower that implies that the output stages can supply double their bias while staying class A. Since the output current of each side of the amp is sourced or sunk by the other, we end up with a maximum of 2.5A PEAK output in class A.

This 2.5V peak output into 8 ohms results in 50 watts peak or 25 W RMS. Toss out a bit to account for the PSU losses, front end and its heavily biased VAS stages along with the slightly higher actual rail voltages and you're down to maybe 20W.

Not that 20W in class A is anything to sneeze at. With reasonably efficient speakers you will probably not exceed that except on the rare peak.

How'd I do, Nelson?

Edit: Note that the class A power output will be less than half what we just calculated in the parts of the spectrum where the impedance is near the stated 3.5 ohm minimum of your speakers.
Click to expand...

Bob,

Thanks a lot for this calculation. Without going into details, it would be great if it is correct, because 25 W of class A would be more than enough .... I guess though, without any hard facts, that something is wrong here. I believe that for 150.5 probably we are talking about only several class A watts ....
 
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