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Upgrade from Pass Labs XA30.8 to X250.8. Worth it?

I am getting ready to make some upgrades to the system. I currently running a DAC into a BAT51SE, then a Pass Labs XA30.8, then finally into Decware DM947 speakers (don't worry if you've never heard of them; not the last word in speakers, but have been doing a solid job for about 5 years now.)

I am contemplating moving up the Pass Labs amplifier line, and the X250.8 looks attractive. I would like to hear more bass authority and definition, but I don't really want to sacrifice the mids and highs of the 30.8. 

I would love to hear some of your impressions of the X250.8, especially if you are able to make some direct comparisons to the XA30.8. I've spoken with both Mark at Reno Hifi, and Kent English of Pass Labs; both are wonderful and informative people. What I am interested in now is the opinions and experiences of end users of either of these amps. 

Thanks! 
willrich47
hk_fan
314 posts
 
The X250.8 may have better bass, but bass is easy - the liquidity and smoothness of the XA in the mids and highs is what's difficult.  If you want bass, keep the the XA and buy a quality powered sub.  Completely serious here.
auxinput
1,538 posts
 

I am still trying to wrap my mind around why bigger AB amps often times have better bass, even though the output may be the same as a class A amp.


What is most likely happening here is that the fully Class A amps require so much constant DC that the power supply just doesn't have enough on tap for the powerful bass hits.  It requires an enormous amount of power supply capacitance to supply enough "smooth DC" power for that constant 30 watt Class A sink -- that it's only just enough to carry through for normal music.  When bass frequencies hit, it will pull even more on the power supply, which is just treading water enough to keep the voltage up (though I suspect voltage will dip anyways on the high power bass hits).  I suppose the exception would be very huge amp, such as the XA200 monoblocks, or something like a Krell FPB300+ monoblocks.

On a large Class AB amp, such as X250.8, the power supply is sized larger than the XA30 with a larger transformer as well so that it can sustain that 250 watts per channel.  However, you hardly use much of that, probably 5-10 watts constant draw during normal music and when the bass hits, the power supply transformer and capacitance has enough current on tap to push that bass frequency waveform through the speaker before it needs to draw on the main A/C current to recharge the power supply caps.  The power supply is able to draw A/C voltage at 60 hz, which is generally fast enough to keep the power supply full for low bass frequencies.

kosst_amojan
2,095 posts
 
Auxinput,
I'm not sure you understand class A, especially in push-pull. The demand on the power supply in a class A push-pull amp hardly changes at all until you're laying out double the rated power. An amp like the XA25 is biased to 25 watts, but it has a class A envelope of 50 watts. As the amp swings voltage, one half turns on more and the other half turns off an equivalent amount. Most class A amps will deliver current way beyond what will melt the outputs. Mine certainly will. Every Pass amp I'm familiar with will. Push-pull class A amps are really nice because they just transition to AB and keep swinging voltage when pushed outside their envelope, and most are designed with power supplies that will do that. That's why the XA25 measures so well way beyond power rating. 
auxinput
1,538 posts
 

kosst, honestly, I really don't trust anything you say anymore.  Now if almarg came in and corrected me, I would believe him.

I do know that Class A amps have a much larger constant A/C draw.  In Class AB an amp might have an idle current of 63 watts, but if biased to something like 30 Watts Class A, the constant "idle" A/C draw may go up to something like 160-170 watts.

kosst_amojan
2,095 posts
 
Auxinput,

Where do you think all that constant power draw goes? It goes straight into the heatsinks. Use some thinking here. If a 30 watts amp is dissipating 120 watts in pure heat just sitting there, do you really think that's anywhere near the full extent of the power supply's capacity? Unlike a class AB amp, that thing needs to deliver close to 5.6 amp constantly assuming 32V rails, which is what you'd want. A 600VA transformer (typical for a 30 watts class A amp) is going to put 12.5 amp per rail. You're pushing that amp WAY out of class A before you peter out the power supply. You can get away with torpidly built transformers in class AB amps that run higher voltages with lower current ratings because they hardly get warm under idle. That's the major reason class AB exists. The most expensive part in the entire amp doesn't need to be as expensive. 

All of that said, a well designed class AB amp with a decent regulated supply can certainly hold it's own against a similar class A amp with a massive unregulated power supply. 

I know people like to think those boards with the transistors on them are the real special, expensive parts, but nothing costs more than the transformer and the chassis. That's why class A is so expensive. 
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