Class A amps

Hi all.

So I’ve been looking at some amps as of late - and I ran across the Pass Labs XA series of amps that are pure class A designs. The xa100.5 @ 8 ohm puts out 100watts in class a then goes into a/b beyond that.
The bhk monos on the other hand stay in class A for 1 watt. Both amps get excellent reviews. Can anyone compare the tonal characteristics of the amps and advantages of either design ? I know the pass puts off heat because of the design. That doesn’t concern me.

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I have the xa100.5:s, they are pure class A, there is no class B stage.

I have the XA60.5 and as Nelson Pass describes in a review on stereophile:
„…operating in class-A mode up to the amp’s rated output and class-AB beyond that point."
As far as I understood, this point is visible when the meters start to move. But in my system this occurs only on levels that will wake up your neighbours (I mean the ones on the other side of the road :wink: )


Not so sure about that but there do seem to be some contradictory signals about the XA100.5 amps. They’re described as “Pure Class A Solid State Monoblock Power Amplifier” but in the same breath “Power: 100 Watts RMS Pure Class A; 175 Watts Class AB (into 8 Ohms)”

Here’s a link to a review in Home Theater for the XA100.5.

There is no confusion here, other than what have been added by reviewers. This is from the manual of the xa100.5:

” With the introduction of the XA100 Pass LaboratoriesTM significantly raises the performance standard of excellence by introducing an entirely new series of power amplifiers. The new “XA” series of amplifiers feature a refined and elegantly simple, very highly biased, pure Class A mode of operation. This is a new and unique circuit topology that we descriptively characterize as being Single-ended Class A.”

I was on the way to write a bit about it, but as I remembered how different my former Gryphon and the Pass Aleph 0 monos and the Lamm M1.1 hybrid monos sounded (all highly biased Class A), I gave up. :wink:

The Aleph 0 sounded closer to very good AB and tube amps than to other Class A amps and tubes in the cirquit are another thing again.

So as you concretely speak of Pass amps I don’t know well, it’s difficult for me to help. If you’d be on the way to buy I Gryphon, I could tell more (but from the past, although their designs stayed very similar through the decades).

The confusion lies in the xa100 and the xa100.5. The.5 I believe introduced the additional B stage after rated output - per your quote … ‘The xa100 ……’.

I have a Gryphon Antileon Evo amplifier that states it offers 150 watts pure class A. It allows you to throttle back to 1/3 or 2/3 or full blast. You save energy at the lower levels and it reduces heat, lots of it. I typically listen at full blast but when I reduce it to 1/3 or 2/3 I don’t really notice any degradation. My speakers are highly efficient so I am guessing I hit 15 watt peaks no matter what level I run at. I could be wrong. Class A gives you a sound like a tube amp with no need to roll or replace tubes. I know, boring.


I would always run at full blast too Lol.

But we got steered away a bit on the xa100 vs xa100.5 topic…. The bhk design seems so on the opposite side of this spectrum. People act like moving out of class A is a sin in the XA series Pass amps.

There also seems to be this belief that pure class A sounds more powerful. So 100 watts of class A brings more to the table. True or false?

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Yes jazznut. From what I understand - the different Pass designs are voiced differently. Re: Gryphon … I’m not sure I have Gryphon money. I have used PassXA series money. :slight_smile:

I have the X250.8. 500 into 4ohm and first 25w Class A. As mentioned above, you have to really try and get the meter to move out of class A. However I’m proud to say I do that on some sessions- rock type music only of course. Speakers are 91 dB. I have owned the PSA 250 but not the 300s. Also I’m not sure what to say referenced the comment above about how Pass may be “voiced”. What I can say is the amp gets warm, never hot. It adds gobs of power while keeping all the detail and musicality from a BHK pre and DS and is the best amp overall I have owned. And there have been a few. As of now what I think works best is a accurate and detailed DAC/Pre with a tube input or component stage to a SS amp out to speakers. The Pass does not get “in the way” of anything it seems and the power is pretty awesome. I was told once as I was deciding that if I wanted overall musicality, go PSA. Detail etc, stay with Pass. The Pass does offer detail in spades and is a great amp. Would be quite a chore to A/B both but would be fun. You can’t go wrong with Pass in my book but that’s just me I suppose.

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Yes, the Aleph 0 were fully Class A biased with much less watt than the XA you mentioned. And their sound was more or less the opposite of the also fully A biased Gryphon. You really have to listen to designs which are different but at similar price points. Circuit design and synergy rule over basic concept mostly.

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I believe 100 watts of pure class A brings more to the table than 100 watts from any other class.
I’m not trying to be classist about it though.


No it did not.

FWIW, without seeing this statement in context, could not one assume that this is but one operational mode of the XA 100? In other words, just because there is a pure Class A mode for this kit, it does not, necessarily follow that there is not also a Class A/B mode.

And “no” I have not read the manual and I am unfamiliar with the XA 100.5 amplifier.

I just get hung up on semantics sometimes so feel free to correct me/ignore this comment.

In any event, I don’t see how one could go wrong with a modern Pass Labs amplifier today.



Furthering the discussion a bit. According to the Pass site the XA100.5 leaves Class A mode at 200 watts.





I’ve had many Pass amps and I currently have an XA30.5. I’ve always found that pure Class A has a particular purity to the sound. This amp is warm and romantic but also very detailed, reproducing lots of ambiance when it’s in the recording.
I had a BHK 250 and liked it but I ended up a fan of Pass designs.
As for the transition from A to AB, Pass builds his amps with enormous power supplies. This enables the amps to go well beyond their rated specs, beyond the spec for Class A and into AB. Usually, the police will have arrived well before one gets to that point, however.



PerL. What speakers do u have hooked to the xa100.5s?

Advantages/disadvantages? Class A runs hot because the output devices are biased on at all times. No switching. Dollar per watt Class A is expensive. Necessarily so because of the large heatsink area and other circuit measures that have to be utilized to keep the amplifier thermally stable. SQ? I’ve been playing with Class A since owning a Bedini 25/25 (there is a trip down memory lane). Completely grainless midrange is my best description of what I hear with Class A. The best Class A amps (Pass does Class A as well as any manufacturer on the planet, Nelson Pass is a master of that design) don’t sound either ‘warm’ or ‘cold’ to me. The Pass X-series really put to rest any notion that Class A has a ‘sound’ beyond being free of grain. I do hear one weakness, well maybe better put as a characteristic of Class A. Class A doesn’t have the bass extension and punch of Class AB. That is why I’m a big proponent of deeply biased Class A/Class AB which combines the best of Class A and AB. The Pass X-series (but not the XA series) are that type of design.