XSP-1 gen 2 with XPA-1L...a way to tell class A operation?

[hcsunshine90]

i see the XSP-1 gen 2 pre amp has a volume control that has a digital decibal indicator. if i were to use the XPA-1L amps with this pre amp, switched to class A operation, is there a specification as to when the XPA-1L transitions into class A/B operation. for example, if you go above -17db on the indicator, it then switches to class A/B. so my question is: it runs in class A operation up until what decibal threshold?

[DYohn]

It has no relation to the setting on a preamp. According to the specs, it puts our Class A up to 35 watts output. This might be achieved at any setting on your preamp depending on the material and the load.

[hcsunshine90]

so I reckon, it's not like when it indicates -17db it's pumping up the amps to 20 watts output and when it says -15db it's pumping up the amps to 25 watts output and so on. my ears likely won't be able to tell the difference, was just wondering if there was a way to tell for sure when the amps were running in class A operation. thanks, john (I see now that the type of speakers I am using has an effect on this too)

[AudioHTIT]

Apr 22, 2014 9:53:14 GMT -5 hcsunshine90 said:

so I reckon, it's not like when it indicates -17db it's pumping up the amps to 20 watts output and when it says -15db it's pumping up the amps to 25 watts output and so on. my ears likely won't be able to tell the difference, was just wondering if there was a way to tell for sure when the amps were running in class A operation. thanks, john (I see now that the type of speakers I am using has an effect on this too)

You could figure this out with detailed specs, but measurements are probably easier. If you took say a 1KHz test tone or noise and (carefully) set the voltage at the speaker terminals to 2.83V, you could then read the display on the XSP-1 and know at what reading your amps generate 1W into 8 Ohms (or 2 into 4). If for instance the XSP-1 read -40 dB you could expect to get near -25 dB and remain in Class A (assuming the 1L's 35 Watts Class A into 8 Ohms).

There are many variables here, most specifically which input and device you use for your test tone. If you played your tones off say a CD you could expect this to be reasonably accurate when playing CDs (though it won't account for peaks). If you played the tones off an LP then you could expect it to be accurate when playing albums. Which is all to say it'll get you in the ballpark.

If you did go to all this trouble I'd also measure the dB SPL at 1 meter and/or your listening position. You then could further correlate the XSP-1's dB reading to an actual dB SPL in your room. Again lots of qualifiers but you might also learn more about your equipment and room, but always take care when playing test tones through your system.

[cd]

The chances are very good that you will almost always be in Class A mode if you are using 87 db or more efficient speakers. Most people don't realize just how little power it takes to produce the volume levels that most listen at. 35 watts of Class A power will deliver pretty high volume for 90% of your listening.

CD

[Gary Cook]

Personally, if my ears couldn't hear a difference (between Class A and Class AB) or my speakers weren't revealing it or my room acoustics were masking it then it wouldn't matter.

Cheers
Gary

[hcsunshine90]

thanks for all the replies. I'm certainly not going go to the lengths of using test tones and measuring the results and so forth. what I was looking for and what helps me the most is akin to "CD's" response: "most people don't realize how little power it takes to produce the volume levels that most listen at. 35 watts of class A power will deliver pretty high volume for 90% of your listening." I myself don't usually listen to very loud music much anymore. as I get older, I find myself searching for tone over volume. cheers and happy listening.

[audiobill]

As reported here before, if you go to the Crown website you can find a calculator that considers listening space volume, speaker sensitivity, desired listening volume, amplifier headroom and solves for required power.

May be useful to you.

[hcsunshine90]

thanks audiobill!

[garbulky]

Apr 23, 2014 8:05:07 GMT -5 hcsunshine90 said:

thanks audiobill!

Also of interest, my listening volume is in the mid seventies. Though at times I will crank it to the 80's. And if I am adventurous the 90's. But I was very surprised that my volume was in the mid-seventies in db. As you probably know the db setting on the XSP-1 does not equal to the db volume you are hearing at listening position. That is affected by your speakers and your listening distance/room acoustics.

[KeithL]

You should remember that "Class A operation" actually describes how the amplifier is operating at a given instant. (The "operating class" simply describes the relationship between the bias level of the output stage of the amplifier and the audio signal being amplified. If, at the moment, the signal level is at a level such that both sets of output devices are conducting throughout the full cycle of the output waveform, then you are in Class A; if the signal level is such that each set of output devices is only conducting for part of the output waveform, then you're in Class A/B. Whether you're "in Class A" or "in Class A/B" is something that changes from instant to instant - depending on the signal level. All Class A/B amplifiers operate in Class A all the time for very small signal levels; the Class A switch on the XPA-1L and the XPA-1 Gen2 simply raises the level at which this "transition" takes place by raising the bias level.)

My point here is that music is very dynamic, and usually has occasional peaks that are many times as loud as the average level. (One "rule of thumb" is that "with typical music" that "isn't highly compressed" - whatever exactly that means - the peaks will be ten to twenty TIMES as high as the average level.) What this means in the context of our amps is that, even if you're playing something VERY loudly on either of those amps, odds are that you are actually listening in Class A for most of the time - and just the occasional peak "dips into Class A/B operation".... and those peaks are almost certainly going to be things like loud cymbal crashes and drum whacks.... so, for all intents and purposes, you're going to be hearing Class A operation the vast majority of the time.  (So, even if you're playing music at what works out to an average level of 100 watts, you're probably still "in Class A" 90% or 95% of the time with an XPA-1L - and the times when you "dip" into Class A/B for a few seconds are those you're least likely to notice the difference - and DO remember that both amps sound so good in Class A/B mode anyway that the difference is subtle to begin with. )

The basic problem with "Class A only" amps - besides ridiculously high prices, weight, and heat generation - is that they are absolutely limited to the power they can produce in Class A, so they have no reserve to produce those occasional very-high-level peaks. (Our amps run in Class A most of the time, with very occasional "forays into Class A/B"; a typical "pure Class A" amp runs in Class A most of the time, with occasional forays into CLIPPING if you try to play it at any level where even occasional peaks exceed it's Class A power rating. We think our solution makes a lot more sense.