The first watts


Okay, let's see if I can ask this question correctly. If you have an class A/B amp and the first 10 Watts is class A and after that it's all class B what happens if you have an inefficient speaker, do the first 10 W get used up real quick so that you're almost never hearing class A amplification, or does it work differently than that?
last_lemming
Unless you listen at very loud levels, say 85dB or higher, your amp will operate in class A most of the time.
You are on the right track. Normally and amplifier will be called A/AB, where it will operate in Class A up to about 5-15 wpc then slide into Class AB. The Class A watts will be the first watts used, so they will go first. Power works on a logarithmic scale, meaning that 100 wpc will sound twice as loud as 10 wpc (+3dB).

So if you have inefficient speakers, that are soaking up 100-200+ watts, you will basically be listening in Class AB. Now with average speakers, at normal levels, you may be listening in Class A, and never switch over to AB. Very few amps operate in pure Class A up to 200 wpc, and they are very expensive.
Onhwy61:
Unless you listen at very loud levels, say 85dB or higher, your amp will operate in class A most of the time.

It's partially right. The other part totally depends on speaker. The current demand can be even at lower volume levels especially at crossover points and lower frequencies.
After transition from class A (both transistors conducting) to class B (only one transistor conducting at given moment) it is already linear and class name doesn't make any difference. Actually, increasing bias point to keep it longer in class A would increase distortion (overbias) since it increases area of double gain (gm doubling) caused by the fact two transistors operate instead of one.
I used to have a full class a amp that one could cook a steak on. I didn't feel the marginal benefits were worth the price, weight, and excessive heat.
for the record I'm using Thiel CS 2.4's
11-21-14: Jmcgrogan2
Power works on a logarithmic scale, meaning that 100 wpc will sound twice as loud as 10 wpc (+3dB).

Whoops, that should read +10 dB. 10 times the power gives you twice the volume, or +10 dB. Sorry about that confusion.

So 10 watts is twice as loud as 1 watt. So your speakers, rated at 87 dB @ 1 watt, should be able to get up to 97 dB in an amp that switches over to Class AB at 10 wpc. Probably most of your listening would be in Class A, unless you are a headbanger.
So your speakers, rated at 87 dB @ 1 watt, should be able to get up to 97 dB in an amp that switches over to Class AB at 10 wpc. Probably most of your listening would be in Class A, unless you are a headbanger.
Sorry, that's not exactly right. The speakers are rated at 87dB/2.83V/m and since the speaker is 4 ohm, that will be 2 watts. Then there is a gain for 2 speakers, and a loss for listening distance greater than 1 meter. So a rough estimate for a distance of 2.5 meters using 10 watts would be ~92dB. Also, even if the average SPL is 75dB, some music can have dynamic peaks of 95dB and higher. If you really want to know, get a sound level meter, and measure the peak SPL when you listen.
If you use an iphone you can download an app with a fairly accurate spl meter. I find I rarely listen much over 70db in a small room. This was a surprise to me.

This makes me think your amp is operating mostly in class A.
I got the app. It seems my listening preference is between 75 and 85 dB. That's 8 to 9 feet away from the listening position to the center of the speakers. I'm not sure what that equates to in Watts
It seems that there is a disconnect between theory and practice here. A speaker rated at 87db is considered to have moderately low sensitivity. I would say that most speakers have a higher sensitivity today.

If 10 watts powers that speaker to 92db, that is loud. Wouldn't a 100 watt amp be a monster amp? They certainly aren't considered that these days. Any ideas why that is?
If 10 watts powers that speaker to 92db, that is loud. Wouldn't a 100 watt amp be a monster amp? They certainly aren't considered that these days. Any ideas why that is?
If 10 watts powers the speakers to 92 db at the listening position, 100 watts will power them to 102 db at that position (assuming the speakers can handle that power level without thermal compression or other problems).

As Tony (Tls49) said, "even if the average SPL is 75dB, some music can have dynamic peaks of 95dB and higher." I have more than a few classical symphonic recordings in my collection which when played at average levels in the 70's will reach occasional brief dynamic peaks of 100 to 105 db, at my listening position.

The ability of the amp to comfortably handle such high volume peaks will be less of a concern, of course, with many or most pop and rock recordings, which are typically compressed to narrow dynamic ranges. Often to the extent of having less than a 10 db difference between the loudest and the softest notes.

Also, I'll add that in the absence of specific technical information about whatever SPL meter is being used to measure these levels, I would not assume that a given meter necessarily responds fast enough to capture the true maximum volume of a brief dynamic peak.

Regards,
-- Al
The unofficial dynamic range data base (Google search will pull it up) illustrates very convincingly (just Enter name of artist or group) how the dynamic range has suffered over the past twenty or so years as subsequent reissues of once dynamic pop and rock albums have been compressed more and more. Even the first releases are hopelessly compressed in many cases.
Sound pressure falls off by the square of the distance. By the time you are 9 feet back this can be significant.

2 meters is thus 1/4 the energy. The speakers, once the conversion is done from sensitivity to efficiency, are 84 db 1 watt/1 meter (87 db 2.83V/1 meter and a 4 ohm speaker).

so 1 watt at 2 meters is 78 db. We're a little further back than that of course. 10 watts is 88 db (10x more power). Add 3 db for the other channel. Discount for absorptive materials in the room- carpet, sofa, etc, as well as being further back.

You may assume that the amplifier is making significant class AB power- the 10 watts is likely getting used up pretty quick unless you have a very lively room. However that first 10 watts is likely helping out with low lever detail.
Note...I wouldn't be too concerned about this...a friend has a switchable A to A/B amp and the differences were nominal...although "golden ear" types will push pure class A...the irony...the majority of normal listening requires very little power...and operates in class A anyways...
^^ For the most part, I would not either, whether the amp is tube or solid state. Due to the rather low efficiency of the speakers, power is likely going to be more important than what class of operation is employed.
Atmasphere nailed it...