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Class A into Class AB

What is the goal of a designer who makes intergrated amps that have class A for x amount of watts before it goes into class AB? Are there any examples of this being implemented well? I get this feeling that it’s kind of just a marketing thing...where people think they are getting some quality class A without the very high price tag. I was particularly looking at the CODA CSiB amps where you have three choices of how much of your first watts are class A. I have since found a few other respectable brands that implement this as well. I have yet to come across anyone who has heard much of difference between AB amps and one’s that’s state "first X amount of watts..." Class A/AB. Anyone have any experience with these kind of integrated amplifiers? Just looking for a little bit of understanding as I’m trying to upgrade my amplifier.
tmac1700
itsjustme
682 posts
 
6 watts is quite a lot of class A power.  Given that most decently recorded music has a peak:average ratio of ~ 10 or more, that means that peaks would be 60 watts if you were, on average running in class A.

And 6W is a lot of power! (surprisingly). As a designer i put a lot of experimental (read that: could blow up) stuff in my system with very expensive speakers.  So i have fused adapters pretty much all the time.  I use 1A fuses. Which means that they blow > 8W. They never blow.
itsjustme
682 posts
 
that in an RMS sense… it is mostly Class-A.

I think you mean in an **average** sense. RMS has to do with how we measure a wave:  peak, average, or "area under the curve" (root mean square or RMS).  It is quite different.  For a sine wave RMS = average. For complex waveforms you need to integrate to get the area.  Eek! calculus!


But your point, if i infer right, remains valid.
uncleang
123 posts
 
The Threshold S550e amp from the early 90's was rated at 250 WPC.
This stasis design had optical bias circuitry and the bias was set at 100mV.
Threshold claimed that it would run up to 20% of it's rated power in class A (that's 50 watts!!)

My NAK PA-7A MKII is also a stasis design but without the optical bias circuit of the S550e of the same era.

Bias specs for the NAK is 40mV but I run it at 50mV. It draws around 200 watts at idle. It's rated at 225 WPC.

My guesstimate is that it runs in Class A for the first 10-12 watts.
itsjustme
682 posts
 
Geez i fell like a detective, and with that data inspector Clouseau to be specific.....
per the threshold - complete the math!!  100mV across what? What's the derived current (which is what matters)?  Across the emitter resistors, which are what value? Without that you don't know the current.  Let's assume 1 ohm.  We shall assume per device, but number of parallel devices unspecified.  If nelson claimed it was A to 50 watts, that's 20V/8ohms or 2.5A; which implies 25 parallel NPN and 25 more parallel PNPs per channel -- minimum.  Some number there does not add up.
uncleang
123 posts
 
Why don't you ask NP on diyAudio.
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