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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
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.
itsjustme
682 posts
 
i don't really care, just trying to figure out what you were posting about and don't have the data.  was not clear what your point was....
unsound
7,333 posts
 
Re: the Threshold 500’s.  Claimed 20 output devices per channel, for 250 Watts per channel into 8 Ohms with about 20% of that being Class A,  the Class AB power output could steadily double down into 4 Ohms, but while doing so the Class A output would halve, the amp was said to be capable of doubling it’s rated steady state output for up to a couple of minutes.  Nelson Pass might have been a bit generous to himself when it came to how he defined Class A. Still, IMHO impressive.
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