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  #1  
Old 01-24-2022, 05:06 PM
paltomare paltomare is offline
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Default Class A Amp Question

I understand class A amps run at 100% regardless of input level, which explains why some of them run hot even at idle.

Why then when I turn up the volume on my pre-amp does the meter on my power conditioner show an increase of amps being drawn? At idle it shows about 1.3 amps, last night I had it cranked up and it was showing a 3.2 amp draw.

Just curious.
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Old 01-24-2022, 08:03 PM
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No amp in your signature line is Class A (maybe the Schiit?). Your preamp may have Class A stages in it, but draws so little power between idle and full output it may not show on your power meter. Your MC275 is a PP (push-pull) amp. The more power you want the more power it will take from the wall socket. A true Class A amp using KT88s will make about 7 or 8wpc/tube (look at Dennis Had's Inspire amps as an example) while your MC275 is rated at 75wpc. Definitely not Class A.

Class A amps do not run at 100% rergardless. Their large heatsinks & power supplies would have to be even larger (aren't they big enough all ready? - my 25wpc INT-25 weighs 50# - once it is delivered). Class A often makes the most heat at idle as the output devices are at an essentially steady-state mid-band. When playing music the average current draw is lower as it moves up and down around that mid-band bias point.
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Old 01-24-2022, 09:38 PM
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100% refers to the relationship of input signal to conduction of current in the output stage. A class-A amp's output will conduct for 360 degrees (the entirety) of the input signal. A class-B amp's output will conduct for only 180 degrees. Class AB is squarely between the two. The lower duty-cycle of class AB or B amp sections (halves) gives them time to cool down thus allowing higher average power. Any amp regardless of configuration will consume more current when the volume is increased.
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Old 01-25-2022, 05:45 AM
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Vidar is A/B
Nad is D
If A-48 is Accuphase A-48, that is Class A.
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Old 01-25-2022, 07:04 AM
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Quote:
Originally Posted by tima View Post
If A-48 is Accuphase A-48, that is Class A.
Indeed. I missed that. Thanks.
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Old 01-25-2022, 09:05 AM
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There has been a lot of creative design and marketing around ‘Class A’ amplifiers that deviates from the technical definition of class A. That is, the amplifying element is biased so the device is always conducting.
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Old 01-25-2022, 10:03 AM
paltomare paltomare is offline
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I should have mentioned I was referring to my Accuphase A-48.

"100% refers to the relationship of input signal to conduction of current in the output stage."

This makes sense now, thanks all.
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Old 01-25-2022, 03:47 PM
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Here are a few articles addressing Class A by some guy named Nelson Pass. I can't vouch for him, but have heard he knows a thing or two on the subject.............

https://www.passlabs.com/technical-articles/
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Old 01-25-2022, 07:00 PM
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Thanks for sharing, some interesting articles in there and from a man that is pretty clued up on this stuff!
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Old 01-26-2022, 12:06 AM
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Yes. I found his explanation of why MOSFETs are best for Class A amps very good. Nelson is one of greats in our hobby.
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