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Old 10th January 2004, 06:01 AM   #1
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I'm a newbie and would like to try one of those Class-A amps I've been hearing so much about...what kinda PSU are wqe looking at since class A has got low noise rejection?? and why do we use Mosfets instead of Bipolars...is Bipolar usuage possible?? and of course are matched stuff very important?? Sorry for a bit of OT...I understand these amps are at best 45% efficient (if it ever gets there...) so I'll need some BIG heatsinks....
And is building WITHOUT a PCB possible?? I.E I just mount everything on the Heatsinks since the circuit I've seen are pretty easy...and what kinda schematic are we looking at for the Aleph3?? sorry again for the many dumb0 questions...
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Old 10th January 2004, 06:17 AM   #2
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Oh...thanks moderator for shifting me to a better place...
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Old 10th January 2004, 09:48 AM   #3
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Quote:
Originally posted by li_gangyi
what kinda PSU are wqe looking at since class A has got low noise rejection??
If the output stage is a source follower or emitter follower then the power supply noise rejection will be very good. Just make sure the earlier (low current) stages have a noise-free supply rail. That's easy.
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Old 10th January 2004, 11:30 AM   #4
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hmm...so what kinda circuit are we looking at for the best noise rejection assuming I'm gonna scrimp on my PSU section for testing...
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Old 10th January 2004, 06:06 PM   #5
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For testing purposes, you can scriimp all you want. In fact,
this will give you a thumbnail value for how clean your supply
will want to be when you build the real thing.
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Old 11th January 2004, 02:46 AM   #6
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Quote:
Originally posted by li_gangyi
I'm a newbie and would like to try one of those Class-A amps I've been hearing so much about...what kinda PSU are wqe looking at since class A has got low noise rejection?? and why do we use Mosfets instead of Bipolars...is Bipolar usuage possible?? and of course are matched stuff very important?? Sorry for a bit of OT...I understand these amps are at best 45% efficient (if it ever gets there...) so I'll need some BIG heatsinks....
And is building WITHOUT a PCB possible?? I.E I just mount everything on the Heatsinks since the circuit I've seen are pretty easy...and what kinda schematic are we looking at for the Aleph3?? sorry again for the many dumb0 questions...
Li,

The answers to most of your questions can be found at the Pass Labs site here:
http://www.passlabs.com/aleph.htm
There are all the owners and service manuals for all the Aleph series amps. You also want to look at some of the early articles on class A designs. The articles on the Zen amps can be found at the PassDIY site here:
http://passdiy.com/amps.htm

Have fun,
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Old 11th January 2004, 04:21 AM   #7
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Use differentials in the front end and you'll acheive a certain amount of immunity to PS noise. Current sources can help also. Class A amps need not be sensitive to noise. After all, the front end in class A/B and class B amps run in class A already. After that, it's only a question of how to bias the outputs.
Certainly bipolars are possible. So are tubes, for that matter. Lots of class A amps were build before MOSFETs became practical devices.
Although I don't generally run point-to-point wiring myself, there's no reason a class A amp can't be done that way.

Grey
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Old 11th January 2004, 04:21 AM   #8
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I've been looking at the DoZ thingies...seems like they are the easiest to implement...probably gonna use some Bipolars instead of MOSFETs...dun really like them...anyone care to "push" me towards MOSFETs?? Hehe...I did use MOSFETs in a Class A Headphone amp once...sound was very clear but at high volumes it sounded kinda "fake" dunno if it's my PSU sagging therefor inducing some compression though...
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Old 11th January 2004, 04:34 AM   #9
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Oh yeah...and thanks to everyone for their help and especially the moderators for helipng me shift this thread...looking forward toi my first Class A big time amp...maybe I'll drop GC and ABs after this...
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Old 11th January 2004, 01:10 PM   #10
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Default Build an Aleph

There is a lot of information. You can build anything from 15 to 100 watts if you have heatsinks. The motherboard is the same except for some resistor changes. Use the MOSFETS. You will like them. If you have a hard time getting them I would be glad to send you a few.
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