Classe Audio CA-400 & Obtuse Bias Procedure (scat included) - diyAudio
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Old 9th May 2010, 07:36 PM   #1
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Default Classe Audio CA-400 & Obtuse Bias Procedure (scat included)

I have this amp and I'm checking it out for a friend. Works fine, but we both want to know that bias and offset are good. Offset is no problem, but I've never seen such a bizarre bias adjust procedure. 24 hours warmup? Calculation formula? Reference to parts that don't exist (R8 and RV1)?

Anyone want to look at this and see if they can make heads or tails of it? GlenB has some experience with these amps, I think. Hopefully he will chime in...

https://www.yousendit.com/download/O...WkI4aU94dnc9PQ

(link good for 7 days or 100 downloads...best I can do)

Last edited by EchoWars; 9th May 2010 at 08:01 PM.
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Old 10th May 2010, 05:34 AM   #2
Glen B is offline Glen B  United States
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The crazy bias procedure in the Classé service manual is of fairly recent vintage. I have the original and more simple bias adjustment procedure dating back from before they created a service manual. I sent a copy to your PM box.
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Old 10th May 2010, 10:56 PM   #3
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Thanks Glen...much appreciated.
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Old 11th May 2010, 03:29 AM   #4
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FWIW, interesting to see that over 70 people jumped in to download the schematic, but only one guy replied with any useful information.
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Old 11th May 2010, 02:57 PM   #5
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A very interesting circuit. Seems quite well protected.

The PS is assembled with 9 sets of 4700uF caps. If the power is 400w/ch into 8 ohms, then that would need some increase.
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Old 11th May 2010, 03:29 PM   #6
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Quote:
Originally Posted by carlmart View Post
A very interesting circuit. Seems quite well protected.

The PS is assembled with 9 sets of 4700uF caps. If the power is 400w/ch into 8 ohms, then that would need some increase.
Well, most multi-watt PA amplifiers have no more then 2x 10000uF

Agree a very interesting circuit. The fact that there are no base stopper resistors at the bases of the power transistors. Also the fact that there is no output inductor/resistor in series, just a parallel Zobel to ground. I did read some horror stories however about burning CA400's wonder if this had something to do with it...

Ps: The soft-start circuit, and on/off latch switch is easy to copy for us DIY-ers and will work very well.

With kind regards,
Bas
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Old 11th May 2010, 04:12 PM   #7
Glen B is offline Glen B  United States
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All the Classé legacy models (DR-00, CA-000 and CA-001 series) share the same lack of base stopper resistors and output indictor/resistor. I haven't heard any stories of Classé amps burning up en-masse. I've been running a CA-300 into a fairly difficult speaker for the greater part of 14 years, and it has been very stable. The CA-300 circuit is identical to the CA-400, just with one less output transistor pair, and smaller power supply.

Last edited by Glen B; 11th May 2010 at 04:15 PM.
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Old 9th September 2011, 02:48 AM   #8
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Hey I realize this is an old thread but I just obtained one of these amps used and I have a question.

Mine runs much cooler than what I'm use to is this normal for the Classe CA-400? My Nakamichi PA-7's run much hotter and those are correctly bias'ed.

I'd like to check the bias on my Classe if someone could provide me with the procedure too I'd be much appreciated.

Thanks, Mark
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Old 9th September 2011, 03:31 AM   #9
Glen B is offline Glen B  United States
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Mark, I sent you a PM.
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