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Old 15th March 2005, 07:05 AM   #1
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Join Date: Jul 2004
Location: close to Hamburg
Default Roten RB 980 BX

Good morning,

does someone know the correct setting for
bias current on the Rotel RB 980 BX Stereo
amplihier?

It uses 2SA1492 and 2SC3856 output devices,
three of tem in parallel.

On www.zero-distortion.com I found a schematic of
a Rotel RB 991, which uses the same Output Transistors,
but more of them. This amplifier uses 7 mV / 0.22 Ohm =
32 mA per device according to the manual.

Compared to other, often referred numbers, this seem quite
low, otoh, if you go into the Datasheet of the output
transistors, you'll find hfe-Linearity over Ic defined beginning
at 20mA, from this point of view, a bias of 32 mA seems reasonable. (For comparison, the often referred replacement/
upgrade transistors MJL1302 / MJL3281 have gain linearity
specified beginning at Ic = 100mA. Would this mean in turn,
the MJE types would need more BIAS to obtain the same
distortion figures ?

There are some who recommend increasing the BIAS in
amplifiers to run in in Class-A at low output. Other sources
claim that distortion will increase if BIAS is set too high.

As i have very high-efficiant loudspeakers i am primarily
concerned on distortion at low output power.
Night-Listening of music uses approx 0,5 mW, loud music
listening is approx. 45 mW.



What are your opinions?

Thanks,
Stephan
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Old 15th March 2005, 09:19 AM   #2
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Join Date: Mar 2002
Location: Aveiro-Portugal
Hi Stephan

I also have a Rotel RB 980 BX and is biased at 50 mA in each output pair. Giving a total bias of 150mA per channel .

Is working like a charm ...
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Jorge
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