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Old 30th January 2007, 11:03 PM   #1
azira is offline azira  United States
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Default Rule of thumb for BIAS current in class A

Obviously the more the better for bias, but as a general rule, what is the minimum BIAS I should consider in a ClassA circuit?

For my Zen, I know that the bias was set to about 3x the peak output current expected (10-watts into 8 ohms ~ 1.25 amps, BIAS set to 3amps).

I just got some HD 555s for Christmas and I'm working out a design for a headphone amp. Given my max output current is more in the 100mA range, I can afford quite a bit more relative bias than in the Zen.

I normally use the "10 is significantly larger than 1" rule in most of my designs so my first instinct is to set my bias to 10x my output current.

What do you think?
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Old 30th January 2007, 11:22 PM   #2
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Actually, 10W into 8 Ohms represents something closer to 1.1Arms. The extra bias is of use for those whose speakers are less than 8 Ohms...which includes nearly everyone since most speakers have impedance dips.
For headphones I'd say you could do quite nicely with an amp of bias. I just described a headphone amp with .5A bias in another thread, albeit with 24V rails. How high you set your rail voltage will factor into your design, too. I chose 24V because MOSFET capacitances increase rather rapidly below that. If I'd gone with bipolars, I could have used lower rail voltages. On the other hand, it would take more current to drive the circuit.
Just a question of which choices you want to make.

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Old 30th January 2007, 11:39 PM   #3
flg is offline flg  United States
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I beleive the real reason for the 2-3Amps of bias is due to the MOSFETs themselves. They start to get pretty linear above 2 Amps or so. However, in a balanced, differential or bridged circuit, the two sides of the amp tend to cancel each others non-linearities
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Old 30th January 2007, 11:52 PM   #4
MikeW is offline MikeW  United States
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Rule of thumb. Put you thumb on the heatsink for more than 5 seconds and the bias is to low.
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Old 31st January 2007, 12:12 AM   #5
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Default Re: Rule of thumb for BIAS current in class A

Originally posted by azira
I normally use the "10 is significantly larger than 1" rule in most of my designs so my first instinct is to set my bias to 10x my output current.

What do you think?
My kind of guy.

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