990 as-designed is not class A output.
John Hardy makes a good modern copy of the '990 and has details of his 2016 builds:
http://www.johnhardyco.com/pdf/990.pdf
It's not just "parts". (Altho if you are just pushing Hi-Fi inputs, most of this is moot.)
John Hardy makes a good modern copy of the '990 and has details of his 2016 builds:
http://www.johnhardyco.com/pdf/990.pdf
It's not just "parts". (Altho if you are just pushing Hi-Fi inputs, most of this is moot.)
I mis-spoke.
We can estimate the power stage idle current by inspection. Total current is 25mA. Q4 passes 2mA, Q7 passes 7mA, Q5 about 1mA, bias network about 2mA, so 12mA. 25mA-12mA is 13mA for the power stage.
Working with +/-15V rails, about 13V peak, it will stay A for loads down to 1K.
But a specific feature of the 990 is driving loads to 75 Ohms. And +/-24V rails. Peak current can be 260mA. That's why the giant output devices. This was useful in older systems and for driving long lines at high level. With 13mA idle the peak current staying A is 26mA, 1/10th of the rated peak current working hard; it is far into class B.
If your loads are only nominal Hi-Fi things, you don't need a '990.
We can estimate the power stage idle current by inspection. Total current is 25mA. Q4 passes 2mA, Q7 passes 7mA, Q5 about 1mA, bias network about 2mA, so 12mA. 25mA-12mA is 13mA for the power stage.
Working with +/-15V rails, about 13V peak, it will stay A for loads down to 1K.
But a specific feature of the 990 is driving loads to 75 Ohms. And +/-24V rails. Peak current can be 260mA. That's why the giant output devices. This was useful in older systems and for driving long lines at high level. With 13mA idle the peak current staying A is 26mA, 1/10th of the rated peak current working hard; it is far into class B.
If your loads are only nominal Hi-Fi things, you don't need a '990.
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