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Old 3rd December 2003, 04:45 AM   #1
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Hi all. I apologize for this second set of silly newbie questions. I'm using this preamp: http://www.bonavolta.ch/hobby/en/audio/12au7lin.htm
I've changed R9 to 600 Ohms to increase the bias which has made the bass better, but I fear that the bias might be too high. What would be a safe value, if 600R is indeed too low? Even at this point, however, the bass is still weaker than my Sonic Frontiers Line1. Secondly, I find the sound to be too sharp for my tastes. Is there anything I can do to change that? I'm just using run of the mill parts.
Thanks for your help.
Regards, H.
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Old 3rd December 2003, 04:58 AM   #2
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Changing just one bias resistor in an SRPP stage will only screw it up. It's a carefully balanced system. If nothing else you have to reduce R7 and R9 together. By ohm's law, R8 will also have to drop to keep the voltages the same. And that then messes with the distortion and gain characteristics of the amp! In addition, increasing bias current will *not* allow more anything, because I can only hope the stage was biased ideally to begin with; any higher current and you lose headroom to saturation, any less and you lose it (and distortion) to cutoff effects.

Bass, midrange and tone are frequency response effects and will not be materially affected by the bias of a stage. Power output and distortion will.

If you need more drive capability, use a beefier tube. (5687 comes to mind, and mind you you'll need a rebiased circuit to try it.) Speaking of which, I wonder why the designer bothered with the hi-z output.. that doesn't make sense.

Tim
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Old 3rd December 2003, 05:30 AM   #3
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Thank you , Tim.
I guess I'll undo the changes and leave it as it was.
Much obliged for your help.
Regards, H.
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