I am using this circuit as the gain stage for a DAC
http://www.diyaudio.com/forums/attachment.php?s=&postid=938884&stamp=1150150590
It sounds remarkably good, but it has been suggested to me that converting it to fixed bias might be a good thing. As I understand it, this would mean removing R2, R3, and C1, jumpering over R3 and C1's spots (that is, tying the cathode to ground) and applying a negative voltage to the grid -- I guess between the grid stopper and the input cap.
So, the questions are (1) does that sound correct, and (2) what is the best way to apply this voltage? It seems that any ripple at all is just going to be quickly amplified, so perhaps a battery with the + terminal tied to ground? And, will there be any current drawn from the biasing voltage? Thoughts?
-d
http://www.diyaudio.com/forums/attachment.php?s=&postid=938884&stamp=1150150590
It sounds remarkably good, but it has been suggested to me that converting it to fixed bias might be a good thing. As I understand it, this would mean removing R2, R3, and C1, jumpering over R3 and C1's spots (that is, tying the cathode to ground) and applying a negative voltage to the grid -- I guess between the grid stopper and the input cap.
So, the questions are (1) does that sound correct, and (2) what is the best way to apply this voltage? It seems that any ripple at all is just going to be quickly amplified, so perhaps a battery with the + terminal tied to ground? And, will there be any current drawn from the biasing voltage? Thoughts?
-d
You are correct, technically. You can do it with a battery, but place a resistor in series with it (1M or something) to prevent short circuiting whatever you plug into it. Batteries are noisy by the way.
You could probably just replace R3 and C1 with an LED instead, it'll give you the same advantages with less of the hassle and noise. Obviously, the colour of the LED will have a baring on the bias voltage it drops, so chose wisely.
You could probably just replace R3 and C1 with an LED instead, it'll give you the same advantages with less of the hassle and noise. Obviously, the colour of the LED will have a baring on the bias voltage it drops, so chose wisely.
Old Orange and Green LED's give 2.0V drop with about 6-8mA of current.
Practically all new RED LED's drop about 2.5V these days. Other colours higher. These are not as good as the old LED's as they also have higher resistance. Salvage from 80-90's equipment might be a good source of the 1.7V Red LED's.
Watch your currents, LED's only sparkle at over 7mA of current, below that they sound flabby.
Shoog
Practically all new RED LED's drop about 2.5V these days. Other colours higher. These are not as good as the old LED's as they also have higher resistance. Salvage from 80-90's equipment might be a good source of the 1.7V Red LED's.
Watch your currents, LED's only sparkle at over 7mA of current, below that they sound flabby.
Shoog
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