Help Fixing Bias

dsavitsk · 23rd June 2006, 07:52 PM

I am using this circuit as the gain stage for a DAC

http://www.diyaudio.com/forums/attac...amp=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

Merlinb · 23rd June 2006, 09:34 PM

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.

lndm · 24th June 2006, 12:35 AM

Come.....join us in the red light district

dsavitsk · 24th June 2006, 01:12 AM

I'm not opposed to LED biasing, but I need about 6V. I suppose this requires a few in series? Any suggestions for which color?

lndm · 24th June 2006, 01:23 AM

I believe SY recommends the standard (old) red types, and I am happy with those too. Series stringing is acceptable. I have even added ordinary diodes to the string to make up parts of a volt with no regrets, but YMMV, LED's are the recommended devices.

ray_moth · 24th June 2006, 03:38 AM

Hi Indm,

What voltage would red LEDs give per diode?

arnoldc · 24th June 2006, 04:08 AM

in my experience-

red = 1.5 - 1.6

orange = 1.8 - 2.0

lndm · 24th June 2006, 04:33 AM

and I get between 1.9 and 2.05 per, depending on the current I pass through them. BTW I've had my LED's sitting around for more than 10 years.

Shoog · 24th June 2006, 09:46 AM

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

23rd June 2006, 07:52 PM	#1
dsavitsk diyAudio Member Join Date: Jan 2005 Location: Chicago	Help Fixing Bias I am using this circuit as the gain stage for a DAC http://www.diyaudio.com/forums/attac...amp=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

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23rd June 2006, 09:34 PM	#2
Merlinb diyAudio Member Join Date: May 2006 Location: York	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.

24th June 2006, 12:35 AM	#3
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	Come.....join us in the red light district

24th June 2006, 01:12 AM	#4
dsavitsk diyAudio Member Join Date: Jan 2005 Location: Chicago	I'm not opposed to LED biasing, but I need about 6V. I suppose this requires a few in series? Any suggestions for which color?

24th June 2006, 01:23 AM	#5
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	I believe SY recommends the standard (old) red types, and I am happy with those too. Series stringing is acceptable. I have even added ordinary diodes to the string to make up parts of a volt with no regrets, but YMMV, LED's are the recommended devices.

24th June 2006, 03:38 AM	#6
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Hi Indm, What voltage would red LEDs give per diode?

24th June 2006, 04:08 AM	#7
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	in my experience- red = 1.5 - 1.6 orange = 1.8 - 2.0

24th June 2006, 04:33 AM	#8
lndm diyAudio Member Join Date: Mar 2006 Location: nsw	and I get between 1.9 and 2.05 per, depending on the current I pass through them. BTW I've had my LED's sitting around for more than 10 years.

24th June 2006, 09:46 AM	#9
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	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

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