Ping Sy: LED for Cathode Bias - Page 5

Shoog · 21st March 2006, 12:36 PM

As a matter of interest I decided to lash up a circuit to boost the current through the Green LEDS. I took AC of the heater and recified it with a fast soft recovery diode. I put it through some capacitance and a LM7805 and then some more capacitance. I then put it through a 330R resistor and finally the LED with a 22uf bypass cap. This gives a final current of 13mA through the LED.
The result is very good, very clear and detailed with plenty of bass. Difficult to say exactly whats what as the bias point has shifted slightly. Still it works. All the previous softening and smudging has gone. Got a bit of hum and don't need the extra gain so don't know if I will keep it, but I think it sounds better than a plain unbypassed resistor.

Shoog

Ryssen · 21st March 2006, 08:04 PM

So when I look for the right Led,if I want 1,5 v at the catod and 15mA current I look at the Led´s Forward current/volt and find one that crosses at 1,5v/15mA? Is that right?

Seccond question-Does anyone have a schema for a CCS for the anode
or can make one?

SY · 21st March 2006, 08:22 PM

Take a look at the CCS I used in the thread "SY gets jiggy" for a good example of an inexpensive plate CCS that works very well.

The LED "curves" take off pretty sharply at a particular voltage rather than being a smooth curve. So look for a color with a Vf near the bias point you want, which will generally be specified at 10mA; there won't be a lot of difference between 10 and 15mA.

rdf · 21st March 2006, 08:48 PM

Hi Shoog, have you tried bypassing the LED with a cap yet? In my measurements the green LED benefited most from it, dropping all harmonics below the noise floor of the sound card and besting both the red and the yellow. Curious whether it makes an audible difference for you. On the bench it took at least 25uF to reach maximum benefit and 6.8uF made little difference. The bypass occurs in the midrange for these values so theoretically it possibly alters the stage frequency response to some degree. How much depends on the degree of degeneration caused by the LED I suppose.

EC8010 · 21st March 2006, 09:31 PM

This might help when choosing LEDs for a specific voltage and current...

EC8010 · 21st March 2006, 09:38 PM

And this illustrates how sharply LEDs take off. It's the same red LED in all three attachments.

Shoog · 21st March 2006, 10:35 PM

"Hi Shoog, have you tried bypassing the LED with a cap yet? In my measurements the green LED benefited most from it, dropping all harmonics below the noise floor of the sound card and besting both the red and the yellow. Curious whether it makes an audible difference for you. On the bench it took at least 25uF to reach maximum benefit and 6.8uF made little difference. The bypass occurs in the midrange for these values so theoretically it possibly alters the stage frequency response to some degree. How much depends on the degree of degeneration caused by the LED I suppose."

I did bypass the Green LEDs with 22uf, but this was more to do with generating a stable 9mA current boost than bypassing audably. Can't say if it has effected the sound as I have nothing to compare it with really. I have been sufficiently impressed with the boosted current version that I have kept it in my preamp. Overall there is a significant amount more detail across the whole audio range compared to an unbiased cathode resistor. Got rid of all residual hum and shifted the bias points to compare more with the original version.
Well worth the effort.

Shoog

arnoldc · 21st March 2006, 11:18 PM

Can somebody please explain this to me.

Last night when playing music, I observed that both the orange LEDs are flashing at a regular interval. After a while, the flashing stoped. I turned my record to the other side, played it and the flashing starts again.

When playing the middle part of the record, the flashing stops.

When I investigated, the record that I'm playing has a horizontal warp at one point. When the cart hits that point, it must be banging left and right on the vinyl groove

and during this, the LEDs flashes.

What the??

Hey ryssen, tubelab has a nice schema of the 10M4 IXYS CCS as an alternative to that of SY. They're cheap and I'll be ordering a couple from RS Components. Pete Millet has also experimented with them, with positive outcome.

SY · 21st March 2006, 11:37 PM

The brightness change is an indication that you're clipping that stage.

Another nice benefit of using a LED, a built-in distortion monitor!

Dave Cigna · 21st March 2006, 11:40 PM

Quote:

Originally posted by EC8010
And this illustrates how sharply LEDs take off. It's the same red LED in all three attachments.

I was about to put together a test setup to make this curve when you posted this.

For anyone that wants to look at these curves themselves with different diodes, there's a really simple setup.

21st March 2006, 08:22 PM	#43
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Take a look at the CCS I used in the thread "SY gets jiggy" for a good example of an inexpensive plate CCS that works very well. The LED "curves" take off pretty sharply at a particular voltage rather than being a smooth curve. So look for a color with a Vf near the bias point you want, which will generally be specified at 10mA; there won't be a lot of difference between 10 and 15mA. __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

21st March 2006, 08:48 PM	#44
rdf diyAudio Member Join Date: Jun 2004 Location: the thermionic past	Hi Shoog, have you tried bypassing the LED with a cap yet? In my measurements the green LED benefited most from it, dropping all harmonics below the noise floor of the sound card and besting both the red and the yellow. Curious whether it makes an audible difference for you. On the bench it took at least 25uF to reach maximum benefit and 6.8uF made little difference. The bypass occurs in the midrange for these values so theoretically it possibly alters the stage frequency response to some degree. How much depends on the degree of degeneration caused by the LED I suppose. __________________ Blame the Manichaeists

21st March 2006, 11:37 PM	#49
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	The brightness change is an indication that you're clipping that stage. Another nice benefit of using a LED, a built-in distortion monitor! __________________ "...we stumble and get up, we are sad, confident, insecure, feel loneliness and joy and love. There is nothing more; but I want nothing more.” - Christopher Hitchens 1949-2011

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21st March 2006, 12:36 PM	#41
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	As a matter of interest I decided to lash up a circuit to boost the current through the Green LEDS. I took AC of the heater and recified it with a fast soft recovery diode. I put it through some capacitance and a LM7805 and then some more capacitance. I then put it through a 330R resistor and finally the LED with a 22uf bypass cap. This gives a final current of 13mA through the LED. The result is very good, very clear and detailed with plenty of bass. Difficult to say exactly whats what as the bias point has shifted slightly. Still it works. All the previous softening and smudging has gone. Got a bit of hum and don't need the extra gain so don't know if I will keep it, but I think it sounds better than a plain unbypassed resistor. Shoog

21st March 2006, 08:04 PM	#42
Ryssen diyAudio Member Join Date: Dec 2003 Location: Sweden--> Here	So when I look for the right Led,if I want 1,5 v at the catod and 15mA current I look at the Led´s Forward current/volt and find one that crosses at 1,5v/15mA? Is that right? Seccond question-Does anyone have a schema for a CCS for the anode or can make one?

21st March 2006, 10:35 PM	#47
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	"Hi Shoog, have you tried bypassing the LED with a cap yet? In my measurements the green LED benefited most from it, dropping all harmonics below the noise floor of the sound card and besting both the red and the yellow. Curious whether it makes an audible difference for you. On the bench it took at least 25uF to reach maximum benefit and 6.8uF made little difference. The bypass occurs in the midrange for these values so theoretically it possibly alters the stage frequency response to some degree. How much depends on the degree of degeneration caused by the LED I suppose." I did bypass the Green LEDs with 22uf, but this was more to do with generating a stable 9mA current boost than bypassing audably. Can't say if it has effected the sound as I have nothing to compare it with really. I have been sufficiently impressed with the boosted current version that I have kept it in my preamp. Overall there is a significant amount more detail across the whole audio range compared to an unbiased cathode resistor. Got rid of all residual hum and shifted the bias points to compare more with the original version. Well worth the effort. Shoog

21st March 2006, 11:18 PM	#48
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	Can somebody please explain this to me. Last night when playing music, I observed that both the orange LEDs are flashing at a regular interval. After a while, the flashing stoped. I turned my record to the other side, played it and the flashing starts again. When playing the middle part of the record, the flashing stops. When I investigated, the record that I'm playing has a horizontal warp at one point. When the cart hits that point, it must be banging left and right on the vinyl groove and during this, the LEDs flashes. What the?? Hey ryssen, tubelab has a nice schema of the 10M4 IXYS CCS as an alternative to that of SY. They're cheap and I'll be ordering a couple from RS Components. Pete Millet has also experimented with them, with positive outcome.

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