Salas hotrodded blue DCB1 build

[Salas]

Originally Posted by ppcblaster

Matching the LED'S, I wire a 1.2K resistor in series with the LED apply 9vdc and measure the voltage across the LED?

Vf what's the f stand for voltage what ?

What voltage am I looking for? I ordered 40LED's and need
16, so I measure all 40 and select the 16 that measure the same
or as close to the same?

Please advise.

Yes you can do that.

Vf= Forward voltage drop. Describes the nominal voltage that any diode develops when sufficient current is run through it given its nominal spec. LED is a glowing diode you can say.

You look for enough measuring as close to the same between them.

[dvb-projekt]

Originally Posted by ppcblaster

Pleas explain the purpose of the LED's.

Are they used as a resistor?

If not matched, what is the consequence?

Originally Posted by Salas

They are the active low noise voltage reference. If not matched, small assymetry in +/- Vout.

Are there any sources out there to get matched LED´s?
Not that it would be difficult, but i think not everyone want to buy a lot of LED´s to get 10 matched, me including.

Perhaps someone would like to do the job and make a GB?

[mach1]

Originally Posted by dvb-projekt

Are there any sources out there to get matched LED´s?
Not that it would be difficult, but i think not everyone want to buy a lot of LED´s to get 10 matched, me including.

Perhaps someone would like to do the job and make a GB?

I think the key issue is to get the STRINGS voltage matched. Having all the individual LEDs voltage matched is not necessary. This gives you a quite a bit of leeway in LED selection.

[ppcblaster]

Originally Posted by mach1

I think the key issue is to get the STRINGS voltage matched. Having all the individual LEDs voltage matched is not necessary. This gives you a quite a bit of leeway in LED selection.

What does that mean? "Strings voltage match"

I am still trying to understand this circuit, using LED's to "they are the active low noise voltage reference"

LED's have no opposition to current flow so having several LED's in series does what to the voltage? what is the voltage going in vs what is coming out

or is the voltage the same only a more stable voltage? How would a reference voltage be achieved if no LED's were used? What circuit would be used?

[AndrewT]

Originally Posted by mach1

I think the key issue is to get the STRINGS voltage matched. Having all the individual LEDs voltage matched is not necessary. This gives you a quite a bit of leeway in LED selection.

well said Mach1.

The LEDs don't need to be matched.
It is helpful to measure the Vf and label them accordingly, but not necessary.

A batch of LEDs will be very close in Vf. I have 5000 off of red LEDs that are 1.55Vf A tolerance of +-0.05V will probably bring 99% of them within 1.5V to 1.599V
I also have 100 red LEDs that are 1.95Vf+-0.05V. This batch are obviously made by a different manufacturing process. I have another batch that are ~1.8Vf

Selecting from these bags allows an almost infinite combination of string voltages.

The LEDs in the DCB1 are in three groups. A singleton, a triplet and a quintuplet.

The singleton is a confirmation that power is available after the 12V regulator.
It can be any LED, red, green, blue, white, orange, yellow.
The triplet is a string of three unmatched LEDs that have a total Vf between 5V and 6V. If the strings of total voltage are made consistent for all your Shunt regulators, it makes for less calculation in setting the current in the CCS part of the regulator. Again it does not matter if the strings are different, but it saves time and measurement and adjustment later if you know beforehand that your strings all measure 5.85V to 5.9V or any other range you come up with.

The purpose of this triplet is to set the Vgs of the CCS mosFET. The Vgs is a very variable parameter of mosFETs. All that time you spent matching your triplet string is thrown out the window because the mosFET has a much wider Vgs tolerance than the LEDs.
Best to just select a random mosFET and a group of three random LEDs and assemble with just one of the parallel resistors fitted. Measure the voltage drop across the CCS resistor. What is the current? How much more current do you want? What value of resistor will you select to go into the parallel location. Job done.

Finally the quintuplet.
This sets the output voltage of the shunt regulator.
The output voltage is the 5LED string Vf plus the Vbe of the adjacent BJT plus the voltage drop of the resistor between LEDs and BJT base.
The resistor Vdrop is very low due to the tiny base current flowing through it. Ignore this.
The Vbe of the BJT will depend on the type of device chosen. Choose a random device from the bag of recommended devices. It's Vbe will be between 550mV and 700mV but it's tolerance will be within about +-10mV for that batch of devices. Lets suppose it's 585mVbe +-10mV.
The string of 5LEDs had a total Vf of ~8.9V
The output voltage will be ~8.9+0.585V +-10mV. Vout = 9.485V+-10mV

Compare this to the other half of the DCB1. Swap a few LEDs in either 5LED string if you feel that the imbalance in regulated voltage is outside your desired tolerance.

IMPORTANT
The tolerance of the CCS current does not affect the regulated output voltage.

[Marra]

When I build my DCB1 to make things "safe" I will need to install a capacitor on the input of my yet to be built F5 [still waiting for the heatsinks; 6 weeks & counting] What value cap do people recommend and would it make any difference if I installed one on the F5 input or the output of the DCB1? I am reading through all relevant threads so hope I havent missed this bit of info.
Thanks.http://www.diyaudio.com/forums/image...s/confused.gif

[Marra]

That did'nt work right.

[AndrewT]

The DCB1 is by definition DC coupled.
The F5 is also DC coupled.

When both the source and the amplifier have no DC blocking capacitor, I believe it is essential to protect the speaker from abuse.

I recommend that a DC detect system be added. On detection of excessive a system to protect the speaker must be added.
I would also recommend, but not essential, that a DC servo be added to minimise the drift in output offset.

[Salas]

*If someone is perfectionist about matching the Verf LED gangs of 5, the best maniac party trick is to use a *12V battery, connect gate and source together in the specific 2SK170s that are going to feed those strings (those Jfets above 1R), feed (+) to the drain, connect g&s node to LEDS string, return string's other end to (-), and swap between enough glow bugs until you got 2 total across 5 LEDS Voltage matches, each to its own Jfet. For the cherry on the cake set DVM that measures diode drop (some got it) and find 2 Vbe matched BC550 & 560.
Practical benefit? Next to nothing. Aesthetic ritualistic benefit? Priceless.

*9V,1.5V,1.5V batteries in series that are common types, maybe already in remote controls on the coffee table, will do for 12V total. And don't post pics from the parking space tapping on the car's stuff.

[Russellc]

Originally Posted by Marra

When I build my DCB1 to make things "safe" I will need to install a capacitor on the input of my yet to be built F5 [still waiting for the heatsinks; 6 weeks & counting] What value cap do people recommend and would it make any difference if I installed one on the F5 input or the output of the DCB1? I am reading through all relevant threads so hope I havent missed this bit of info.
Thanks.http://www.diyaudio.com/forums/image...s/confused.gif

I've been asking around about that too. The suggestions I've had is to put it on the input of the F-5. I would prefer to leave the F-5 as is, and put it on the output of the B-1, although it wont be exactly DC then, which was the point, but I like a little insurance. The person who was commenting seemed to suggest it would be better on the input of the F-5, I'm not clear on why, so I continue to ask around. Nelson apparently chose to put the caps in the B-1 (standard non DC version of course) for what ever reason.

Russellc