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Old 9th November 2012, 04:04 PM   #1231
DRZ1 is offline DRZ1  Canada
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And if i connect the pot in parallel with the diode string ?????

I think that this set up could work better than in series.

It upset me a bit because my first FC-100 work very well (super well compensated) with the exact same transistor (0302-0281) from an other batch.
I have tried it with NJL1302-3281 and it work very well too, but a prefer the sound with the first ones.

So friends wants a FC-100 too and wee purchased a pretty big batch of NJL and they seems to have a problem with this diode....

I don't have time to work on it this week, but i will give it some time next week.

Wait and see.
Denis

Last edited by DRZ1; 9th November 2012 at 04:15 PM.
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Old 9th November 2012, 04:44 PM   #1232
roender is offline roender  Romania
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Quote:
Originally Posted by DRZ1 View Post
And if i connect the pot in parallel with the diode string ?????

I think that this set up could work better than in series.

It upset me a bit because my first FC-100 work very well (super well compensated) with the exact same transistor (0302-0281) from an other batch.
I have tried it with NJL1302-3281 and it work very well too, but a prefer the sound with the first ones.

So friends wants a FC-100 too and wee purchased a pretty big batch of NJL and they seems to have a problem with this diode....

I don't have time to work on it this week, but i will give it some time next week.

Wait and see.
Denis
Too bad that ONsemi f..ked up some production lots of NJLs. I'm convinced that they put by mistake other type of diode inside which requires other compensation current in order to correct track the transistor Vbe over temperature.
I do not recommend to bypass all the diodes string with the pot. Just use what Andrew recommend, one resistor over the diode with way out Vf than the others.
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Old 10th November 2012, 08:50 AM   #1233
AndrewT is offline AndrewT  Scotland
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Checked the diode Vf.
I used the 4281/4302, the diode drops are PNP 440 to 442 NPN are 440 & 443 Total = 2.21V hot. It has been idling all night.
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regards Andrew T.
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Old 10th November 2012, 02:02 PM   #1234
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Originally Posted by AndrewT View Post

What are the diode voltages? Is the 33k on the diode with the highest voltage? or another?
Depending on which you did, it may be prudent to add a resistor to another high Vf diode and dial in a little resistance with the 500r Vr.
When running for around 10 minutes I am seeing Ttrak diode voltages of around 480mV. The one I shunted with the resistor was not selected as being higher than others, although I believe I have shorted the highest one.

By snipping through the resistor while powered the Vf came up just 1mV (477mV to 478 mV) I did not want to have the temperature change during the exercise. This is telling me that one diode shorted is almost enough to put the bias in range but it needs the slightest tweek to get it over the line. I will try a lower shunt R and some pot R.

Keith
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Old 14th November 2012, 10:44 PM   #1235
DRZ1 is offline DRZ1  Canada
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Well, it's worse than i thought. At lightly warm temp NPN diode = 0.67V and PNP = 0.52V.

If i want about 2.5V total with 5 diodes i have to bypass too much current from the NPN diodes and they don't do their jobs ( i must put a 200 ohms across it to drop it to 0.5V).

I thought that the compensation diode was part of the transistor dye, so i'm surprised with an error like that.

any suggestions ? Schotky diode in place of them ?

Look bad....


Denis

Last edited by DRZ1; 14th November 2012 at 10:49 PM.
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Old 14th November 2012, 11:25 PM   #1236
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by DRZ1 View Post
Well, it's worse than i thought. At lightly warm temp NPN diode = 0.67V and PNP = 0.52V.

If i want about 2.5V total with 5 diodes i have to bypass too much current from the NPN diodes and they don't do their jobs ( i must put a 200 ohms across it to drop it to 0.5V).

I thought that the compensation diode was part of the transistor dye, so i'm surprised with an error like that.

any suggestions ? Schotky diode in place of them ?

Look bad....


Denis
No reason not try bypassing two diodes (2npn, 2pnp, or one of each). Or you might bypass two and use an external series diode of lower drop (on the heatsink) to get the correct voltage drop. You can play quickly by leaving all the diodes unbypassed, and just shorting them in different combinations with clip leads. One suggestion: Avoid bypassing the center device in the row of three. It will run a little hotter than the outer two, so will provide a little more compensation.

Sheldon
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Old 14th November 2012, 11:53 PM   #1237
DRZ1 is offline DRZ1  Canada
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I have tried some of your suggestion, and it have under compensation if i use only 4 diodes.

I think the use of some external diodes will be the only solution.
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Old 15th November 2012, 03:16 AM   #1238
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by DRZ1 View Post
I have tried some of your suggestion, and it have under compensation if i use only 4 diodes.

I think the use of some external diodes will be the only solution.
The total voltage drop across the diode string, plus series pot, should be very close to 2.5V. As long as the series resistance required is lower than about 100R, then compensation should be adequate.

in my amp, the pot is set at about 50R. The diode string passes about 2mA, so that means the diodes drop about 2.4V total.

By my calculations you can get close to that with two NPN and two PNP diodes.

Sheldon
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Old 15th November 2012, 09:51 AM   #1239
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Quote:
Originally Posted by roender View Post
Too bad that ONsemi f..ked up some production lots of NJLs. I'm convinced that they put by mistake other type of diode inside which requires other compensation current in order to correct track the transistor Vbe over temperature.
I do not recommend to bypass all the diodes string with the pot. Just use what Andrew recommend, one resistor over the diode with way out Vf than the others.
When they were manufactured at the beginning, they did the same. It was good if I had read this, couple of weeks before. I would have dropped my RMI-FC100 project.

Now the thermaltrack order has already gone.
MAY GOD BLESS ME and I do not get these f.ke tranys.

Gajanan Phadte
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Old 15th November 2012, 09:52 AM   #1240
diyAudio Member
 
Join Date: Jul 2003
Location: Dona paula, Goa
Quote:
Originally Posted by roender View Post
Too bad that ONsemi f..ked up some production lots of NJLs. I'm convinced that they put by mistake other type of diode inside which requires other compensation current in order to correct track the transistor Vbe over temperature.
I do not recommend to bypass all the diodes string with the pot. Just use what Andrew recommend, one resistor over the diode with way out Vf than the others.
When they were manufactured at the beginning, they did the same. It was good if I had read this, couple of weeks before. I would have dropped my RMI-FC100 project.

Now the thermaltrack order has already gone.
MAY GOD BLESS ME and I do not get these f.ke tranys.
Gajanan Phadte
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