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Old 8th May 2011, 03:56 PM   #1
diyAudio Member
Join Date: May 2011
Default Leach amp: bias current


I am testing my leach amp (pavouk's njl version).

The test with the 100 Ohm resistors in the fuse holders was successful. I can measure about 1.45V on these resistors. And nothing started smoking...

But when i leave one fuse out, and measure the current trough fuse holder F2 i measure about 15mA.

Leach testing procedure says that you must adjust this current with P1, but when i am twisting P1, the current remains approximatly at 15mA.

This is measured with in and output disconnected.

Someone who can help me ?
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Old 8th May 2011, 07:04 PM   #2
djoffe is offline djoffe  United States
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Join Date: Oct 2008
Default The difference is the bias diodes

Looking for a common datapoint to compare bias point...can only find 0.1 Amps. It's not ideal, but it still indicates the nature of the problem.

Original amp uses 1N4004 diodes, which have 0.78V across with 0.1 Amp through

Your version uses the diodes from the thermal trak device, NJL3281, which has 0.67 Volts with 0.1 Amp through it.

Multiplying by 4 diodes, the difference would approach 0.48 Volts. That's enough so that the output transistors will never really turn on, thus your bias current never starts to draw on the output devices.

Now, the difference probably isn't quite this big at the actual operating current, but I think this is the heart of the matter. You'll have to change diodes or tweak the bias circuit resistor values.

Hope this helps...
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Old 8th May 2011, 08:15 PM   #3
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Join Date: May 2011
Thanks for your help djoffe.

I understand the bias problem, but i do not understand why pavouk would not have this problem?

I must say that i have used the resistors from version 4.6.
As example:
R34, R35: leach 4.3 = 270 Ohm
pavouk = 330 Ohm.

I have used the 270 Ohm resistors, maybe this is my fault ?

With R15,R16,R13, and R14 i did the same.
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Old 8th May 2011, 10:16 PM   #4
djoffe is offline djoffe  United States
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Join Date: Oct 2008
Default Critical issue...

I assume that we're both looking at the same schematic...there are two on your link...I haven't checked part for part that they are the same...with that caveat...

Set up a DC meter across C12, or any equivalent place. Do it with the power off, in some good way that won't slip off and destroy things. Power up with no signal and no load. What is the voltage across C12? There are 6 junctions in the biasing loop, so you'll need roughly 6*0.6=3.6 volts across C12 to turn on the output transistors. This is kind of approximate, but if you have just 3 V, you're probably out of luck.

To chase it further, you can carefully measure the voltage drop across the 4 diodes in the bias loop, to see if they make sense.

Finally, Could it be that the POT has opened up?
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Old 9th May 2011, 06:12 AM   #5
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Thanks again djoffe.
I measure 3,102V across C12.

On D7: 3,02V
On D8: 2,4V
On D5: 15mV
On D6: 15mV

I guess the voltage on D5 and D6 are to low ?

What do you mean with " Could it be that the POT has opened up" ?.
The pot is set to max. resistance, i measure approximately 1.8 k across the pins of it.
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Old 25th May 2011, 06:10 PM   #6
Claudiu is offline Claudiu  Romania
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Join Date: Jan 2006
Location: Cluj-n
change r27 from 1..1,2kohm in 330ohm and it will work
I have NJL0302 and NJL1281 in power stage
Have a good day
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