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Old 14th October 2004, 11:30 PM   #1
audioPT is offline audioPT  Portugal
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Default :: Lm3875 Gc Problem ::

Hi!

I've made 10 LM3875 GC and all of them have the same problems:

1- 250mV DC at output with load and 300mV DC without load.
2- When I turn off the power and after the caps discharge (10.000uF / rail) I have no 'bump' but a sweep at high frequencies (puiiiii).

Only 5 of them are connected at the same time (AV amp). I've tryed one at each time, and the result is the same.

Is this normal??

I have the attached config

Regards

Pedro Martins
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Old 14th October 2004, 11:34 PM   #2
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The problem is with your 100k shunt resistor at the input. The maximum shunt resistance shouldn't be more than the value of feedback resistor, in that case 22k. The lower the value of shunt resistor, the less offset.
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Old 14th October 2004, 11:43 PM   #3
audioPT is offline audioPT  Portugal
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Thanks PD.

I'll try that
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Old 15th October 2004, 01:11 AM   #4
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Hi Pedro!

Yes, use a 22K shunt resistor on the input, and also change the input cap for 2.2uf.
Use drain resistors after the big PSU caps.

When do you come to Lisbon, man?
I've got some things to show you.
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Old 15th October 2004, 08:49 AM   #5
audioPT is offline audioPT  Portugal
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Hi Carlos

Tell me, the drain resistors are...??

I attach the thing I think it is.

And Lisbon? I've been there last thursday with my wife, but with very hurry... I've told her that we will go meet you . And before Christmas
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Old 15th October 2004, 11:34 AM   #6
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Quote:
Originally posted by audioPT
I attach the thing I think it is.
That's it.
1k resistors should do the job.
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Old 15th October 2004, 01:50 PM   #7
audioPT is offline audioPT  Portugal
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Following Ohm's Law, if I have 30 V and a 1 k resistor means:

I = 30 V / 1000 Ohm = 0.03 A = 30 mA
P = 30 V * 0.03 A = 0.9W

So, the resistor must be 1kOhm / 2W at least, right?
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Old 15th October 2004, 02:03 PM   #8
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Yes, 2W is fine.
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