:: Lm3875 Gc Problem ::

audioPT · 14th October 2004, 11:30 PM

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

Peter Daniel · 14th October 2004, 11:34 PM

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.

audioPT · 14th October 2004, 11:43 PM

Thanks PD.

I'll try that

carlosfm · 15th October 2004, 01:11 AM

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.

audioPT · 15th October 2004, 08:49 AM

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

carlosfm · 15th October 2004, 11:34 AM

Quote:

Originally posted by audioPT
I attach the thing I think it is.

That's it.
1k resistors should do the job.

audioPT · 15th October 2004, 01:50 PM

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?

carlosfm · 15th October 2004, 02:03 PM

Yes, 2W is fine.

14th October 2004, 11:34 PM	#2
Peter Daniel diyAudio Member Join Date: Jul 2002 Location: Toronto, Canada	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. __________________ www.audiosector.com “Do something really well. See how much time it takes. It might be a product, a work of art, who knows? Then give it away cheaply, just because you feel that it should not cost so much, even if it took a lot of time and expensive materials to make it.” - JC

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14th October 2004, 11:43 PM	#3
audioPT diyAudio Member Join Date: Aug 2002 Location: Right here	Thanks PD. I'll try that

15th October 2004, 01:11 AM	#4
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	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.

15th October 2004, 01:50 PM	#7
audioPT diyAudio Member Join Date: Aug 2002 Location: Right here	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?

15th October 2004, 02:03 PM	#8
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	Yes, 2W is fine.

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