First GC Amp .... something wrong?

sl_1800 · 23rd December 2004, 10:34 AM

I have built my first GC amp using Brian's boards. I measured for DC offset with not load resistor and measured a negative 110mv. How can I be getting a negative reading? And yes I'm sure I had the meater hooked up correctly and the outputs on the board are correct. Is there really a problem or should I check with a load resistor? All other voltages are correct.

I was pleased when I pluged it in and got no smoke and no flames but I really am confused on the negative reading on the DC offset.

jaudio · 23rd December 2004, 10:43 AM

what are the measurement for supply rails. check solder joints

kafka · 23rd December 2004, 10:52 AM

I really don't understand Your confusion... If You can have positive DC offset You can have negative as well, cos' the power supply is symetrical.

jaudio · 23rd December 2004, 11:07 AM

kafka right but 110mv is kinda high

peranders · 23rd December 2004, 11:21 AM

If you calculate what you _can_ get this is not extremely high.

Let's say that you have between offset voltage (according to datasheet) 1-10 mV and assume that the input bias currents are small in relation with chosen resistor values.

The gain is 30 => offset 30-300 mV!

sl_1800 · 23rd December 2004, 03:53 PM

Ok, it this negative DC offset is not out of the question then how do I go about reducing it from the 110mV to something a little closer to 0 ???

I have heard about a Ci cap which would be between Ri and ground, correct? If so then I'm assuming I would have to pull the ground end of Ri and insert a non polar cap inbetween ??

Also will a different value resistor for Ri have a effect ?

Thanks for the help.

boholm · 23rd December 2004, 08:14 PM

It can have something to do with your (?) choice of values of the resistors. Can you please post a schematic with resistor values in it?

sl_1800 · 23rd December 2004, 08:20 PM

The values used are the ones that come with the BrianGT standard LM 3875 kit.

boholm · 23rd December 2004, 08:37 PM

If I am looking at the right schematic - and I am not very familiar with BrianGTs products - there is a 220 ohm resistor between input and pin 7 of the LM3875.

This is the resistor to experiment with. In theory it should be 601 ohms ( 620 // 20k ). You may try with a trimmer (1 kohm) if you got one.

sl_1800 · 23rd December 2004, 09:44 PM

I can give that a try, but won't that change the gain of the amp or not?

23rd December 2004, 10:34 AM	#1
sl_1800 diyAudio Member Join Date: Dec 2004 Location: Altus, OK	First GC Amp .... something wrong? I have built my first GC amp using Brian's boards. I measured for DC offset with not load resistor and measured a negative 110mv. How can I be getting a negative reading? And yes I'm sure I had the meater hooked up correctly and the outputs on the board are correct. Is there really a problem or should I check with a load resistor? All other voltages are correct. I was pleased when I pluged it in and got no smoke and no flames but I really am confused on the negative reading on the DC offset.

23rd December 2004, 11:21 AM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	If you calculate what you _can_ get this is not extremely high. Let's say that you have between offset voltage (according to datasheet) 1-10 mV and assume that the input bias currents are small in relation with chosen resistor values. The gain is 30 => offset 30-300 mV! __________________ /Per-Anders (my first name) or P-A as my friends call me

23rd December 2004, 08:14 PM	#7
boholm diyAudio Member Join Date: Apr 2004 Location: Copenhagen	It can have something to do with your (?) choice of values of the resistors. Can you please post a schematic with resistor values in it? __________________ Best regards Bo

23rd December 2004, 08:37 PM	#9
boholm diyAudio Member Join Date: Apr 2004 Location: Copenhagen	If I am looking at the right schematic - and I am not very familiar with BrianGTs products - there is a 220 ohm resistor between input and pin 7 of the LM3875. This is the resistor to experiment with. In theory it should be 601 ohms ( 620 // 20k ). You may try with a trimmer (1 kohm) if you got one. __________________ Best regards Bo

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23rd December 2004, 10:43 AM	#2
jaudio diyAudio Member Join Date: Oct 2004	what are the measurement for supply rails. check solder joints

23rd December 2004, 10:52 AM	#3
kafka diyAudio Member Join Date: Jun 2004 Location: Lublin	I really don't understand Your confusion... If You can have positive DC offset You can have negative as well, cos' the power supply is symetrical.

23rd December 2004, 11:07 AM	#4
jaudio diyAudio Member Join Date: Oct 2004	kafka right but 110mv is kinda high

23rd December 2004, 03:53 PM	#6
sl_1800 diyAudio Member Join Date: Dec 2004 Location: Altus, OK	Ok, it this negative DC offset is not out of the question then how do I go about reducing it from the 110mV to something a little closer to 0 ??? I have heard about a Ci cap which would be between Ri and ground, correct? If so then I'm assuming I would have to pull the ground end of Ri and insert a non polar cap inbetween ?? Also will a different value resistor for Ri have a effect ? Thanks for the help.

23rd December 2004, 08:20 PM	#8
sl_1800 diyAudio Member Join Date: Dec 2004 Location: Altus, OK	The values used are the ones that come with the BrianGT standard LM 3875 kit.

23rd December 2004, 09:44 PM	#10
sl_1800 diyAudio Member Join Date: Dec 2004 Location: Altus, OK	I can give that a try, but won't that change the gain of the amp or not?

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