DC offset question

Nicks · 29th November 2005, 08:34 PM

i just connected my P2P chipamp. Its an LM3886TF.

So i thought it would be wise to measure before connecting it to any speakers.

So when i first measured i got 20.2 VDC on the output!!!

Then i shorted the input and got 0.5VDC on the output.

Why is it like this?

it might be a stupid question, but i ask since i dont know

Best regards

Nicks

JonHarrison · 29th November 2005, 08:41 PM

Nicks,

What do you measure AC on the output ?

Jon

Nicks · 29th November 2005, 08:47 PM

No, i measure DC at the output.

I think its supposed to come out clean and nice AC in the form of music from the output.

jaudio · 29th November 2005, 09:00 PM

Check the grounding. Check the grounding in the power supply.

boholm · 29th November 2005, 09:22 PM

Please post the schematics. the one, that you have build your GainClone after, where *all* the components are showed - also the "secondary" (and not so important) ones.

If you do not have a resistor from + input to ground, then you will have such problems. The inputs needs a reference, when nothing is connected. When connecting a cd-player or a pre-amp, this will provide a reference.

peranders · 29th November 2005, 09:23 PM

The problem is two:

1) From each input the IC must "see" the same DC resistance (to signal ground, = real ground or output) if the value is "big" compared to the input bias currents. A large value is > 1 kohms (most likely).

2) If the DC gain is equal the AC gain the output offset will get the gain times more. If you have the gain set at 30 the offset will be 30 times higher which is 1-10 mV (according to datasheet) times 30 = 30-300 mV. On top of this you have input offset bias current also.

The input must ALWAYS be tied to ground via a resistor.

Have you checked the datasheet and application note AN-1192 carefully? If not do that because there you have almost everything you ought to know.

Nicks · 30th November 2005, 09:08 AM

Thanks gentlemen,

I have made a slight error in my thinking.

Im building a BPA 200 but with 3 chips in parallell.

Then i use rod elliots bridge circuit instead of building 3 inverted and 3 non inverted.

So the input resistor is before the bridge circuit.

I only measured one chip without the bridge circuit.

I will measure again with the input resistor.

best regards

Nicks

peranders · 30th November 2005, 10:38 AM

This was valuable information the you left out.

We are not mind readers you know.

tiltedhalo · 30th November 2005, 03:29 PM

Input has to have a referanced to ground 1k input 22k to ground on each chip in parrellel. Make sure your inverted non inverted driver circuit is not introducing DC just one mv will be multiplied by the gain limits set by the amp.

Nicks · 30th November 2005, 04:48 PM

Sorry guys...

i was so focused on the dc offset that i forgot the "golden rule" i.e present the WHOLE situation... not just the glitch...

THANKS anyway, this forum is really great!

Best regards

Nicks

29th November 2005, 08:34 PM	#1
Nicks diyAudio Member Join Date: Jan 2005 Location: South of Sweden	DC offset question i just connected my P2P chipamp. Its an LM3886TF. So i thought it would be wise to measure before connecting it to any speakers. So when i first measured i got 20.2 VDC on the output!!! Then i shorted the input and got 0.5VDC on the output. Why is it like this? it might be a stupid question, but i ask since i dont know Best regards Nicks __________________ It's so darn fun with HUGE machines!!!

29th November 2005, 08:47 PM	#3
Nicks diyAudio Member Join Date: Jan 2005 Location: South of Sweden	No, i measure DC at the output. I think its supposed to come out clean and nice AC in the form of music from the output. __________________ It's so darn fun with HUGE machines!!!

29th November 2005, 09:22 PM	#5
boholm diyAudio Member Join Date: Apr 2004 Location: Copenhagen	Please post the schematics. the one, that you have build your GainClone after, where all the components are showed - also the "secondary" (and not so important) ones. If you do not have a resistor from + input to ground, then you will have such problems. The inputs needs a reference, when nothing is connected. When connecting a cd-player or a pre-amp, this will provide a reference. __________________ Best regards Bo

29th November 2005, 09:23 PM	#6
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	The problem is two: 1) From each input the IC must "see" the same DC resistance (to signal ground, = real ground or output) if the value is "big" compared to the input bias currents. A large value is > 1 kohms (most likely). 2) If the DC gain is equal the AC gain the output offset will get the gain times more. If you have the gain set at 30 the offset will be 30 times higher which is 1-10 mV (according to datasheet) times 30 = 30-300 mV. On top of this you have input offset bias current also. The input must ALWAYS be tied to ground via a resistor. Have you checked the datasheet and application note AN-1192 carefully? If not do that because there you have almost everything you ought to know. __________________ /Per-Anders (my first name) or P-A as my friends call me

30th November 2005, 09:08 AM	#7
Nicks diyAudio Member Join Date: Jan 2005 Location: South of Sweden	Thanks gentlemen, I have made a slight error in my thinking. Im building a BPA 200 but with 3 chips in parallell. Then i use rod elliots bridge circuit instead of building 3 inverted and 3 non inverted. So the input resistor is before the bridge circuit. I only measured one chip without the bridge circuit. I will measure again with the input resistor. best regards Nicks __________________ It's so darn fun with HUGE machines!!!


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29th November 2005, 08:41 PM	#2
JonHarrison diyAudio Member Join Date: Mar 2004 Location: UK	Nicks, What do you measure AC on the output ? Jon

29th November 2005, 09:00 PM	#4
jaudio diyAudio Member Join Date: Oct 2004	Check the grounding. Check the grounding in the power supply.

30th November 2005, 10:38 AM	#8
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	This was valuable information the you left out. We are not mind readers you know. __________________ /Per-Anders (my first name) or P-A as my friends call me

30th November 2005, 03:29 PM	#9
tiltedhalo diyAudio Member Join Date: Sep 2005 Location: Melbourne Florida	DC offset Input has to have a referanced to ground 1k input 22k to ground on each chip in parrellel. Make sure your inverted non inverted driver circuit is not introducing DC just one mv will be multiplied by the gain limits set by the amp.

30th November 2005, 04:48 PM	#10
Nicks diyAudio Member Join Date: Jan 2005 Location: South of Sweden	Sorry guys... i was so focused on the dc offset that i forgot the "golden rule" i.e present the WHOLE situation... not just the glitch... THANKS anyway, this forum is really great! Best regards Nicks __________________ It's so darn fun with HUGE machines!!!

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