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-   -   dc offset question (regarding LM3886 Chipamp kit) (http://www.diyaudio.com/forums/chip-amps/126394-dc-offset-question-regarding-lm3886-chipamp-kit.html)

prospero21 15th July 2008 02:42 AM

dc offset question (regarding LM3886 Chipamp kit)
 
Hey all,

I'm making my first little baby-steps on the path of diy audio building. As a safe start, I chose a 3886 kit from chipamp.com

The building process went very smoothly, I had some decent soldering lessons from a friend of mine. Assembling the amp in a case also went like a charm.

I think I made my first mistake right after that. Everything was hooked-up, inputs and outputs. So in my enthousiasm I fired up my Zero24 pre-amp and turned on some music... It sounded wonderful !! I was delighted beyond words, somewhat emotional even :P But then I remembered something... I should've checked the dc-offset on the outputs before connecting my wonderful Diamond 9.1's

So I grabbed my MM and starting measuring.. Then things got strange;

With the input connectors shorted (I used a rca-cable for this. Just made a loop from one input to the other using once channel. Is this correct ?) and no load on the speaker outputs, I measured 102mv on the left channel and 104 on the right ! :eek:

So I decided to connect my pre-amp again and then the output measured 27mv on the left and 48mv on the right...

Now I am all confused, since Ive read that the "correct" method of measuring dc-offset is with shorted input and no load on the output. How is it possible that I have these very high dc-offset figures using the first method, and very acceptable figures while using the pre-amp ?

Nuuk 15th July 2008 07:46 AM

Shorted input means connecting the signal and ground tabs of the input socket to each other NOT the left channel to the right.

Also measure the DC offset across a 10 ohm resistor.

So your amp would appear to be OK with typical DC offset for the LM3886 chip. ;)

AndrewT 15th July 2008 09:12 AM

Yes, it's quite possible to measure the DC output offset with the source connected and it probably measures different from shorted input.
But, what are you measuring?
The DC from the source component, or the output offset of the power amp, or the interaction of the power amp input with a non zero source impedance, or a combination of all three?

Build a few shorting plugs from RCA plugs.
A pair of zero ohm to start with.

You did build a light bulb tester for the first transformer "power on"?
And then used it again when the amp was powered up?
And it's waiting on the shelf for the next modification you plan to power up.

Finally,
measure the output offset in stages, shorted input, source connected but switched off, source connected but switched on, source connected and experimenting with on/off cycles.
When you are satisfied that there are no potentially damaging pulses/offsets, connect the speaker.

prospero21 15th July 2008 12:19 PM

Quote:

Originally posted by Nuuk
Shorted input means connecting the signal and ground tabs of the input socket to each other NOT the left channel to the right.

Also measure the DC offset across a 10 ohm resistor.

So your amp would appear to be OK with typical DC offset for the LM3886 chip. ;)

Ah ok, I misunderstood the directions I read on this forum then. It does seem more logical to short the signal and ground. Ive tried this, and the dc-offset went down to 10mv on the left channel and 41mv on the right channel.

Judging by what I read on this forum, those are indeed typical offset figures for this chip :)


Quote:

Originally posted by AndrewT
Yes, it's quite possible to measure the DC output offset with the source connected and it probably measures different from shorted input.
But, what are you measuring?
The DC from the source component, or the output offset of the power amp, or the interaction of the power amp input with a non zero source impedance, or a combination of all three?

Build a few shorting plugs from RCA plugs.
A pair of zero ohm to start with.

You did build a light bulb tester for the first transformer "power on"?
And then used it again when the amp was powered up?
And it's waiting on the shelf for the next modification you plan to power up.

Finally,
measure the output offset in stages, shorted input, source connected but switched off, source connected but switched on, source connected and experimenting with on/off cycles.
When you are satisfied that there are no potentially damaging pulses/offsets, connect the speaker.

To be honest, I read about the lightbulb tester _after_ i did all the assembling *shame*. But it seems like very good advice, so I'll start building one right away and keep it on the shelf for future amps (which are a certainty, since Ive seriously contracted the DIY virus ;) Currently building my first cmoy headamp)

One thing I noticed just a few minutes ago... My pre-amp has 2 functions, one is a "simple" dac which doesnt do anything "to" the signal in regards of attenuation. The other function is a pre-amp with attenuation, mainly used for the headamp output, but if no headphones are connected it lets met control the volume on the rca-outputs. When in the latter mode, the dc-offset on both channels is about 30mv... Is it a good thing that both channels have about the same offset ? In that case I'll just leave the pre-amp in this mode and be happy with it...


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