DC offset and gain on GC LM3886 - diyAudio
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Old 22nd January 2012, 09:47 PM   #1
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Default DC offset and gain on GC LM3886

first of all i must admit i am new in the DIY thing (but i love it already). With to much time on my hand i decided to build a GC with the LM3886 dual mono kit from chip amp.com.
because of my little problem with concentrating reading is not a hobby an because of my trembling hands soldering is not either.....

But i managed to put it al together you can see the pics here..https://picasaweb.google.com/Xerxes75/GaincloneLM3886

i must say a few tears where there when it turned on.....it worked. i do however have slight problem.
the DC offset with the volume down is one side -32mV and on the other side -22mV? This DC thing is new for me so if anybody can turn the light on for me..please.

and then there is the massive volume! it plays with the volume slightly on very, VERY loud!. (turn down the volume on my iMac to play at a sensible volume)

the DC offset with the volume complete on is off the scale...

besides the kit there are two 225 2x25v toroids (gives 27.1v) on the rail i measure 37.8v (both sides) the source is my iMac with hrtII streamer.
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Old 22nd January 2012, 10:03 PM   #2
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Recheck that you have all components in the right places and that the resistors are values that match the schematics (that I assume were sent with the kit).

If you don't want it so loud you can do one of two things. Put a log scale potentiometer on the input as a volume knob, or reduce the amp's gain by changing one or both of the resistor values shown on the schematic between the chip's output and its inverting input, they form a voltage divider that determines gain. This is detailed in the LM3886 datasheet linked below.

Do you have any DC offset when the audio source (iMac?) is not plugged in? Measure source output DC offset. The kits do not appear to include AC coupling (DC blocking) capacitors on the audio input which would remove DC offset before it is multiplied by the (chipamp) gain stage.

On the LM3886 datasheet you can see an example of placing a POT on the input in the typical application schematic, pg 1., and a 10uF coupling capacitor in the single supply application circuit pg 6.

http://www.ti.com/lit/ds/symlink/lm3886.pdf
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Old 22nd January 2012, 10:40 PM   #3
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Sorry i forgot to write that i did THE measurement without speakers or input.
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Old 22nd January 2012, 10:50 PM   #4
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The 10uf coupling cap sounds good, but i am planning passive preamp with stepped attenuator is this cap still needed?
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Old 22nd January 2012, 11:27 PM   #5
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Quote:
Originally Posted by Xerxes75 View Post
The 10uf coupling cap sounds good, but i am planning passive preamp with stepped attenuator is this cap still needed?
I would leave it in!!! just to be sure no dc voltage gets to the inputs especially if you are using a passive preamp.
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Old 23rd January 2012, 01:40 AM   #6
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A passive preamp, if we are thinking about the same thing, will have no way to correct DC offset so if the music source outputs DC offset significant enough that the (power amp gain) * (DC offset) = (too high a DC offset to speakers), then you need an AC coupling cap "somewhere" in the audio chain.

Some people would build as if two subsystems are forever married together and only use one cap/channel, while others would treat the separate stages as separate components which might be used without each other and put a coupling cap on both of them. Generally speaking, since the higher the gain is the more a DC offset will matter, it is safer /more compatible to put the cap at the beginning of the (sub)circuit for your last, highest gain stage in the audio path from source to speaker.

Otherwise, I have not built that kit and don't have the schematic for it, consider contacting chipamp.com

It's sort of a shame chipamp.com doesn't provide enough support to even show good pics of the PCB or links to manuals. Are they THAT worried someone is going to copy their design or something? It's not as though there aren't plenty of other people sharing different but viable designs that if someone were included to use, would be a person wanting something other than a kit to build.

Last edited by !; 23rd January 2012 at 01:43 AM.
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Old 23rd January 2012, 08:31 AM   #7
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There is a pdf article explaining the project in detail. They are using the default schematic on the national semiconductor website with few modifications.

DC offset upto 100mV positive / negative is fine.

Advice:

1. Use a 20K log potentiometer / stepped attenuator.

2. Use DC blocking input capacitors 1 microfarad and above polypropylenes will do.

3. Add some speaker protection / DC protection circuit. Cost $10 to protect those beautiful bookshelves.
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Old 23rd January 2012, 09:14 AM   #8
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Thanks for your reply. I must admit the support and THE manual could be better. But since i have the kit already thats passend station for me. I am planning second kit and thinking about different manufacturer audiosector came to mind.

In THE manual http://chipamp.com/docs/lm3886-manual.pdf they are speaking of placing the feedback cap and placing THE resistor R3 differently.

Is this the Same principal as the 10uf cap?

About THE passive preamp, i want to keep THE amp part seperatly from it so i indeed am free to move about with the amps.
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Old 23rd January 2012, 09:43 AM   #9
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I think i got the picture, can i place this coupling cap directly on the input or are There better ways to apply this.

And yes i would be shame to dammage my Jamo Concert 8's. But most protectionkits i've seen are huge. Do you knowhow simple circuit i can build?
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Old 23rd January 2012, 10:28 AM   #10
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Can you tell us which is the exact circuit for your amp?

At least that way we can know what might be the problem, and not just guess it.

From the photos I can see you already have a motorized pot (value?), so that is half a passive-preamp. The other half is the input switch.

Another question: the 3886 is totally isolated from the heatsink? It should be.
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