crazy DC offset LM386 amp

Rembrant · 3rd January 2009, 08:13 AM

I built the MiniDisc.org headbanger amp about 2 years ago. This was my first amp and I still use it today for my PC. I decided to break out the meter and measure the DC offset.

Imagine my surprise when the meter read 4V! Yes FOUR VOLTS! From LO to OG and from LR to OG. WTF? This shouldn't be happening. The output has a 470uf cap on it.
This is the schematic.
http://www.minidisc.org/schem.gif

Does this value change when headphones are plugged in. Should I be measuring it with phones plugged in? I was just using a mini to mini connector and measuring from tip to barrel and ring to barrel.

I have had phones plugged into this thing and I haven't burnt up a set yet.

pacificblue · 3rd January 2009, 08:46 AM

That is absolutely normal, because the amplifier is working from a single power supply. The output must therefore be at half rail voltage ~4,5 V, when no signal is present, otherwise you would not get anything out of it. In your case the DC offset is the deviation from the exact half rail voltage.

Don't worry about your amp or your headphones. The blocking caps will not let DC currents pass through.

mjf · 3rd January 2009, 08:48 AM

when the headphones are plugged in the offset has gone i think.because there is no resistor at the output,so the cap has to be loaded with your headphones.
build in a resistor at the output(1kohm or so).......
regards...........

AndrewT · 3rd January 2009, 09:34 AM

Quote:

Originally posted by mjf
....build in a resistor at the output(1kohm or so)..................

10k or even 100k would add less loading to the chip output.

Rembrant · 3rd January 2009, 07:48 PM

Few, that eases that fear! It makes since too. Thanks guys. I did measure it with headphones plugged in and got zero V.

3rd January 2009, 08:13 AM	#1
Rembrant diyAudio Member Join Date: Mar 2007	crazy DC offset LM386 amp I built the MiniDisc.org headbanger amp about 2 years ago. This was my first amp and I still use it today for my PC. I decided to break out the meter and measure the DC offset. Imagine my surprise when the meter read 4V! Yes FOUR VOLTS! From LO to OG and from LR to OG. WTF? This shouldn't be happening. The output has a 470uf cap on it. This is the schematic. http://www.minidisc.org/schem.gif Does this value change when headphones are plugged in. Should I be measuring it with phones plugged in? I was just using a mini to mini connector and measuring from tip to barrel and ring to barrel. I have had phones plugged into this thing and I haven't burnt up a set yet.

3rd January 2009, 08:46 AM	#2
pacificblue diyAudio Member Join Date: May 2008	That is absolutely normal, because the amplifier is working from a single power supply. The output must therefore be at half rail voltage ~4,5 V, when no signal is present, otherwise you would not get anything out of it. In your case the DC offset is the deviation from the exact half rail voltage. Don't worry about your amp or your headphones. The blocking caps will not let DC currents pass through. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

3rd January 2009, 08:48 AM	#3
mjf diyAudio Member Join Date: Jan 2009 Location: upper austria/near linz	headamp when the headphones are plugged in the offset has gone i think.because there is no resistor at the output,so the cap has to be loaded with your headphones. build in a resistor at the output(1kohm or so)....... regards...........

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3rd January 2009, 07:48 PM	#5
Rembrant diyAudio Member Join Date: Mar 2007	Few, that eases that fear! It makes since too. Thanks guys. I did measure it with headphones plugged in and got zero V.

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