Intermittent DC Offset - Source or Amp?
Hello DIYers - I appreciate any and all help with my issue. Sorry, I am a newbie and it’s a long post...
I blew up a pair of speakers testing my "minimized" AudioSector LM3875 amp in the first few hours of its life and I am trying to figure out why. I checked and tested everything other than DC offset before hooking up my first pair of test speakers (8 ohm) and input source (a Microsoft Zune MP3 player working off battery only). I cycled the power at least 20 times before hearing the loud hum from the power supply and a crackle and hum in the speakers that resulted in blown speakers.
I posted my question on the commercial-gainclone-kit thread here:
http://www.diyaudio.com/forums/audi...it-building-instructions-165.html#post3483962
After reading info AndrewT replied with, I put the power supply in a box with good grounds, including a ground-break circuit and tested everything again. This time I also tested DC offset as explained in the commercial-gainclone-kit thread. DC offset as measured was 47/69. Once again the amp didn't send anything other than sweet sounds to the speakers through many on-off cycles before the horrible hum shot through attempting to foil my listening efforts for a second time. This time I had 4 ohm single driver car speakers hooked up and they survived the hum.
Could this problem have been caused by DC at the input from my Microsoft Zune MP3 player that was amplified and sent to the speakers? I have not used the Zune since the second time this has happened. I instead tried an NAD 1600 Tuner-Preamp and Denon CD player and they seem to be a good match for this amp. Nothing but sweet sounds so far, but I want to make sure I understand what happened to blow up my speakers before getting too comfortable with this amp.
Here's what I built:
The AudioSector board layout is here:
http://www.audiosector.com/images/lm3875_se_pcb.gif
I followed the instructions here:
http://www.diyaudio.com/forums/audi...cial-gainclone-kit-building-instructions.html
My power supply uses a 330va toroid with two 25v secondary windings into one rectifier board. I didn't have C1 and C2 installed when the first speakers blew, but I did have C1 and C2 installed the second time the hum shot through almost blowing up a second pair.
On the amp boards, I have R1 installed with the supplied 200 ohm resistor. In post #3, Peter Daniel mentions installing a small 4.7uF electrolytic cap in place of R1 to "protect the amp from DC that may be produced by a source component".
I did not install Rz or Cz as these components are not included in the kit. I understand these values should be Rz 2.7R and Cz 0.1uF if I choose to install them. I did a little reading about Zobel networks and it seems installing this won’t impact the sound and could also save my speakers.
I built a star ground on the amp boards using 14ga copper wire between the output grounds (OG) on each main board. I have power going through an umbilical to the power supply rectifier board. I tied PG+ and PG- together with a 14ga copper wire and used one wire in the umbilical from the rectifier ground to the star ground point in the amp. I have another ground wire in the amp going from the star ground point to the amp chassis.
So what do you think? Could it be the source injecting DC at the inputs, or is there some intermittent issue with my amp build that is waiting to destroy another pair of speakers?
Thanks for reading! -Glen
Hello DIYers - I appreciate any and all help with my issue. Sorry, I am a newbie and it’s a long post...
I blew up a pair of speakers testing my "minimized" AudioSector LM3875 amp in the first few hours of its life and I am trying to figure out why. I checked and tested everything other than DC offset before hooking up my first pair of test speakers (8 ohm) and input source (a Microsoft Zune MP3 player working off battery only). I cycled the power at least 20 times before hearing the loud hum from the power supply and a crackle and hum in the speakers that resulted in blown speakers.
I posted my question on the commercial-gainclone-kit thread here:
http://www.diyaudio.com/forums/audi...it-building-instructions-165.html#post3483962
After reading info AndrewT replied with, I put the power supply in a box with good grounds, including a ground-break circuit and tested everything again. This time I also tested DC offset as explained in the commercial-gainclone-kit thread. DC offset as measured was 47/69. Once again the amp didn't send anything other than sweet sounds to the speakers through many on-off cycles before the horrible hum shot through attempting to foil my listening efforts for a second time. This time I had 4 ohm single driver car speakers hooked up and they survived the hum.
Could this problem have been caused by DC at the input from my Microsoft Zune MP3 player that was amplified and sent to the speakers? I have not used the Zune since the second time this has happened. I instead tried an NAD 1600 Tuner-Preamp and Denon CD player and they seem to be a good match for this amp. Nothing but sweet sounds so far, but I want to make sure I understand what happened to blow up my speakers before getting too comfortable with this amp.
Here's what I built:
The AudioSector board layout is here:
http://www.audiosector.com/images/lm3875_se_pcb.gif
I followed the instructions here:
http://www.diyaudio.com/forums/audi...cial-gainclone-kit-building-instructions.html
My power supply uses a 330va toroid with two 25v secondary windings into one rectifier board. I didn't have C1 and C2 installed when the first speakers blew, but I did have C1 and C2 installed the second time the hum shot through almost blowing up a second pair.
On the amp boards, I have R1 installed with the supplied 200 ohm resistor. In post #3, Peter Daniel mentions installing a small 4.7uF electrolytic cap in place of R1 to "protect the amp from DC that may be produced by a source component".
I did not install Rz or Cz as these components are not included in the kit. I understand these values should be Rz 2.7R and Cz 0.1uF if I choose to install them. I did a little reading about Zobel networks and it seems installing this won’t impact the sound and could also save my speakers.
I built a star ground on the amp boards using 14ga copper wire between the output grounds (OG) on each main board. I have power going through an umbilical to the power supply rectifier board. I tied PG+ and PG- together with a 14ga copper wire and used one wire in the umbilical from the rectifier ground to the star ground point in the amp. I have another ground wire in the amp going from the star ground point to the amp chassis.
So what do you think? Could it be the source injecting DC at the inputs, or is there some intermittent issue with my amp build that is waiting to destroy another pair of speakers?
Thanks for reading! -Glen
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