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Old 20th May 2013, 02:28 AM   #1
glenv6 is offline glenv6  United States
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Default Intermittent DC Offset? Source or Amp?

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:
Commercial Gainclone kit- building instructions

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:
Commercial Gainclone kit- building instructions

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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Last edited by glenv6; 20th May 2013 at 03:26 AM.
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Old 20th May 2013, 10:36 AM   #2
AndrewT is offline AndrewT  Scotland
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Have you wired the MAG to Chassis bridge rectifier correctly?
Have you kept the dual secondaries separate to feed the 8 rectifier diodes?
Are you using a 3pole power output socket, with +ve, Zero Volts and -ve?

Can I see a third wire entering the bolted PE connection?
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Old 20th May 2013, 10:38 AM   #3
AndrewT is offline AndrewT  Scotland
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Insert a temporary DC block between your Source and your Power Amplifier (both channels)
A 1uF would do to start with, 10uF would allow for low Zin of the Power Amplifier.
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Old 20th May 2013, 01:10 PM   #4
glenv6 is offline glenv6  United States
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Hi AndrewT,
Thanks for taking some time to comment! Sorry, but I am not familiar with some of the acronyms, but here are my answers...

Have you wired the MAG to Chassis bridge rectifier correctly?
*The Chassis bridge rectifier is wired correctly. Sorry, what does "MAG" mean?

Have you kept the dual secondaries separate to feed the 8 rectifier diodes?
*Yes. Two wires from each secondary go to the rectifier.

Are you using a 3pole power output socket, with +ve, Zero Volts and -ve?
*Are you referring to the XLR socket? That is a 5 pin. I am using one pin for +ve, one pin for -ve, one pin for Zero Volts, and the ground lug to connect the amp chassis to the physical ground lug on the power supply chassis. This assures that the ground between the amp chassis and power supply touches first. If you're asking about the power plug to mains power, yes, that has 3 pins.


Can I see a third wire entering the bolted PE connection?
*Sorry, not sure what you mean by "PE"?
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Old 20th May 2013, 01:14 PM   #5
AndrewT is offline AndrewT  Scotland
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PE = Protective Earth. That big bolted connection with the two green wires that are far too long.

MAG = Main Audio Ground. More often referred to as Star Ground.
I see yellow connections as well as green and yellow wires from adjacent terminals.
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Old 20th May 2013, 01:17 PM   #6
glenv6 is offline glenv6  United States
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Quote:
Originally Posted by AndrewT View Post
Insert a temporary DC block between your Source and your Power Amplifier (both channels)
A 1uF would do to start with, 10uF would allow for low Zin of the Power Amplifier.
Thanks again AndrewT! Question: Should this be an electrolytic cap? Also, since the problem is intermittent, how will I know that this has done anything?

BTW, I have to scrounge up some caps, so this will take a while.
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Old 20th May 2013, 01:20 PM   #7
AndrewT is offline AndrewT  Scotland
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electro will block.
Keep it in circuit for an extended test period.
If you can get a DC detect circuit to monitor the output with a big flashing LED or a Klaxon to tell you the DC is coming from before or after the blocker.
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Old 20th May 2013, 01:45 PM   #8
glenv6 is offline glenv6  United States
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Quote:
Originally Posted by AndrewT View Post
PE = Protective Earth. That big bolted connection with the two green wires that are far too long.

MAG = Main Audio Ground. More often referred to as Star Ground.
I see yellow connections as well as green and yellow wires from adjacent terminals.
I followed this article below and an article on Decibel Dungeon to build the ground circuit. I don't have any audible issues with the grounding, what is the effect of those wires being too long? They are currently about a couple of cm in length. The green wires go from the mains earth to the chassis ground lug, and the ground on the xlr connector to the ground lug.
Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

For MAG, I have one black wire going from the star ground in the amp to the PG point on the rectifier. That yellow wire goes from the PG point on the rectifier to the ground break circuit, as in the documentation I followed. From there it goes to the chassis ground lug.
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Old 20th May 2013, 01:48 PM   #9
glenv6 is offline glenv6  United States
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Quote:
Originally Posted by AndrewT View Post
electro will block.
Keep it in circuit for an extended test period.
If you can get a DC detect circuit to monitor the output with a big flashing LED or a Klaxon to tell you the DC is coming from before or after the blocker.
Okay, thanks...

So far you have only been concerned with grounding and input. Any comment on Rz, Cz? Should I add that Zobel?
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Old 31st May 2013, 04:07 AM   #10
glenv6 is offline glenv6  United States
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Default Update

Well, no update really other than to say I haven't changed the amp. It is working too well right now!

I built a pair of 72" high PVC Transmission Line speakers using Dayton Audio RS100-8 4" full range drivers and it all sounds really good together. I am still messing a bit with the baffle step correction on the speakers and I plan to add super tweeters soon.

I got a couple of replies to the pictures I posted on the chip amp picture forum suggesting I add bigger caps to the PSU. So I am going to try some caps offered to me to see if I like the sound.

So at this point, I don't have a problem with this amp. I think what happened was DC amplified from my Zune. Nothing but sweet sound with the NAD 1600 preamp hooked up, so that's what I'm going to use with this amp. Thanks again for the help on this forum.
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