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Old 1st February 2011, 03:53 AM   #1
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Default DC offset 3875?

Hi everyone. I built a LM3875 a while back and was amazed with the result. I used Peter Daniels basic kit and followed the guide. The amp was arranged as described below.:

Star gounding throughout.
Seperate psu (mains switch, Fuse's, safety earth and 270va traffo)
AC in the umbilical (rectifier in the amp chassis)
Half bridge using jumpers and 22-0-22 CT. (10UF FROM KIT)

This worked perfectly for me. I did not test anything as it was noiseless even at full tilt no input was v.quiet.
Only concern was reliability as i rushed the final bits. So i decided to re-solder / wire and generally tidy the build up (already good). This i did to a visably higher standard and was convinced of improvements all round.

While doing this i decided to build a big cap rectifier board using carlosfm schematic with snubbers. this i installed in the psu box this time allong with a new umbilical for the dc voltage. other than renewed wire this is the only mod. upon testing the PSU i had a stable 29v+ and 29v- ref to ground (CT / safety).

upon hooking up to the amp i did some testing. Like i sid before i didnt test the original so only have these figures but something is wrong.

DC offset 6ohm load each side. No input
L ch. 184mv!
R ch. 40mv

The dc is audible at "tick over" on the left and the amp is no where near as quiet. The sound has changed as i expected for the snuubber psu, tighter bass with no loss in the mids. I used ELNA 4700ufs on psu and agree with most people when they say the sibilance is a bit harsh.

To sum up im pleased with the rectifier mod but cant understand the dc figures and im at the limits of my knowledge so far. What could cause this. i dont know whether just to go bck to the original or what now... Help
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Old 1st February 2011, 05:49 AM   #2
jaycee is offline jaycee  United Kingdom
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If there is any flux around the joints, clean it off.
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Old 1st February 2011, 06:09 AM   #3
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Maybe its grounding related. You say you used star grounding throughout, so that would mean the star has to be in the PSU box - you've got a centre tap there right?
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Old 1st February 2011, 09:24 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by tom wesley View Post
DC offset 6ohm load each side. No input
L ch. 184mv!
R ch. 40mv
the amplifier output offset is measured with the input shorted to signal ground and with the output open circuit.
If adding the source (tested with it both ON and OFF) changes the output offset, then DC is getting from the source to the amplifier.
Get this sorted first before you attach a dummy load and recheck before you attach speakers.
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Old 1st February 2011, 02:24 PM   #5
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Quote:
Originally Posted by abraxalito View Post
Maybe its grounding related. You say you used star grounding throughout, so that would mean the star has to be in the PSU box - you've got a centre tap there right?
On original the rectifier was in amp chassis so i had a mains safety earth at the inlet on psu chassis. i connected this to the CT with pirelli earth cable. from here i also sent a slightly smaller earth cable in the umbilical to the amp chassis star where i had both OG's and CHG's from boards and a pair going to the rectifier. Like i said previously this was more than acceptable to my ears. I own 23 amps and this sounded best so cant be wrong can it? I realise i have created a ground loop of sorts but wanted to be safe and based it on a sansui board which had alogical "star of Stars" system.
#

The only difference is the rectifier is now in traffo box so initially i just moved erything "back" to the psu and kept the grounding estianilly the same. The closer proximity to the traffo of the rectifier could obviously cause increased noise but dosnt explain the dc at output. which certinly wsnt there before.

I will check all solder joints today.

Thanks Andrew for the test method but i have ruled out the input (i think) as the dc noise is still present with preamp unpluged (pwr and signal)

Im starting to think a v.small short somewhere.

Could a damaged component cause this?

One other observation is that with the new rectifier it runs alot cooler is this the norm???
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Old 1st February 2011, 04:25 PM   #6
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Hi,
You can do a check to see if the noise is coming from earth ground. Remove it at the amplifier chassis and see what happened. Sometime the earth ground may introduces an unwanted noises.

Tauro0221
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Old 1st February 2011, 04:30 PM   #7
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Thank you for inviting me to your thread Tom. I'm not sure I can be of any help but I will try some logical exercises and perhaps something can be gained from the effort.
I do mostly amp repair, so if I sound layman about anything please consider my background, and field of experience.

As I gather the DC problem was only diagnosed after the mods you installed, so anything before that is just a guess This throws a big question mark into the picture.
I am not suggesting that you undo any of your mods at this point although they may be clouding our vision of this symptoms and issues.

After you have completed all of the other suggestions from the other posters please post your findings so we can all be on the same page together.

Just jumping in like I am I feel that a AB comparison of your current chip and a replacement chip might glean more conclusive comparison info. To justify my comment, if two individual different chips perform similarly then I think you have a circuit layout design issue.
I realize this may not be a economical test method but it might also rule out a simple defect in the silicon and point us in the correct direction of the problem.
As for the noise issue that appeared after your mod well I am guessing that something is a miss in the power supply additions and the signal is being injected from that source. A scope would be helpful to track this. Again this is an assumption since the issue was not present before the mod, and it is just more clouds of the vision of what is really happening here.

A link to all of the add on mods would be helpful so I can actually see their designs. I have seen simple filter mods create switching / pulsating power supply noise. The chip is supposed to have very good PSU noise rejection figures but if the signal is large enough and gross enough no amount of PSU noise rejection will be of any importance.

Things happen in any project and the best efforts of us all have folder over on any of us at any given time. To error is to be human, and I am suspect of chip damage first case, and I would try to rule this out by simple AB swap out and test.
If this is inconclusive then I am highly suspect of all the rest.
Most all of these Silicon one chip amp designs can be damaged easily by static and or irregular voltage and heat issues.
And a economical O-scope would be a very handy tool right now to visualize what is going on in your project.
In my experience DC offset is related to damaged Silicon, failed or damaged components of one sort or another as most of my work deals with pre-designed works of art of others where simple problems have been engineered out by re-design and rework. Weather or not this project had issues up front is still a unknown, so their will be a lot of speculation and experimentation to resolve all the ghosts that are present currently... I sincerely hope some of my input with be helpful...Cheers...C

PS I have seen chips oscillate after being damaged and this also causes noise to the speakers regardless of the power supply design.
Also if after this project has been cleared of defects and the like their is still a un-acceptable DC offset, I would like to suggest a simple two resistor and pot voltage divider source be added across the rails to supply a low level Dc source to the input of the chip so any residual minimal DC offset can be canceled out by simple DC injection to the input of the amp chip. Lots of amp designers do this and it works on simple DC offset issues that do not fluctuate with input drive signal levels.

Last edited by 1moreamp; 1st February 2011 at 04:35 PM. Reason: spelling
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Old 1st February 2011, 04:45 PM   #8
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Quote:
Originally Posted by tom wesley View Post
Like i sid before i didnt test the original so only have these figures but something is wrong.

DC offset 6ohm load each side. No input
L ch. 184mv!
R ch. 40mv

The dc is audible at "tick over" on the left and the amp is no where near as quiet. The sound has changed as i expected for the snuubber psu, tighter bass with no loss in the mids. I used ELNA 4700ufs on psu and agree with most people when they say the sibilance is a bit harsh.

To sum up im pleased with the rectifier mod but cant understand the dc figures and im at the limits of my knowledge so far. What could cause this. i dont know whether just to go bck to the original or what now... Help
As he said, he didn't measure the original, and offsets like that are still possible, although very rare.

What I suggest, is to measure offsets again, with input shorted to ground. They should be much lower.

You can also check each board separately, by disconnecting the other one. If none of the boards produce noise by itself, it means you have ground wires problem.

Some pics would be helpful too.
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Old 2nd February 2011, 04:55 PM   #9
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BUMP, any updates today ?
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Old 3rd February 2011, 12:38 AM   #10
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Thanks Guys and sorry for the late response to your excellent advice. I got laid off work so have had to go job hunting.

Update on amp. I will post pics asap. I still have a stock rectifier board from the original so im going to try that in the new posision, i.e. in the psu. This could help rule that out. I have thoughrly checked the amp boards for bad joints etc and they seem good. Continuity also good. More tomorow in more detail. Thanks again
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