Lamm LL2.1 DC Offset problem - diyAudio
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Old 22nd March 2014, 11:53 AM   #1
gino22 is offline gino22  Indonesia
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Unhappy Lamm LL2.1 DC Offset problem

I need help to solve problem with Lamm LL2.1 preamp.

Not sure whether this is a DC offset problem, but the outputs used to produced a high DCmV fluctuation, exceeding +-300mV.

I replaced the output caps and coupling in both channel (Vishay MKP1841 2.2uF 400V & Electrocube 0.033uF 100V), resulting a good Left Channel measurement and steady at 0.7mV. However the Right Channel remain in problem, it still produces a fluctuative DCmV exceeding +-30mV and causing speakers woofer moving back and forth.

Appreciate your help and thought.
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Old 22nd March 2014, 01:59 PM   #2
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Wow a Lamm,
you must be a rich man.
If its not the output-caps maybe there is an issue with the pulldown-resistor after the caps.
Perhaps it will be usefull to see a schematic?

Hilmar
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Old 22nd March 2014, 02:08 PM   #3
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This looks like a very simple and straightforward design, meaning that as long as the output caps are ok, there is just no dc offset. Could it be the replacement caps are also defective?
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Old 22nd March 2014, 04:26 PM   #4
gino22 is offline gino22  Indonesia
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Dear Hilmar,

Thanks for the advice. Absolutetly Iam not rich man . It is a sad story, I spend years doing saving for this Stereophile's highly recomended unit, yet I have to strugle with the problem since I bought this brand new unit a year ago.
I checked all resistors in both signal path, and measurements showed all in good condition. Perhaps I need to double check, particularly the pull-down resistors in both input and ouput caps.
Unfortunately, I dont have the schematic, have tried to get it from Lamm but no reply. Let me see whether I can decode it and share with you.
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Old 22nd March 2014, 04:35 PM   #5
gino22 is offline gino22  Indonesia
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Quote:
Originally Posted by analog_sa View Post
This looks like a very simple and straightforward design, meaning that as long as the output caps are ok, there is just no dc offset. Could it be the replacement caps are also defective?
Dear Analog_sa,

Thanks for your advice. I replaced the right channel output caps twice, so this is the second replacement, unlike the left channel.

however, I still have 2 spares, perhaps it is worth to retry.
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Old 22nd March 2014, 08:29 PM   #6
DF96 is offline DF96  England
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Wandering DC can be caused by mains voltage fluctuations, poor LF stability (verge of motorboating) and some component being on the edge of breaking.

A common modern problem is coupling caps which are too large so the amp LF rolloff is below the HF rolloff of the PSU filtering. This leaves a band where voltage variations come straight through into the output.
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Old 22nd March 2014, 09:55 PM   #7
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Hi,

Quote:
A common modern problem is coupling caps which are too large so the amp LF rolloff is below the HF rolloff of the PSU filtering. This leaves a band where voltage variations come straight through into the output.
A lot of wisdom in there.

If DC blocking caps break down repeatedly then something is likely to be wrong with the voltage at their input.
So check the DC voltage at the input and further upstream. It may be a faulty tube as well so check those too by swapping them L to R channel and so forth.

In the mean time I'd suggest you disconnect the amps/speakers as the DC current may end up toasting the speaker coils.

Ciao,
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Old 22nd March 2014, 10:04 PM   #8
kevinkr is offline kevinkr  United States
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Could also be a bad tube.. The Lamm gear has a reputation for being very well built with non gimmicky parts..
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Old 22nd March 2014, 10:13 PM   #9
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Hi,

Quote:
being very well built with non gimmicky parts..
Some of those parts I do not particularly like. Good on paper but soundwise not very convincing if you'd ask me.

Ciao,
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Old 23rd March 2014, 03:58 AM   #10
gino22 is offline gino22  Indonesia
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Dear All,

Many thanks for your advices, really appreciated.
I've tried new tube replacements (at least 3 new combination) and lots of swapps, so this is not the tubes.

Ciao, the left channel took only one replacement and the problem permanently fixed. You might be right regarding the parts, it was surprising when I measured the old output and coupling caps, they showed a good condition. it is different with the right channel, two times replacement of the output and coupling caps didn't fixed the problem at all. I did also replace the input stage caps, but problem remains.

DF96, I agree with you, this caused by main voltage fluctuation. I did simultaneous measurement to the unit and main outlet, a small rapid change in main outlet (as small as 0.3V) can caused rapid change in DC exceeding +-60mV. The problem is, how to find the breaking components? or some that might be in the breaking edge?. Further, I need your advice regarding poor LF stability, PSU Filtering, and how to handle this. Is it about the design itself? or would the problem be solved by replacing some component in the path? I started being nervoeus that this is about the desgn.
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