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Old 18th May 2017, 10:43 PM   #1
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Default Lehmann Audio Black Cube SE II question

Hello all,
I have a lehmann Audio Black Cube SE II that had a blown channel.
Got the replacement opamp THAT 1510. When I went to replace it there was a resistor across pins 1&8 only on one channel.
This is supposed to set the gain, but wouldn't there be one on both channels?


BTW it sounds balanced

thanks
mdr
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Old 18th May 2017, 11:04 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by mdelrossi View Post
there was a resistor across pins 1&8 only on one channel.
Are you sure? The two channels seem to be laid out the same.
Is a resistor is missing in one of the footprint positions?
http://www.audiocostruzioni.com/r_s/...I/dett%202.jpg
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Old 18th May 2017, 11:55 PM   #3
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Definitely, the resistor was soldered to the backside of the pcb ( bottom)
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Old 19th May 2017, 01:13 AM   #4
rayma is offline rayma  United States
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Quote:
Originally Posted by mdelrossi View Post
Definitely, the resistor was soldered to the backside of the pcb ( bottom)
If one resistor is really missing, you should install one that matches the other channel.
Is there any sign that one was soldered in place and removed, or are the pads solder free?
If one was never installed, you should tell Lehmann about this, they'll appreciate it.

Last edited by rayma; 19th May 2017 at 01:16 AM.
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Old 19th May 2017, 06:27 AM   #5
Calvin is offline Calvin  Germany
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Hi,

a resistor on the bottom is certainly a modification not present in the original.
The fixed MM-gain resistor between pins 1 and 8 of the THAT chip is a 220R 1% metal film.
A second resistor of 22R can be switched via Dip-switch pos1 of SW101/201 for the higher MC-gain.
The only off-the-shelf modificatioon on the PCB bottom side are the two soldered wire bridges that put the input stage into unbalanced mode.

jauu
Calvin
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Old 19th May 2017, 02:09 PM   #6
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Thanks for all the replies.
Upon further investigation the power supply isn't balanced. one side outputs 15v the other side 15.4. the higher side is where the resistor is attached to the back of the PCB.
Attached is the closeup of the adjusting resistors for the power supply.
one side is 120 and 1.3k the other is 240 and 2.7k. this doesn't make much sense.Click the image to open in full size.

Here is the top of the Cube , all resistors match side to side
Click the image to open in full size.

Last edited by mdelrossi; 19th May 2017 at 02:11 PM.
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Old 19th May 2017, 02:40 PM   #7
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Underside of PCB 2.9k?
Click the image to open in full size.
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Old 19th May 2017, 04:41 PM   #8
Calvin is offline Calvin  Germany
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Hi,

iIrc the datasheet of the LM317/337 regs shows feedback resistor values around 240R-270R and around 2k5-3k3 for the upper feedback resistors.
The set of 120R/1k3 results in similar output voltage, but represents probabely not the original values.
The soldering of the modded gain resistor looks very non-professional ... heck even the casings of both(!) nearby Dip switches are molten.

jauu
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Old 19th May 2017, 05:33 PM   #9
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Quote:
Originally Posted by Calvin View Post
Hi,


The soldering of the modded gain resistor looks very non-professional ... heck even the casings of both(!) nearby Dip switches are molten.

jauu
Calvin
ooops, my bad soldering angle on resistor and dip switches
But the resistor didn't look that much better before I replaced the opamp.

Any idea on why the values are so far off? Other than a mistake?

do you think they tried to compensate for the added voltage with the extra resistor on the back?

I'll order a few resistors that should conform better too the data sheet.
Or should I get a multi turn pot, and dial it in?

thanks

mdr

Last edited by mdelrossi; 19th May 2017 at 05:52 PM.
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Old 19th May 2017, 06:26 PM   #10
Calvin is offline Calvin  Germany
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Hi,

I don't know why the second set of resistors is lower in value .... maybe those resistors had been flying around.
For that added bottom side resistor .... it looks as if the colour code would read brown-white-black-red-violet, hence 19k 0.1%.
Paralleled with the original 220R the difference in gain is probabely only measurable but sonically irrelevant.
Still though I'd rather change back to the original state.

jauu
Calvin
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