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Old 2nd March 2006, 08:58 PM   #881
DaveM is offline DaveM  United States
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Hi Peter,

I have now gotten my LM3875 premium kit up and running. Things are humming away nicely on a set of nice little heatsinks and the proper transformer should be in the mail to me today (22v 400Va as opposed to the 18v 750Va intended for an Aleph X). Now I am considering adding a pot to make an integrated with the possibility of adding extra sources as well. I read back in the archives that you have tried several different pots and a stepped attenuator as well. What would you suggest for pot values (10K, 50K) and brand (Noble, PEC)?

Thanks

DaveM
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Old 2nd March 2006, 09:19 PM   #882
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I would suggest 25K - 50K Noble pot.

There is also another project coming out: a remote controlled volume attenuator with 130 steps and highest quality available. This will be better than all the pots and better than most of that type units available presently on the market.

You can read more about here: http://www.diyaudio.com/forums/showt...425#post839425

The boards went already into production, I'm still waiting for my assosiate to develop programming.

Here's the schematic (there's always only 7 resistors in a signal path: series and shunt positons):

Click the image to open in full size.
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Old 2nd March 2006, 09:32 PM   #883
DaveM is offline DaveM  United States
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Thanks for the heads up. I was aware of that and I am eyeing that to use in my X-BOSOZ that should have parts comming my way soon from the endless group buy. I believe that a decent pot will do just fine as this amp will probably end up in a room where the spousal acceptance factor must be high, and the end placement of the speakers will probably be less than optimal.
But we will see. I'm going to make sure to be able to short around the pot if needed, so that when the pre is done, I can use it as a straight amp. Now I just need some time in the shop to get the chassis completed.

Thanks

DaveM
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Old 3rd March 2006, 11:05 AM   #884
hayena is offline hayena  Czech Republic
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Oh yeah, the caps come from one czech high-end speaker company Xavian. I replaced them for normal 1500mF electrolyts yeasterday and it's defenetely the MKPs causing the problem with RF!

The size might be the issue, they act as an antenna....quite expensive, but useless idea anway

I will possibly look for those BlackGates N Peter uses in his patek. They are hard to get here in Czech rep though...
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Old 3rd March 2006, 11:26 AM   #885
karma is offline karma  Canada
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no sheilding inside the cap only plastic.

it sure looked good
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Old 3rd March 2006, 01:36 PM   #886
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Quote:
Originally posted by hayena
Oh yeah, the caps come from one czech high-end speaker company Xavian. I replaced them for normal 1500mF electrolyts yeasterday and it's defenetely the MKPs causing the problem with RF!

The size might be the issue, they act as an antenna....quite expensive, but useless idea anway

You may try to shield your big MKP caps with copper foil (connect shield to ground) and use additional small bypasses directly on amp's board, like 1 - 10uF caps perhaps. Otherwise, with MKP connected with wires, there is no capacitance directly at amp's supply pins and that may be causing problem as well.
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Old 3rd March 2006, 07:05 PM   #887
karma is offline karma  Canada
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copper foil works wonders
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Old 4th March 2006, 11:55 AM   #888
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Peter, just wanted to update on what happened to my LM4780. It was my own stupidity. After endless searching, what was wrong, it turned out to be a dry joint on the signal input on PCB side.

Now it's playing nicely on both channels and no hum. Thanks for all the help. Now onwards to the LM3875.
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Old 5th March 2006, 04:37 AM   #889
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Hi Peter,

What is the status of your Buffer Kits, and also the Integrated Chassis?

Robert
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Old 5th March 2006, 06:24 AM   #890
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I got all the components for the buffer in stock, boards as well, just not enough time to sort it out.

The chassis should be ready soon as well.
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