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Old 25th February 2005, 03:46 AM   #1
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Question BrianGT 3875 parts changes

From the Rev A to the Rev C boards, a couple of parts changed. For instance, the 4.7uF caps in the pS changed into 10uF. Also, the 220R resistor seems to have become 1K.

Does using the parts listed with the Rev A board with the Rev B board cause problems? Are there advantages to the Rev C parts, or are they imcompatable? And if I use the Zobel network parts listed with the C board, does this require use of the Rev C parts, or would the A/B parts be okay?

Really the issue is I have all of the A/B parts (+ the Zobel parts listed with C), but I'll need to pick up the resistors and caps for the C parts list, so I'd rather not have to.

Here are links to the 2 parts lists
Rev A http://www.chipamp.com/nigc_kit-users_guide.pdf
Rev C http://www.chipamp.com/lm3875.shtml

Thanks,

-d
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Old 25th February 2005, 04:56 AM   #2
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The original premium kit was based on attached schematic, that I posted once on a forum. This is also the same circuit and parts choices that I'm still using in Amp-1.

The Caddock 220R was used instead of Vishay, because it was cheaper and sounded equaly good.

4.7 BG N caps were used originaly, as they improved the performance of 1500u Panasonics. I tried (and later chose) those caps here, because I had some left over stock from using them previously for coupling in inverted version of the amp.

When Brian decided to come up with budget oriented version of a kit, 4.7u BG caps were substituted with 10u FC Panasonics, but this did not affect performance in any major way and those caps are a good choice as well.

Later, LM4780 kit was introduced, and since it was using 1k input series resistors, those were probably also used with the current LM3875 kit version to reduce amount of parts variation. Now, my choice of original 244R was strictly arbitrary (as I just happened to have Vishay in that value and I thought it would be suitable). I just needed some series resistor with amp's input as in certain setups I noticed that increased DC level was produced (when 50K pot was fully open). But this series resistor may as well be 1K and will not affect performance of the amp (although higher resistance might have stronger sonic signature influence).

You can mixed parts, and it will not cause any problems. You might even come up with a better sounding amp

For instance, presently, I'm not using any series resistance with the amp's input, and the sound is better (but only because the rest of the setup allows it). I used to like Vishays here, as they tend to attenuate brightness somewhat (especially with 1000u BG STD caps), but with 100u BG N I'm using now, none of it is neccessary.

Also, don't take it for granted that Zobel is an upgrade. In majority of well balanced systems it is actualy a degradation. Try it and decide for yourself.

PS: The speaker's polarity should be reversed, as when I was drawing that schematic, I was still under influence of inverting GC topology
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Old 25th February 2005, 05:48 AM   #3
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Like this : O)
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Old 25th February 2005, 01:34 PM   #4
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There was actually a brief period of time when I prefered listening to the amp with an iverted phase, but it didn't last long.

If someone is curious how MUR860 got into popularity, here's how it started. It was exactly 2 years ago

http://www.diyaudio.com/forums/showt...309#post134309

http://www.diyaudio.com/forums/showt...530#post136530

Here's more comments about 4.7u caps:

http://www.diyaudio.com/forums/showt...593#post136593
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