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Old 11th March 2007, 01:45 AM   #1
smbrown is offline smbrown  United States
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Default Finsihed LM3875 kit, wow!

I bought the kit from Audiosector a couple months ago but just got around to building it. I built it as a stereo amp. My background so far has primarily been tubes and tube based systems, well, at least for the last 10 years. So I'm comparing the LM3875 to some pretty cool stuff, like my parafeed SET 45 amp using nickle output transformers. All together about $1200 for parts alone. How did the LM3875 kit do? Embarassingly well. It was a bit deeper on bass (not necessarily a good thing as it really emphsized turntable rumble), a bit more extended on the highs. I think the vocals are smoother on the 45 amp, and bass has more weight, though not as deep. Overall, a very close call. Something I like the SS amp better for, overall listenability, still goes to the 45. I think one issue that I may be able to fix on the 3875 to move it even closer is to try and tame a bit of a hard edge in the upper mids. This is most noticable on piano (what isn't?) above middle C (guessing here). The notes have more of a hardness, kind of carnival sound that is a bit annoying. I tried to put in a carbon volume pot to see what that did. It took the edge off, but made everything else a bit muddy too. Next I may try taking that out and chaning the 200 ohm series R to the + input to a carbon comp or carbon film and see what that does, not much room on the board for those old "normal size" parts, however. Anyway, it's been loads of fun so far. Any suggestions would be welcome. The amp has about 15 hrs on it right now.
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Old 11th March 2007, 08:05 AM   #2
G is offline G  United States
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Default Re: Finsihed LM3875 kit, wow!

Quote:
Originally posted by smbrown
I bought the kit from Audiosector a couple months ago but just got around to building it. I built it as a stereo amp. My background so far has primarily been tubes and tube based systems, well, at least for the last 10 years. So I'm comparing the LM3875 to some pretty cool stuff, like my parafeed SET 45 amp using nickle output transformers. All together about $1200 for parts alone. How did the LM3875 kit do? Embarassingly well. It was a bit deeper on bass (not necessarily a good thing as it really emphsized turntable rumble), a bit more extended on the highs. I think the vocals are smoother on the 45 amp, and bass has more weight, though not as deep. Overall, a very close call. Something I like the SS amp better for, overall listenability, still goes to the 45. I think one issue that I may be able to fix on the 3875 to move it even closer is to try and tame a bit of a hard edge in the upper mids. This is most noticable on piano (what isn't?) above middle C (guessing here). The notes have more of a hardness, kind of carnival sound that is a bit annoying. I tried to put in a carbon volume pot to see what that did. It took the edge off, but made everything else a bit muddy too. Next I may try taking that out and chaning the 200 ohm series R to the + input to a carbon comp or carbon film and see what that does, not much room on the board for those old "normal size" parts, however. Anyway, it's been loads of fun so far. Any suggestions would be welcome. The amp has about 15 hrs on it right now.
I just bought the same kit today. Let me know how the carbon film sounds at the input. From what I understand some people like it without any series resistor at the input. I am still debating on whether to use carbon film resistors in the amp. I too am a tube guy. I hope that I am not dissapointed with the kit, but if so oh well. What preamp are you using? What phono stage?
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Old 11th March 2007, 08:14 AM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
is that 200r input resistor
a gain setting resistor for an inverting chipamp?
or
is it the R part of an RF input filter in a non-inverting chipamp?

Changing the resistor material/assembly method could make a little difference to the sound.
Omitting it could completely change the amp design.
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Old 11th March 2007, 08:31 AM   #4
G is offline G  United States
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It is the series resistor at the positive input on the non inverting design.
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Old 11th March 2007, 08:37 AM   #5
Salas is online now Salas  Greece
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I am surprised. In my system the gainclone was just so much inferior to tubes.
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Old 11th March 2007, 10:18 AM   #6
Nordic is offline Nordic  South Africa
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These babies perfrom pretty well after a valve buffer/preamp stage....Generaly we are talking a new league of class. Maybe you want to look at that....

I wouldn't worry to much about the carbon comps, the best , basic IGC I build was with the cheapest stuff I could find (before I learned how to source parts) even stuff like regular Non-polar electrolytic caps on the input...

Now my best advice is to make sure to use low ESR caps on power pins, this tends to take away any glare (hisslike bright distortion on edges of treble sounds) good quality cap on input ( I like those big thumblike axial lead ones).

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Old 11th March 2007, 11:45 AM   #7
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I experienced a similar thing just yesterday at our monthly audio club. Entire room full of tube gear and one tiny gainclone (chipamps-3886). We ran the clone with a passive pre and then used a tube-pre and everyone was shocked. The most common comment was, "very smooth" which sounds like a good compliment from tube guys.

I still think it is all personal taste but I quite happy considering the simplicity of this project.

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Old 11th March 2007, 11:53 AM   #8
smbrown is offline smbrown  United States
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Gavin, you asked, "I too am a tube guy. I hope that I am not dissapointed with the kit, but if so oh well. What preamp are you using? What phono stage?"

I am using a line stage based on Allen Wright's FVPA design. Really a wonderful preamp. Nice low output impedance. The phono stage is based on an Audio Inovations P2. These are both tube based designs. I tried the GC with input directly from my CD player and preferred it with the tube pre up front, by a wide margin.
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Old 11th March 2007, 11:56 AM   #9
smbrown is offline smbrown  United States
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Nordic, you suggested a cap on the input, my GC kit did not have an input cap. Is this something you found helpful? I was actually thinking of adding one to try and limit the LF extension below, say 30hz to control rumble and feedback from my turntable.
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Old 11th March 2007, 12:29 PM   #10
Nordic is offline Nordic  South Africa
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I find it very unlikely your kit would have no DC protection on the input... Ultimately the best cap in the signal path (where no DC is present) is, no cap. It is posible to have a cap on the output side still doing the filtering and DC blocking I guess.

So depending on how low the DC offset of the source is, you may well get away without. Wouldn't plug a pc into that though.... not unless you can easily replace the speakers.

I didn't have a particulartly good valve buffer, nice enough tube, but minimalist circuit / iron. It still sounded better than just the GC. I have since built Pedja Rogic's Jfet buffer which is up there in the tube league where quality is concerned... it is still sounding better to me everyday.... Much more detail than my valve setup, but still slightly less listenable... The valves have a much more romantic warmth to the sound, I have heard a pretty clinical sounding aikido too to prove the rule...

NOW GET OUT THAT WEBCAM AND LETS SEE!!!!!!!!!!

Ps you need a cap and a resistor to make the highpass filter you mentioned... I think 30Hz may be a little high

0.1uf (100nf) + 47k will have its knee at 34Hz
0.1uf + 60k will be at 27Hz.....
75k @ 21Hz...

play with those values and see how low you can go before the rumble comes back...

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