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Old 21st May 2007, 12:06 PM   #1
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Default Help needed for my preamp

Hi
Well after 2 months of building I powered up my preamp based on the Comet Octal Phono preamp courtesy of Hagerman Technology and,for the line amp, a 6sn7 SRPP.
I have managed to build a dual mono power supply utilising 1 5u4g 1 6080 1 AT7 and 1 5651 per channell.
2 questions.
1. the phono preamp has a 300k resistor to ground at its output. this output is connected to the phono section of the input selector switch. The line amp takes the input from the selector strait into the grid via a 100k pot (alps Blue).
I have the feeling that the impedances are not matched in this case. Is this correct? If this is the case then how should the output of the phono connect to the input of the line amp with respect to matching impedances?
2.my amp's tubes are all above the enclosure... the input tubes for the phono amp (6SL7) are obviously getting some induction in them.. if i put my hand over them the noise is louder... The next thing i am going to try is cooper canisters over them.. does anyone have some hints as to what else may help reduce this phenomenon?
I will post some pics of my amp.. now to building my parallel 300B monoblocs with outboard power supply.....
Thanks A bunch in advance for any help
regards
Nick
Australia
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Old 21st May 2007, 12:31 PM   #2
SY is offline SY  United States
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Why do you want to match impedances? In a voltage drive situation not involving transmission lines (which is the case for 99.999% of home audio), you want the lowest possible source impedance and the highest practical load impedance; one hits diminishing returns pretty quickly once the ratio of the two is larger than 20 or so.

I don't have a schematic of your particular unit, but I'd guess that the 300k resistor is largely irrelevant to the source impedance- it's probably there to prevent pops and bangs when switching sources.
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Old 21st May 2007, 01:13 PM   #3
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Default Thanks

Thanks for that
I am attaching schematics for you to peruse
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File Type: pdf cornetoctal.pdf (43.0 KB, 129 views)
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Old 21st May 2007, 01:15 PM   #4
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Default and the line amp

Here is the line amp stage
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File Type: gif 6sn7_sig.gif (5.6 KB, 266 views)
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Old 21st May 2007, 01:46 PM   #5
SY is offline SY  United States
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Yup, exactly as I suspected. the source impedance is about 2k or so, fine for driving the line stage. The 300k resistor is indeed there to prevent pops.
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Old 21st May 2007, 01:54 PM   #6
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Default Thanks for that

Thanks for explaining that to me.. I think i will the next 4 lifetimes trying to grasp all about tubes and their implementation..
Regards
Nick
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Old 21st May 2007, 02:11 PM   #7
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It's not that hard, really. Get a good book or two (broken record: I think the best currently-available text is Morgan Jones's "Valve Amplifiers"), spend some time reading and experimenting and before you know it, you'll be a guru.
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Old 21st May 2007, 05:31 PM   #8
Salas is offline Salas  Greece
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Default Re: and the line amp

Quote:
Originally posted by duderduderini
Here is the line amp stage

This one has been built by a friend recently with a tube rectified / regulated PSU. One of the best 6SN7 line stage sounds he ever got. It sounded better than a grid lick sans CK one and CC CF one with same supply.
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Old 22nd May 2007, 12:00 AM   #9
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Default I agree

I must admit that i have never been more mesmerised sonically than from this simple but beautiful sounding line stage.. to think that 1 tube and a hanful of components can leave everything i have heard behind is mind boggling but then again does keeping the signal path simple surprise me? Not at all.
The phono section is equally brilliant.. although i need to reduce rfi in the 1st stage (6sl7) i will try copper tubes over them....
If anyone has any ideas then please let me know
Meanwhile i will enjoy the amazing sound...
I must advocate tube regulated power supplies as well.. the difference is palpable...
Regards
Nick
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