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Old 21st April 2010, 12:00 PM  
SY is offline SY  United States
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Join Date: Oct 2002
Location: Chicagoland
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Default His Master's Noise: A Thoroughly Modern Tube Phono Preamp

After more than 25 years of faithful service, it seemed that it might be time to redo my phono system. After all, I like to think that I've picked up a few tricks in the intervening years... The old system consisted of a VPI HW17-II, a Linn Ittok LVII tonearm, and a Troika cartridge. The Troika was...

Last edited by Variac; 2nd April 2011 at 12:17 AM.
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22nd April 2010
Sheldon
diyAudio Member
I would think that the MM version would just replace a step up transformer with a 1:1 line level transformer. A maximum signal level is specified, but no minimum. Don't see why it shouldn't work. Something like this: http://www.jensen-transformers.com/datashts/11p1.pdf

Sheldon
22nd April 2010
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Pano
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Yeah Sheldon, that might do the trick.

Gotta re-read the article to see how much gain is coming from the transformer, how much from the 1st stage.
22nd April 2010
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SY
diyAudio Moderator
20dB from the transformer (10x). As it happens, I've been working on an MM version...
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
22nd April 2010
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Pano
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Thanks SY. I did go back to look and saw the 10:1 transfo ratio.
Since a typical MM cart has an output of what ~ 3-5mV? There should be more than enough gain with a 1:1 input transfo.
22nd April 2010
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SY
diyAudio Moderator
What's not trivial is the input capacitance issue. But I think there are some interesting work-arounds. On va voir.
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
22nd April 2010
Sheldon
diyAudio Member
Around the other side of the transformer, maybe?
22nd April 2010
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SY
diyAudio Moderator
Not sure what you mean, Sheldon.

The high Miller capacitance of the input tube means that, first, the transformer will not be easily de-ringed with a 47k load, and second, the cartridge will not be happy because of the high input C. Dropping the 47k resistance to get the ringing out will make the cartridge even unhappier.

Moving coils are much better about load capacitance and resistance.
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
22nd April 2010
Sheldon
diyAudio Member
I see the problem now. I was referring to the strategy of applying various loads to either side of the transformer. I've seen some discussion of that before. However, that was with MC step up transformers and may not be useful at 1:1. But, maybe with a modest step up. And maybe playing with series resistance?

Sheldon

On further reflection, and fiddling with the numbers, I see the problem more clearly. Already in the range of typical load capacitance with just the input grid and other connections. Adding in a transformer will be tough. On the other hand, the higher output of most MM cartridges makes the balanced input less significant, as far as noise goes. I guess one could use an FET based balanced to single ended converter, but I'm not sure it's justified.
22nd April 2010
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event horizon
diyAudio Member
Very nice SY I see your comments that you made on a thread here came to fruition (might have been the bybee thread) where you mentioned a way of getting around the input noise issue. I can see how did it now. In fact in just the same way as has been mentioned in an article on "Grounding" i was reading earlier. Balanced - unbalanced, thus one input device & lower noise, neat!

Now if only my MC cartridge worked on both channels....
"Never let your morals prevent you from doing what is right!" Salvor Hardin
23rd April 2010
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Globulator
diyAudio Member
Thanks for posting this SY, great article and you are dead right about balanced line for the cartridge signals. Most arms have that anyway - with a separate earth lead for the arm. One just needs an isolating RCA socket on the pre-amp and most setups would not need modding.

Except mine as it is an old RB300, but that needs modding anyway!
Useful to see the current sources too with the 1k resistor - they should be good for 400V I think which makes them useful perhaps in my Sweet Peach FU50 driver circuit too....

Now to save up for the input transformers

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