John Curl's Blowtorch preamplifier part II - Page 488 - diyAudio
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Old 6th July 2010, 02:06 PM   #4871
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EUVL, there is no need for an input coupling cap with jfets on the front end. A moving coil cartridge cannot produce DC, and the leakage of the jfets is very low. SOME bipolar transistor circuits could create significant bias current with potentially problematic results, but not always. The Levinson JC-1, for example, provided bias cancellation to a first order, and we had no complaints about biasing current problems.
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Old 6th July 2010, 02:42 PM   #4872
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Quote:
Originally Posted by Rod Coleman View Post
Constructors wondering about assembling matched JFETs into a long-thermal time constant module may enjoy Jim William's application note AN124 at LT. He shows a pair of K369s epoxy-moulded into the cap of an apple-juice bottle, in a note aimed at measuring sub-uV noise levels in voltage references. The general construction techniques Jim uses are, as usual, works for admiration.

http://cds.linear.com/docs/Applicati...ote/an124f.pdf



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It's always nice to wade through all the subtle stuff but in the end floating a low noise preamp & A to D on the reference and doing all the signal processing in the digital domain would get the job done. Quantec had an excess noise fixture (noise on top of a 10V DC) that used ordinary off the shelf components.
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Old 6th July 2010, 06:50 PM   #4873
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One colleague of mine, who uses 2SK389's for a very low noise front end, uses 2 pairs of these devices with the same DC offset, and then mates them face-to-face for virtually 0 offset. He then adds thermal shielding to keep any change in temperature from effecting the readings to below perhaps 100 seconds or better.
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Old 6th July 2010, 06:56 PM   #4874
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"He singeth a quiet song"

Brilliant! I love him!

H&H and Jim's book jostled for shelf space ...
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Old 8th July 2010, 12:58 PM   #4875
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Quote:
Originally Posted by scott wurcer View Post
It's always nice to wade through all the subtle stuff but in the end floating a low noise preamp & A to D on the reference and doing all the signal processing in the digital domain would get the job done. Quantec had an excess noise fixture (noise on top of a 10V DC) that used ordinary off the shelf components.
Scott,

Nice to see you again.

So now that I have to buy 64 bit computers, does that mean we will soon be getting 64 bit a/d's and d/a's?

I can only imagine how good they will sound!



ES
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Old 10th July 2010, 02:34 PM   #4876
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The Blowtorch as a preamp is 130 watts . . . really?!?

"the unit draws over 300W on turnon and then a steady 130W in use"

-

A headphone out would be another 130 watts? And another 100 lbs??? lol


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Last edited by johnferrier; 10th July 2010 at 02:42 PM.
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Old 10th July 2010, 03:57 PM   #4877
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Less than 30W continuous, which is plenty already, considering the absence of volume/source switching current.
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Old 14th July 2010, 01:53 PM   #4878
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Originally Posted by simon7000 View Post
Scott,
Nice to see you again.
So now that I have to buy 64 bit computers, does that mean we will soon be getting 64 bit a/d's and d/a's?
I can only imagine how good they will sound! ES
Sorry Scott,

I thought I gave you a nice straight line. But for those who didn't get the joke at 64 bits you can go from the thermal noise level of a 150 ohm resistor at room temp to the energy required to remove the atmospheres of all the planets in the solar system and still have some oomph left!

ES
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Old 14th July 2010, 05:20 PM   #4879
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Well, back to the real world. The Blowtorch preamp is like an expensive automobile. If you can't afford the electricity, don't buy it. Why does it take so much current for a 'simple' preamplifier stage? This is because we want the amplification to be as effortless as possible, just like a high torque engine in responding to a change in conditions.
A secondary factor, especially for the phono stage, when included, is for lowest noise from the paralleled jfets on the input.
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Old 14th July 2010, 06:19 PM   #4880
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Quote:
Originally Posted by simon7000 View Post
Sorry Scott,

I thought I gave you a nice straight line. But for those who didn't get the joke at 64 bits you can go from the thermal noise level of a 150 ohm resistor at room temp to the energy required to remove the atmospheres of all the planets in the solar system and still have some oomph left!

ES
I've been away a lot lately. I don't remember who, but one of the great tape recording gurus (Doug Sax or Keith Johnson) once said in print that it would probably take 256 bits to equal their tweeked 30ips recorders.
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