John Curl's Blowtorch preamplifier part II - Page 202 - diyAudio
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Old 29th October 2009, 12:25 AM   #2011
iko is online now iko  Canada
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Quote:
Originally Posted by syn08 View Post
That would essentially significantly increase the noise. The Johnson noise of that resistor should be added to the JFET noise.
OK, I'm not disagreeing with you, but I'm trying to make sense of this. The same formulas don't apply, I think.

The output conductance when a resistor Rs is used between G and S is given by
go = gos^2 / (1 + Rs (gos + gfs))

Where for the j201 gos = 1uS and gfs = 0.5mS. Let's say Rs = 2k, then
go = (0.000001)^2/(1+2000*(0.000001+0.0005))
or
go = 0.00000000000049950049 S

Would the self noise amplify differently?
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Old 29th October 2009, 12:37 AM   #2012
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And here's a quick pass on a randomly selected LM329. Anode current is 1mA, reference voltage was measured as Vref=7.1V You would of course note the missing 60Hz peak, a good sign that the circuit has a low impedance. Also no filtering (electrolytic or otherwise) was used.

It happens that the noise is, at 108nV/rtHz, as good (or bad) as the best JFET Norton reference...

To follow in the next days, time permitting...
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Old 29th October 2009, 12:40 AM   #2013
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Originally Posted by ikoflexer View Post
OK, I'm not disagreeing with you, but I'm trying to make sense of this. The same formulas don't apply, I think.

The output conductance when a resistor Rs is used between G and S is given by
go = gos^2 / (1 + Rs (gos + gfs))

Where for the j201 gos = 1uS and gfs = 0.5mS. Let's say Rs = 2k, then
go = (0.000001)^2/(1+2000*(0.000001+0.0005))
or
go = 0.00000000000049950049 S

Would the self noise amplify differently?
On top of my head, I don't know, you can pick up, do yourself the math and make sense. I chosed to do the measurements at Vgs=0 because that's how JC decided to implement the Norton reference in the power supply buffers in his Vendetta Research (and Blowtorch) RIAA preamps. Also, the ~19V output voltage is not a coincidence.

Last edited by syn08; 29th October 2009 at 12:42 AM.
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Old 29th October 2009, 12:44 AM   #2014
Salas is offline Salas  Greece
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Quote:
Originally Posted by syn08 View Post
And here's a quick pass on a randomly selected LM329. Anode current is 1mA, reference voltage was measured as Vref=7.1V You would of course note the missing 60Hz peak, a good sign that the circuit has a low impedance. Also no filtering (electrolytic or otherwise) was used.

It happens that the noise is, at 108nV/rtHz, as good (or bad) as the best JFET Norton reference...

To follow in the next days, time permitting...
When a leds fast pass?. Low impedance we like. We want leds, we want leds, we want leds.
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Old 29th October 2009, 01:03 AM   #2015
syn08 is offline syn08  Canada
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Originally Posted by Salas View Post
When a leds fast pass?. Low impedance we like. We want leds, we want leds, we want leds.
Ok, family is going to kill me anyway:

Red LED, manufacturer is unknown (got them from a Chinatown store) @5mA. Voltage across the LED is 1.86V No filtering (electrolytic or other) and the LED is fed via a 2.7kOhm resistor. I'll do a JFET current source next time, but I'm not exepecting to be anytime better.

About 50nV/rtHz and now I really have to substract the HP89410A noise. SQRT(50^2-35^2)=35.7nV/rtHz You need 4pcs to get to 7V, this will make it to a total of about 70nV/rtHz So far, the best of all, but I wouldn't hold my breath. As I've already said, LEDs have huge variations from manufacturer to manufacturer and from batch to batch. Also, the output impedance is not that low...

Gotta go now...
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File Type: jpg RedLED.jpg (20.7 KB, 235 views)

Last edited by syn08; 29th October 2009 at 01:05 AM.
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Old 29th October 2009, 01:07 AM   #2016
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With Jfet will do better. It will at least skip some of the 60Hz. Thanks syn! You are a good sport!
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Old 29th October 2009, 01:08 AM   #2017
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syn08, it is very nice of you to take time to run these experiments. I hereby volunteer a case of your favorite beer (if you drink beer), which is too little a reward, you deserve more, but hey, that's my offer

I think this would be very valuable for a lot of people. Would you agree to test j201 (low Idss, pinch-off and, go) and 2n3819 (high Idss, pinch-off, and go). I can drop a few of them off anywhere downtown TO around lunch time.
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Old 29th October 2009, 01:15 AM   #2018
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leds respond to light, will generate noise from any varying light i.e. flouresent or monitor, so do diodes and transistors in glass packages
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Old 29th October 2009, 01:24 AM   #2019
Salas is offline Salas  Greece
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I keep em under the hood in the phono. Open it picks up nasties from fields anyway. Spray them black if they must be exposed?
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Old 29th October 2009, 01:41 AM   #2020
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Originally Posted by syn08 View Post
On top of my head, I don't know, you can pick up, do yourself the math and make sense. I chosed to do the measurements at Vgs=0 because that's how JC decided to implement the Norton reference in the power supply buffers in his Vendetta Research (and Blowtorch) RIAA preamps. Also, the ~19V output voltage is not a coincidence.
Fair enough. Unless I made a mistake, my calculation shows output noise of about 456nV/sqrtHz for the j201.
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