John Curl's Blowtorch preamplifier part II - Page 193 - diyAudio
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Old 27th October 2009, 07:25 PM   #1921
iko is offline iko  Canada
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Quote:
Originally Posted by john curl View Post
Ikoflexer, it is the wrong piece of equipment for the job. Just use an AC voltmeter with a 10K rolloff filter added. Look for Total noise up to 10KHz. Is it 7uV or 70 uV?
Why is it wrong? I'm curious.

The hp3581a is a selective voltmeter as well; it has a 300Hz wide filter that can be moved up to 50kHz. OK, it doesn't give me the noise up to 10kHz all at once, but it'll give me a good idea of what's going on, no?
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Old 27th October 2009, 08:03 PM   #1922
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Quote:
Originally Posted by ikoflexer View Post
Why is it wrong? I'm curious.

The hp3581a is a selective voltmeter as well; it has a 300Hz wide filter that can be moved up to 50kHz. OK, it doesn't give me the noise up to 10kHz all at once, but it'll give me a good idea of what's going on, no?
There is a app note that says noise figures are meaningless for audio design. Nat Semi or Analog Devices.

What are you trying to achieve ?

Last edited by Fanuc; 27th October 2009 at 08:06 PM.
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Old 27th October 2009, 08:10 PM   #1923
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Originally Posted by Fanuc View Post
There is a app note that says noise figures are meaningless for audio design. Nat Semi or Analog Devices.
What are you trying to achieve ?
Use a low noise voltage reference (such as discussed in the last few pages of this thread) in a voltage regulator to obtain a low noise psu.
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Old 27th October 2009, 08:43 PM   #1924
Fanuc is offline Fanuc  United Kingdom
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Originally Posted by ikoflexer View Post
Use a low noise voltage reference (such as discussed in the last few pages of this thread) in a voltage regulator to obtain a low noise psu.
a low noise psu ?? what's that exactly ?

it's job is to reject ac ripple! How can it have low noise if it's PSRR is 70db and it's self generated noise is -130db.

Never did get that one!

PS. I think the definition used by the switch mode designers (PSRR+noise) is better.

Last edited by Fanuc; 27th October 2009 at 08:49 PM.
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Old 27th October 2009, 10:03 PM   #1925
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Please everyone, don't play on the thread. If someone doesn't know what we are talking about, then educate yourself with Google, using key words. I don't teach basic electronics here.
We are NOT talking about noise figure, here. If you don't know the definition of noise figure, please don't pose an opinion about it. Yes, and by the way, I own 2 HP 3581 analyzers, and used one on my bench for years, you can TRY to measure the broadband (10-10,000Hz) noise with the HP 3581, but it will probably lead you to confusing results, since you don't see or can't independently compute the answer to the measurement in the first place. How will you know how to add your measurements to compare to National's? If you don't know, you should trust the opinions of people who do. What is in it for us, to mislead you?

Last edited by john curl; 27th October 2009 at 10:11 PM.
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Old 27th October 2009, 10:29 PM   #1926
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IF you insist on using an HP 3581, then just set the bandwidth control to 300 Hz, AND measure. You should see about 1 uV of noise. Let's see if you can figure out why.
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Old 27th October 2009, 11:23 PM   #1927
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3581A can go down to 3Hz BW.

Fanuc is right. It's hard for anybody to understand why there's such a desperate need for a pathologically low noise power supply. I've mentioned myself, that's only because the Vendetta input stage has virtually no PSRR. To me it's wrong design decision, but then I cannot debate your "sounds good" class of arguments. I've had enough subjectivism in the cable thread.

I'm asking again, can I post the Vendetta schematic so that anybody could understand what you are talking about?

Last edited by syn08; 27th October 2009 at 11:40 PM.
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Old 27th October 2009, 11:57 PM   #1928
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How much more psrr your cascode adds in your stage 2 phono with the 4 parallel 2SKs and common source resistor, on top of what just a load resistor could do for that input stage, and then again how much more your stage 3 with the folded cascode, syn08? Also how much it cuts on input capacitance by cascoding with so low degeneration help by the necessarily diminutive Rs for keeping their noise contribution negligible? Does the topping BJT contribute its residual noise directly to the shorted input noise of the stage and also the Vref noise for its base bias, or those two factors they should be non linearly added? Thanks.
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Old 28th October 2009, 12:52 AM   #1929
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For the record, all I have to do is to find a 'perfect' current source, with about 40ma Iq, to replace a pair of resistors that provide current to the first and second stage of the complementary folded cascode input stage. Right now, distortion is OK, noise is OK, but I could save a few cents on the power supply buffer, by making it noisy.
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Old 28th October 2009, 01:10 AM   #1930
Salas is offline Salas  Greece
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Could not parallel two G to S shorted 20mA Idss BF862s?
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