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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 28th March 2013, 06:45 PM   #37481
1audio is offline 1audio  United States
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Originally Posted by simon7000 View Post
Thanks for the info. I'll start with the $25 signal generator as we have been using a discrete piece in my test kit.

Now can you tell me how to measure jitter to better than 200 ps for less than $10,000!
OK-
QA400 ($200) Jtest signal (found on the web, generator posted on DIYaudio somewhere, free)

The Minidac is pretty bad, but it was on the bench. What is clear is that getting a measurement with a jitter floor low enough to see anything relevant does not need to cost a lot.
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Old 28th March 2013, 06:45 PM   #37482
john curl is offline john curl  United States
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Coresta, why did you use another type fet?
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Old 28th March 2013, 06:54 PM   #37483
simon7000 is offline simon7000  United States
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OK-
QA400 ($200) Jtest signal (found on the web, generator posted on DIYaudio somewhere, free)

The Minidac is pretty bad, but it was on the bench. What is clear is that getting a measurement with a jitter floor low enough to see anything relevant does not need to cost a lot.
Close, but I have a 48K clock that I want to measure the "Phase Noise" or jitter. But since the QA400 looks like a really spiffy toy I'll get one. I need to have at least 1/2 as much test equipment as you Dick, if I want to keep up!

I am eBay following a scope.

But do try to convince me to shell out for a Stanford analyzer with jitter capability. The AP current version I was quoted at $27K and to be honest watching the AP system 2 drop in value so fast has me put off on the line.
 
Old 28th March 2013, 07:01 PM   #37484
Mooly is offline Mooly  United Kingdom
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John Curl's Blowtorch preamplifier part II
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Originally Posted by PMA View Post
Check the schematics thoroughly, this buffer has almost no DC output offset .... like 150 microvolts or so. Depends on JFET matching.
Thanks for the sim results Pavel. I guess I'd have to build and test one for real but I just know its going to be opamps I use.

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Originally Posted by john curl View Post
Mooly, I am not trying give you a specific design for YOUR needs, this schematic is over 20 years old and is intended for another company. It works fairly well down to 2K ohms or so, but 10K ohms and above is preferred.
It is just an ALTERNATE APPROACH to using an OP AMP for a buffer. It replaced the AD712 in one design with great success. It is not an 'idiot proof' design like an op amp is. It does have limited current drive, will NOT drive headphones, can be biased wrong and have lots of 2'nd harmonic, much more than one would want. Only high current versions of the 2SK170 or the 2SK389, etc will work optimally.
I realise that John, and I can see your whole design ethos is totally different to mine, but as I say to Pavel, it has to be opamp based really (for me).
 
Old 28th March 2013, 07:18 PM   #37485
jacco vermeulen is offline jacco vermeulen  Netherlands
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Sensitive to shaking and mechanical impacts.
Back in the Piher horror days, a veritable artisan could be recognised by traces of paint, and a seal ring on the pinky finger.

Last edited by jacco vermeulen; 28th March 2013 at 07:20 PM.
 
Old 28th March 2013, 07:23 PM   #37486
bcarso is offline bcarso  United States
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Back in the Piher horror days, a veritable artisan could be recognised by traces of paint, and a seal ring on the pinky finger.
How well I know those little black pots. Although, in their defense, they were significantly more reliable than the Piher dual pots with shafts.
 
Old 28th March 2013, 07:33 PM   #37487
john curl is offline john curl  United States
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Mooly, IF I had an 'ideal' op amp recommendation for your need, I would gladly give it to you. What amused me was that people who are always telling me that op amps are the way to go, gave you almost random advice as to which op amp to use. They don't (in general) know as much about op amps as I do, but they are quick to give advice.
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Old 28th March 2013, 07:48 PM   #37488
bcarso is offline bcarso  United States
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General comment on this buffer side-topic. The dual JFET approach is of course tried and true, with some appreciable input capacitance change with input signal (one distortion mechanism that is sensitive to source impedance), and some appreciable sensitivity to loading. However with good duals at least, the output offset voltage will be small.

There are ways with more parts to alleviate both sources of error. A bootstrapped cascode on both upper and lower FETs will remove the drain-gate C variation almost completely, although will entail high frequency instability depending on the source impedance as there will be a negative input capacitance at high frequencies.

For dealing with the loading effects, for a fixed load impedance one can provide an auxiliary complementary negative impedance using a negative immitance converter fashioned from an opamp. Or, the lower device(s) can provide an injection point for a compensating current based on the buffer output voltage and load. The JFETs etc. will still be the dominant buffer, effectively, unless one is trying to drive fiercely low impedances.

If the loading is variable as discussed, from a potentiometer, a second potentiometer section ganged with the main load pot can provide a variable negative impedance when used with the negative immitance circuit.

I used an LSK170 as the input device on one power amp design where I wanted high input Z (LSK because of availability in SOT-23). Rather than provide a matched current generator in its source I used cascode bipolars and an overall d.c. servo. The negative impedance was resistive and one of the sections of a LME49720 was used for the negative immitance converter. I also needed some gain so the overall circuit is more complicated than a unity-gain buffer. The overall front end was also quasi-differential, and a future project is to precisely derive the equations for the resistor values, which were determined by tweaking simulations. But overall the circuit performed beautifully, and allowed the power amp section to dominate the distortion and most of the noise. Alas, four of us jumped off the ship due to conflicts with the company founder.
 
Old 28th March 2013, 07:53 PM   #37489
Mooly is offline Mooly  United Kingdom
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John Curl's Blowtorch preamplifier part II
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Originally Posted by john curl View Post
Mooly, IF I had an 'ideal' op amp recommendation for your need, I would gladly give it to you. What amused me was that people who are always telling me that op amps are the way to go, gave you almost random advice as to which op amp to use. They don't (in general) know as much about op amps as I do, but they are quick to give advice.
Well this is one project where I'm actually beginning to contemplate using different devices for the different stages based on (what I deem for each) the most suitable attributes.

Thanks again John. Hopefully in the weeks to come I'll be able to tell you all what I finally settle on.
 
Old 28th March 2013, 08:35 PM   #37490
coresta is offline coresta  France
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Coresta, why did you use another type fet?
because i have these on hand : E421 dual , 2N4393 . Are the resistive values correct ? Offset is correct but working temp is very high !
 

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